Keywords: Men who have sex with men, HIV, transactional sex, French Antilles

The French Antilles (Martinique, Saint Martin, Guadeloupe) and French Guiana are the French territories most affected by the HIV epidemic. Some population groups such as men who have sex with men (MSM), especially those involved in transactional sex, are thought to be particularly vulnerable to HIV but few data exist to help characterize their health-related needs and thus implement relevant prevention interventions.

To fill this knowledge gap, we used data collected from an HIV/AIDS Knowledge, Attitudes, Behaviours and Practices survey conducted in 2012 among MSM living in the French Antilles and French Guiana and recruited through snowball sampling. Our objectives were to compare social and demographic characteristics and sexual behaviours between MSM engaging in transactional sex and MSM not engaging in transactional sex and to identify factors associated with transactional sex involvement using a logistic regression model A total of 733 MSM were interviewed, 21% of whom reported to undergo transactional sex. Their behaviour and social and demographic characteristics were different from other MSMs' and they were more exposed to factors that are recognized to potentiate HIV vulnerability, at the individual, community, network, and structural levels. The variables positively associated with sex trade involvement were having ever consumed drug (OR=2.84 [1.23-6.52]; p=0.002), having a greater number of sex partners than the median (OR=8. 31 [4.84-14.30] ; p<0.001), having experienced intimate partner violence (OR=1.72 [0.99-3.00] ; p=0.053), and having undergone physical aggression because of sexual orientation (OR=2.84 [1.23-6.52] ; p=0.014). Variables negatively associated with sex trade Health Sciences involvement were being older (OR=0.93 [0.90-0.97] per year; p=0.001), having a stable administrative situation (OR=0.10 [0.06-0.19] ; p<0.001), having a stable housing (OR=0.29 [0.15-0.55] ; p<0.001) and being employed fulltime (OR= 0.29 [1.23-6.52] ; p=0.002).

Introduction

The French Antilles (Martinique, Saint Martin, Guadeloupe) are located in the Caribbean, the region with the highest HIV prevalence after Sub-Saharan Africa. Although it is lower than in other states of the region, HIV incidence is much higher than in mainland France, reaching 59/100 000 PY in 2009-2010 for the three French Departments of America (French Guiana, Guadeloupe, Martinique), versus 17/100 000 in mainland France (Le [START_REF] Vu | Population-based HIV-1 incidence in France, 2003-08: a modelling analysis[END_REF]. In all settings where they have been studied, men who have sex with men (MSM) are disproportionately impacted by HIV compared to other heterosexual groups (S. D. [START_REF] Baral | Male sex workers: practices, contexts, and vulnerabilities for HIV acquisition and transmission[END_REF][START_REF] Beyrer | Global epidemiology of HIV infection in men who have sex with men[END_REF]. Beyrer et al. did a comprehensive review of available data for HIV prevalence and found that it reached 25,4% in the MSM population of the Caribbean [START_REF] Beyrer | Global epidemiology of HIV infection in men who have sex with men[END_REF].

In mainland France, self-declared HIV prevalence varies between 12% and 15% according to the Presse Gay and Baromètre Gay studies [START_REF] Velter | Baromètre gay 2005 : enquête auprès des hommes fréquentant les lieux de rencontre gay franciliens. Numéro thématique. Infections sexuellement transmissibles et VIH : les comportements à risque toujours d'actualité[END_REF][START_REF] Velter | Baromètre gay 2005 : enquête auprès des hommes fréquentant les lieux de rencontre gay franciliens. Numéro thématique. Infections sexuellement transmissibles et VIH : les comportements à risque toujours d'actualité[END_REF]. The Prevagay study conducted among MSM attending commercial gay venues in Paris showed that HIV seroprevalence was 18% and highlighted that 20% of MSM were unaware of their HIV positive status (Le [START_REF] Vu | Biomarker-based HIV incidence in a community sample of men who have sex with men in Paris, France[END_REF]. Le Vu et al.further showed that in 2008, incidence in MSM was of 1006 per 100 000 person-years. While overall HIV incidence decreased in the period between 2003 and 2008 it remained high and stable among MSM [START_REF] Beyrer | Global epidemiology of HIV infection in men who have sex with men[END_REF]. In the French Departments of America, incidence is estimated at around 1000 per 100 000, 28 times higher than in heterosexual men [START_REF] Vu | Population-based HIV-1 incidence in France, 2003-08: a modelling analysis[END_REF].

Public health research has typically treated the MSM population as a relatively homogenous group. However, some studies indicate that levels of exposure to HIV vary among MSM. In particular, MSM engaging in transactional sexhttp://mc.manuscriptcentral.com/ac-phm-vcy those who exchange sex for drugs or money-are thought to be at increased risk towards HIV acquisition and transmission.

Various studies have shown that transactional sex was associated with increased vulnerability to HIV among women but, compared to this substantial body of research, information on men who trade sex remains sparse. Recently, a meta-analysis of 33 studies from 17 countries compared HIV prevalence among MSM who engaged in transactional sex (MSM-TS) to those who did not (MSM-NTS) [START_REF] Oldenburg | Transactional Sex and the HIV Epidemic Among Men Who have Sex with Men (MSM): Results From a Systematic Review and Metaanalysis[END_REF]. Results showed that engagement in transactional sex was associated with a significant elevation in HIV prevalence (OR=1.34, 95 % CI=1.11-1.62), notably in Latin America (OR=2.28, 95 % CI=1.87-2.78).The increased vulnerability to HIV associated with transactionnal sex is thought to be the result of multi-level factors, intervening at the individual level (high number of sexual partners, inconsistent condom use, psychological distress, substance use) but also at the structural level (socio-economic precariousness, limited access to healthcare and rights, multi-layered stigma) (S. D. [START_REF] Baral | Male sex workers: practices, contexts, and vulnerabilities for HIV acquisition and transmission[END_REF][START_REF] Beyrer | Global epidemiology of HIV infection in men who have sex with men[END_REF].

To our knowledge, few, if any, have studied the factors associated with HIV risk among the MSM population in the French Antilles and French Guiana, where homosexual behaviours are stigmatized even more than in mainland France [START_REF] Halfen | Les habitants des Antilles et de la Guyane face au VIH/SIDA et à d'autres risques sexuels[END_REF].

To fill this knowledge gap, a KAPB (Knowledge, Attitudes, Practices and Behaviours) study on HIV and STIs was conducted in 2012 among MSM in the French Antilles and French Guiana by the CIC (Clinical Investigation Center) Antilles-Guyane a single INSERM structure that bridges Guadeloupe, Martinique and French Guiana.

Data collected were then analysed to explore HIV vulnerability of MSM engaged in transactional sex in order to better target prevention interventions.

The objectives were to compare socio demographic characteristics and sexual behaviours between MSM-TS and MSM-NTS using the analytical tool developed by [START_REF] Baral | Modified social ecological model: a tool to guide the assessment of the risks and risk contexts of HIV epidemics[END_REF]and to identify factors associated with transactional sex involvement.

Methods

Participants and Procedures

MSM were considered eligible to participate in the study if they were 18 years of age or older, had engaged in anal and/or oral sex with another male in the 12 months preceding the survey, and had provided written informed consent to participate.

Given that MSM represent a hidden population with no available sampling frame, snowball sampling, a nonprobabilistic method was used to recruit participants [START_REF] Heckathorn | Snowball versus respondent-driven sampling[END_REF]. The interviewers first had to identify and recruit a few members of the MSM population that fulfilled the study's inclusion criteria. These individuals referred to as "seeds" had to be diverse in terms of social and demographic characteristics. They participated in the survey and, at the end of the interview; these "seeds" were asked to refer one MSM from their social network. This process was repeated until the targeted sample size was reached. Appointments were made so that the individuals who accepted to take part in the study could choose when and where they wanted to be interviewed. The interviews were mainly carried out in private spaces (the participant's homes). They were conducted face-to-face by trained and multilingual interviewers familiar with the MSM population after the eligibility criteria were verified and informed written consent was obtained.

After the interview, which took from 40 to 60 minutes to complete, participants received an HIV prevention kit containing documentation on HIV/AIDS, male condoms and food vouchers.

The study was conducted in partnership with the Regional Coordination for the Fight against HIV and NGOs working with MSM (AIDES, ADER, Entr'Aides, Mama Bobi, An NouAllé, Rainbow Gwada, Paroles autour de la Santé). It

Study Instrument

The anonymous questionnaire used in the study was adapted from internationally validated Behavioural Surveillance Transactional sex involvement was based on two questions "Think about the commercial partners (partners with whom you bought or sold sex in exchange for money, services, goods and drugs you had in the previous month). In total how many were partners to whom you sold sex in exchange for money or drugs?». The second question used to construct the variable was "what is your main source of money/income at the moment?", sex work being one of the modalities. MSM were considered involved in transactional sex if they had exchanged sex for money or goods at least with one partner in the previous month or it they had declared sex work as their primary source of income."

Independent variables included socio demographic characteristics such as age, nationality, administrative status, housing status, sexual behaviours. For the "nationality" variable, a "Caribbean" category was created, including the individuals from the Dominican Republic, Haiti, Dominica or St Lucia. Individuals who declared having ever tried marijuana, crack or another drug were considered as "having ever consumed drugs".

Stable administrative status was defined as having French nationality or having a long-term (10 years) residence permit, whereas the individuals without residence permit, with a short-term residence permit (3 months or 1 year) or asylum seekers were considered as having an "unstable administrative status". Stable housing included individuals living in their own personal/familial house, in a rented house/apartment, or in social housing. People living in shared apartments, in informal settlements, on the streets, in a squat, and homeless people were included in the "unstable housing" category. A participant was considered as having undergone intimate partner violence if he had ever been humiliated, threatened, injured or insulted by a partner.

Sexual history and behaviour (notably condom use) with different types of partners was explored: regular partners were those with whom the participant "had sex for six months at least", commercial partners were those with whom the participant "bought or sold sex in exchange for money, services, goods or drugs" and occasional partners were those who were not regular partners nor commercial ones."

Statistical Analysis

First, a descriptive analysis was conducted to compare socio-demographic characteristics and sexual behaviours between 2 groups: MSM involved in transactional sex (MSM-TS) and MSM not involved in transactional sex (MSM-NTS). Mean and standard deviation for normally distributed variables, and frequencies and percentages for qualitative variables were calculated.

Categorical variables were compared between groups using Pearson's χ 2 test or Fisher's exact test when appropriate.

Comparisons of continuous variables were carried out using Student's t-test. Results of the descriptive analysis were Health Sciences framed with the modified social ecological model, a flexible analytical tool developed by [START_REF] Baral | Modified social ecological model: a tool to guide the assessment of the risks and risk contexts of HIV epidemics[END_REF] to guide the assessment of multi-level drivers of HIV infection risk.

Then, univariate and multivariate logistic regression analysis were performed to identify factors associated with transactional sex involvement. Given the large number of variables, the selection of independent variables for the multivariate model retained biographically pertinent variables and variables with a p<0.05 in univariate logistic regression. To obtain the final multivariate model, a backwards elimination procedure was used. All candidate variables were entered in the model, they were then removed one by one starting with the variable with the largest p value until all variables in the model are p<0.05.A 'missing data" category containing missing data and "does not know/no response" was created for each variable to avoid losing observations, and thus statistical power in the final multivariate model.

Analyses were conducted with Stata 13.0 (StataCorp, College Station,TX)

Results

Overall sample description

The final sample size consisted of 733 persons recruited and interviewed in the main cities of the four different sites:

344 were interviewed in Martinique, 223 in French Guiana, 124 in Guadeloupe, and 42 in Saint-Martin. Transgender people were also interviewed, but they were excluded in the present analysis and will be analyzed elsewhere.

The median age was 29 years ([IQR] = 24-37). Among the 733 MSM who completed the survey, 153 (20.8%) were involved in transactional sex (MSM-TS). Their characteristics were compared with those of MSM not involved in transactional sex (MSM-NTS).

Comparisons between MSM involved in transactional sex (MSM-TS) and other MSM (MSM-NTS)

Comparisons revealed significant differences with respect to socio demographic characteristics and sexual behaviours between the two groups. MSM-TS were more likely to have been interviewed in French Guiana and Saint-Martin.

They were also younger (27 years vs 32 years, p<0.001) and less likely to have French nationality. Table 1 shows that unstable administrative situation was common among male involved in transactional sex (60.1% vs 12%, p<0.001), as well as unstable housing (48.4% vs 14.3%, p<0.001). Only a small proportion of male involved in transactional sex were employed full-time (11.8% vs 60.7%, p<0.001).

Substance use was found to be higher among MSM-TS compared with MSM-NTS. Male who engaged in transactional sex were more likely to have consumed 5 or more drinks at one occasion in the previous month (83% vs 60%, p<0.001). At last sexual intercourse with casual partner, 80.7% of male who reported transactional sex had consumed alcohol, vs 48.1% in the other group (p<0.001). However, no significant difference was found for drug consumption history (having ever tried marijuana, or crack, or another drug) between the two groups.

Our results outlined differences in reported condom use between MSM-TS and MSM-NTS. Male involved in transactional sex were significantly more likely to have used a condom at last sexual intercourse, and to report systematic condom use with the regular partner. However, no significant difference was found between MSM-TS and MSM-NTS regarding frequency of condom use with the occasional and commercial partner: the proportion of individuals never using a condom, or using it only "sometimes" were high in both groups. One of the reasons why MSM-TS had not used a condom during their last commercial intercourse was because "the client had paid more for it".

Our results also highlighted the substantially high level of violence experienced by males involved in transactional sex: 40% of them had experienced intimate partner violence (vs 22.9% in MSM-NTS, p<0.001). Additionally, males involved in transactional sex were also more likely to have been injured or insulted due to their sexual orientation.

Multivariate analysis

Table 2 shows the results of the logistic regression analysis of independent factors associated with transactional sex involvement. The variables positively associated with sex trade involvement were having ever consumed drug Individual level factors refer to biologic or behaviours associated with higher vulnerability to acquire or transmit HIV.

(OR=2.
Our results showed inconsistent condom use among MSM-TS as well as MSM-NTS. Notably, 33.6 % of MSM-TS did not systematically use condom with their commercial partners. A focus on the underlying reasons why MSM-TS had not used a condom during their last commercial intercourse showed that this was because "the client had paid more for it". Non-use of condom because of client offering additional money has been reported by various studies among male sex workers [START_REF] Lau | Psychosocial factors in association with condom use during commercial sex among migrant male sex workers living in Shenzhen, mainland China who serve cross-border Hong Kong male clients[END_REF][START_REF] Reisner | Tricks of the trade: sexual health behaviors, the context of HIV risk, and potential prevention intervention strategies for male sex workers[END_REF]. Our results outlined higher substance consumption in MSM-TS than in other MSM. In the literature, alcohol use before or during sex is associated with increased sexual risk behaviour such as increased risk of unprotected anal intercourse [START_REF] Lane | Alcohol and sexual risk behavior among men who have sex with men in South african township communities[END_REF] and decreased ability to negotiate protection measures [START_REF] Liu | Recreational drug use: an emerging concern among venue-based male sex workers in China[END_REF].

Network level factors include the characteristics of sexual networks like their size or density. In our sample, the mean number of partners in MSM-TS was five times higher than other MSM. Large sexual networks have been associated

with unprotected anal intercourse [START_REF] Choi | High levels of unprotected sex with men and women among men who have sex with men: a potential bridge of HIV transmission in Beijing, China[END_REF][START_REF] Choi | An opportunity for prevention: prevalence, incidence, and sexual risk for HIV among young Asian and Pacific Islander men who have sex with men, San Francisco[END_REF] and provide increased probability to be exposed to HIV positive potential partners [START_REF] Smith | Associations between the sexual behaviour of men who have sex with men and the structure and composition of their social networks[END_REF].

Community -related factors analysis underlined that MSM-TS were more frequently victims of violence than MSM-NTS. In the French Antilles and French Guiana, same sex practices are not criminalized as in many Caribbean countries, but homosexuality remains highly stigmatized [START_REF] Halfen | Les habitants des Antilles et de la Guyane face au VIH/SIDA et à d'autres risques sexuels[END_REF], more so than in mainland France.

Various studies have shown that homophobic violence is associated with deleterious mental health outcomes such as psychological distress and depression [START_REF] Mills | Distress and depression in men who have sex with men: the Urban Men's Health Study[END_REF] and lower self-esteem and increased suicidal ideation [START_REF] Huebner | Experiences of harassment, discrimination, and physical violence among young gay and bisexual men[END_REF]. Homophobia also affects MSM's vulnerability to HIV in an indirect way : some studies suggest that the experience of discrimination and stigma associated with sexual orientation is strongly associated with fear of seeking health care services [START_REF] Fay | Stigma, health care access, and HIV knowledge among men who have sex with men in Malawi, Namibia, and Botswana[END_REF] and may act as a barrier to HIV testing (Pyun et Structural factors linked to HIV vulnerability should take into account the high proportion of MSM-TS having unstable administrative situations, with no or short-term residence permit. This may prevent them from accessing the legal employment market and could thus lead them to engage in transactional sex. Unstable administrative situations may also increase HIV vulnerability because it further complicates access to healthcare and prevention [START_REF] Bourdier | Malades et maladies en exil : les migrations brésiliennes vers la Guyane à l'épreuve du sida[END_REF]. Any undocumented migrant with limited resources living in France for at least three months may be eligible for State Medical Assistance (Aide Médicale d'Etat), a program covering health care expenses. However, in practice, only some of them have access to it. In the French Antilles and French Guiana, especially in French Guiana, a few studies have highlighted administrative practices that restrict access to healthcare and rights (request for abusive supporting documentation, loss of files, excessive delays, etc.) [START_REF] Carde | Adapter les pratiques médicales au terrain : maternité et VIH en Guyane à Saint-Martin[END_REF][START_REF] Jolivet | Migrations, santé et soins en Guyane[END_REF].

Finally, all these factors linked to HIV vulnerability should be interpreted within the context of the disproportionately high burden of HIV affecting MSM when compared to other population groups: in such an active epidemic context, any high-risk practice may have important epidemiological repercussions.

We further wanted to determine factors associated with engagement in transactional sex. Prior research has shown that young age, homelessness, regular and heavier use of alcohol and unemployment [START_REF] Bobashev | Transactional sex among men and women in the south at high risk for HIV and other STIs[END_REF][START_REF] Oldenburg | Transactional sex among men who have sex with men in Latin America: economic, sociodemographic, and psychosocial factors[END_REF][START_REF] Weber | Sex trade involvement and rates of human immunodeficiency virus positivity among young gay and bisexual men[END_REF] were independently associated with trading sex for money, drugs, housing or other goods. Our final multivariate model is consistent with these results, and reveals a profile of MSM-TS characterized by multidimensional precariousness, substance use and being the victim of violence.

Some limitations and biases should be taken into account when interpreting our results. The questionnaire was designed to assess the Knowledge, Attitudes and Practices towards HIV and STIs, not to specifically explore the issue of transactional sex. Therefore the analysis was lacking elements that may have provided some useful information such as frequency of sex trade, age at first commercial intercourse, or reasons for engaging in the sex trade.

In this study, MSM were recruited through snowball sampling. This method was chosen because it was adapted to hidden populations such as MSM [START_REF] Heckathorn | Snowball versus respondent-driven sampling[END_REF] and it was the best alternative given the human, financial and time resources of the project team. Selection biases may occur because people with large social networks have a greater probability to be sampled. To minimize this bias, interviewers who knew the MSM population were employed, and they had to recruit "seeds"(initial recruits) with contrasted characteristics. However, despite these efforts, snowball sampling remains a non-probabilistic method and as such, it makes the extrapolation of the findings to the broader MSM population from the French Antilles and French Guiana questionable.

Given the declarative nature of the data and face to face interviews, our data may also be subject to social desirability bias and thus sexual risk behaviours may have been underreported. However, the investigators were all close to the field, coming from NGOs or involved with the MSM population possibly minimizing this bias.

Finally, it is worth mentioning that since the study was cross-sectional, statistically significant relationships between sex trade involvement and the explanatory factors cannot be interpreted in terms of causality.

Despite these limitations and biases, this study remains the first to focus on the MSM population of the French Antilles and French Guiana. Our results highlighted some interesting points that may help optimizing prevention interventions for this at-risk subgroup of MSM. For instance, substance consumption or fighting against homophobia are potential priority targets that should be specifically addressed for increased impact.

However, it is worth reminding that MSM involved in transactional sex are not a homogenous group, and any prevention program will require further discussion with local actors (NGOs, health professionals, etc.) to be adapted to the diversity of social, economic and cultural contexts.

Conclusion

Transactional sex among females is a well-known prevention priority; MSM is also another well identified vulnerable group worldwide. However, MSM who sell sex are currently a blind spot of prevention programmes. Our results show a high proportion of MSM were involved in transactional sex with social vulnerabilities and high-risk practices that

  was funded by the European INTERREG Caribbean IV Fund, by the Hospital of Cayenne and the University Hospital of Pointe-à-Pitre. Study procedures were reviewed and approved by the Comité Consultatif sur le Traitement de l'Information en matière de Recherche dans le domaine de la Santé (CCTIRS n° 12.080), the Comité d'Evaluation Ethique de l'Inserm (CEEI n° 12-055) and the Commission Nationale Informatique et Liberté (CNIL n° DR-2012-264).

  Surveys and modified for local context with the help of non-governmental associations (NGOs). It was translated into five languages (French, Spanish, English, Portuguese, and Haitian Creole) and consisted of 115 questions exploring socio demographic characteristics, sexual history and behaviour with different types of partners (regular

  require specific attention. HIV prevention interventions should not only target individual factors but also the social context in which male involved in transactional sex live, as well as their sexual networks and their community environment. Recently, France approved free pre-exposure prophylaxis (PrEP) via a Recommendation of Temporary Use (RTU). Such antiretroviral-based prevention approaches could be relevant at the individual level: in a context of low condom use, it would give MSM engaging in high risk transactional sex an alternative to control their HIV risk. At a community and structural level, interventions that alleviate background vulnerability factors (fight against homophobia, access to rights and healthcare) should be reinforced.

  Baral et al. conceptualized a theoretical framework -the modified social ecological model-to characterize and analyze multiple levels of HIV risks among vulnerable populations (S.[START_REF] Baral | Modified social ecological model: a tool to guide the assessment of the risks and risk contexts of HIV epidemics[END_REF]. Five levels of risks are included: individual, network, community, policy and stage of the HIV epidemic. The HIV epidemic among MSM is driven by a complex set of factors at multiple levels, including individual, social and structural factors. Accordingly, Baral's model appeared a relevant conceptual framework to analyze multi-level vulnerability to HIV in the subpopulation of MSM involved in transactional sex.

	84 [1.23-6.52] ; p=0.002), having a greater number of sex partners than the median (OR=8.31 [4.84-14.30] ;
	p<0.001), having experienced intimate partner violence (OR=1.72 [0.99-3.00] ; p=0.053), and having undergone
	physical aggression because of sexual orientation (OR=2.84 [1.23-6.52] ; p=0.014). Variables negatively associated
	with sex trade involvement were being older (OR=0.93 [0.90-0.97] per year; p=0.001), having a stable administrative
	situation (OR=0.10 [0.06-0.19] ; p<0.001), having a stable housing (OR=0.29 [0.15-0.55] ; p<0.001) and being
	employed full-time (OR= 0.29 [1.23-6.52] ; p=0.002).

Discussion

al., 2014)

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Table 1 : Comparative Analysis http

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vcy For Peer Only Health Sciences Page 12 of 27 No transactional sex

			Transactional sex	p-value †
		(n=580)	(n=153)
		n (%)	n (%)
	Nationality (n = 733 )
	French	492 (84,8)	58 (37,9)	<0,001
	Guyanes e/Surinam es e	22 (3,8)	15 (9,8)
	"Caribbean"	18 (3,1)	11 (7,2)
	Brazilian	39 (6,7)	54 (35,3)
	Other	9 (1,6)	15 (9,8)
	Stable administrative situation (n=733)
	No	70 (12,1)	92 (60,1)	<0,001
	Yes	510 (87,9)	61 (39,9)
	Stable housing (n=733)
	No	83 (14,3)	74 (48,4)	<0,001
	Yes	497 (85,7)	79 (51,6)
	Full-time employment (n=733)
	No	228 (39,3)	135 (88,24)	<0,001
	Yes	352 (60,7)	18 (11,76)
	Condom at last anal sex (n= 709)
	No	184 (32,9)	32 (21,5)	0,007
	Yes	376 (67,1)	117 (78,52)

Condom use frequency with regular partner (n= 562)

	Never	163 (34,8)	18 (19,2)	0,012
	Every tim e	160 (34,2)	41 (43,6)
	Som etim es	145 (40,0)	35 (37,2)
	Condom use frequency with occasionnal partner (n=376)
	Never	12 (3,8)	4 (7,0)	0,271
	Every tim e	232 (72,7)	36 (63,2)
	Som etim es	75 (23,5)	17 (29,8)

Condom use frequency with commercial partner (n= 201)

	Never	0	5 (3,4)	0,259
	Every tim e	32 (61,5) *	99 (66,4)
	Som etim es	20 (38,5) *	45 (30,2)
	Important alcohol consumption (< 5 drinks on one occasion) (n=733)
	No	232 (40,0)	26 (17,0)	<0,001
	Yes	348 (60,0)	127 (83,0)

Drug consumption history (n= 733)

	No	500 (86,2)	124 (81,0)	0,11
	Yes	80 (13,8)	29 (19,0)

Alcohol consumption at last intercourse with commercial partner (n= 190)

	No	22 (47,8)	69 (47,9)	0,991
	Yes	24 (52,2)	75 (52,1)

Alcohol consumption at last intercourse with casual partner (n=354)

	No	154 (51,9)	11 (19,3)	<0,001
	Yes	143 (48,1)	46 (80,7)
	Intimate partner violence (n= 704)
	No	430 (77,1)	88 (60,3)	<0,001
	Yes	128 (22,9)	58 (39,7)
	Has previously been insulted because of sexual orientation (n=722)
	No	434 (76,0)	97 (64,2)	0,004
	Yes	137 (24,0)	54 (35,8)

Has previously been physically assaulted because of sexual orientation (n= 716)

	No	536 (94,7)	123 (82,0)	<0,001
	Yes	30 (5,3)	27 (18,0

) † p calculated with χ2 Tes t or Fis her's exact Tes t for qualitative variables , Student's Tes t for quantitative variables * individuals included in this category are thos e who reported having paid for s ex (com m ercial s ex as clients )

The KABP MSM study is part of the project "Mise en place d'un Observatoire Intercaribéen du VIH" supported by the University Medical Center of Guadeloupe.

Funding

The KABP MSM study was supported by the European INTERREG Caribbean IV Fund, by Cayenne General Hospital and the University Medical Center of Guadeloupe.


Generation of Shear Waves by Laser in Soft Media in the Ablative and Thermoelastic Regimes.

Pol Grasland-Mongrain 1 , Yuankang Lu 1,2 , Frederic Lesage 2,3 , Stefan Catheline [START_REF] Catheline | The Journal of the Acoustical Society of America[END_REF] , Guy Cloutier 1, 3,[START_REF] Fromageau | [END_REF] (1) Laboratory of Biorheology and Medical Ultrasonics, Montreal Hospital Research Center, Montreal (QC), H1X0A9, Canada (2) Departement of Electrical Engineering, École Polytechnique of Montreal, Montreal (QC), H3C3A7, Canada (3) Institute of Biomedical Engineering, École Polytechnique and University of Montreal, Montreal (QC), H3T1J4, Canada (4) Laboratory of Therapeutic Applications of Ultrasound, Inserm u1032, Inserm, Lyon, F-69003, France (5) Departement of Radiology, Radio-oncology and Nuclear Medicine, University of Montreal, Montreal (QC), H3C3J7, Canada (Dated: December 5, 2016) This article describes the generation of elastic shear waves in a soft medium using a laser beam. Our experiments show two different regimes depending on laser energy. Physical modeling of the underlying phenomena reveals a thermoelastic regime caused by a local dilatation resulting from temperature increase, and an ablative regime caused by a partial vaporization of the medium by the laser. Computed theoretical displacements are close to experimental measurements. A numerical study based on the physical modeling gives propagation patterns comparable to those generated experimentally. These results provide a physical basis for the feasibility of a shear wave elastography technique (a technique which measures a soft solid stiffness from shear wave propagation) by using a laser beam.

When a laser beam of sufficient energy is incident on a medium, the absorption of the electromagnetic radiation leads to an increase in the local temperature. Due to thermal effects, displacements occur in the medium, which can then propagate as elastic waves. Elastic waves within a bulk can be separated into two components: compression waves, corresponding to a curl-free propagation, and shear waves, corresponding to a divergence-free propagation [START_REF] Aki | Quantitative Seismology[END_REF] . Measures of the transmission characteristics of compression and shear waves are useful for inspecting solids, such as a metal, to reveal potential cracks or defects [START_REF] Shan | [END_REF] . In biological tissues, induction of compression waves by laser has been studied with the development of photoacoustic imaging 23 . Elastic waves used in photoacoustic imaging are typically of a few megahertz; at this frequency, shear waves are quickly attenuated in soft tissues, typically over a few microns, and only compression waves can propagate over a few centimeters.

While the induction of surface acoustic waves by laser in soft tissues was recently demonstrated by Li et al. 9 , 10 , a similar phenomenon with shear waves in bulk medium has never been described. This is of great interest for the shear wave elastography technique. As its name indicates, shear wave elastography comprises the techniques used to map the elastic properties of soft media using shear wave propagation 18 , 15 , 4 . These techniques typically use low frequency (50-500 Hz) shear waves so that their propagation can be observed over a few centimeters. The shear waves are currently generated using either an external shaker or a focused acoustic wave. However, alternative shear wave generation methods have drawn important attention recently. For example, it has been demonstrated that one can use natural motion of the medium 8 , 24 , the Lorentz force 3 , 7 , 6 or electrolysed-induced bubbles 14 . Compared to these gen-eration sources, a laser presents the advantage of being fully remote, without need of coupling gel; and of being miniaturizable at low cost using an optical fiber.

In our experiment, illustrated in Figure 1, we used a Qswitch Nd:YAG laser (EverGreen 200, Quantel, Les Ulis, France) to produce a 10-ns pulse of 10 to 200 mJ energy at a central wavelength of 532 nm in a 5-mm diameter circular beam. The laser beam was absorbed in a 4x8x8 cm 3 black mat phantom composed of water, 5% polyvinyl alcohol, and 1 % black graphite powder. Two freeze/thaw cycles were applied to stiffen the material to a shear modulus of 25±5 kPa 5 . To observe the resulting shear waves, the medium was scanned simultaneously with a 5-MHz ultrasonic probe, consisting of 128 elements, connected to a multi-channeled ultrasound scanner (Verasonics V-1, Redmond, WA, USA), and placed on the other side of the sample. The probe was acquiring 4000 ultrasound images per second during 30 ms. Due to the presence of graphite particles, the medium presented a speckle pattern on the ultrasound image.The computation of displacements along the Z axis (the ultrasound axis) was computed by tracking the speckle spots with the Lucas-Kanade method [START_REF] Lucas | Proceedings of Imaging Understanding Workshop[END_REF] . This method solves basic optical flow equations by least squares criterion in a window of 64*5 pixels centered on each pixel. Displacements over time were then filtered from 200 to 800 Hz using a 5th-order Butterworth filter and averaged over four experiments.

Figure 2 illustrates the resulting displacement amplitude maps observed along the ultrasound axis at 1.0, 1.5, 2.0, 2.5, and 3.0 ms after laser emission for two laser beam energies (10 and 200 mJ). Displacements reached an amplitude of 0.02 µm for the 10-mJ laser beam and 2.5 µm for the 200-mJ laser beam. They propagated at a velocity of 5.5±0.5 m.s -1 , which is typical for a shear wave, but far slower than the usual velocity of a compression wave (about 1500 m.s -1 in soft tissues). Shear modulus can then be calculated using the relationship v s = µ/ρ. Supposing the medium density, ρ, at 1000 kg.m -3 (water density), the propagation velocity corresponds to a shear modulus of 30±5 kPa, which is in the range of the expected value for this phantom. Shear wave frequency spectrum was centered at 500 ± 50 Hz.

Careful observation reveals several differences in the propagation patterns of the two laser beam energies. At low energy, first central displacements are directed towards the outside of the medium (left arrow), and three half cycles are observed. Conversely, at high energy, first displacements are directed towards the inside of the medium (right arrow), and only two half cycles can be observed. Let's examine now the physical phenomena involved in these experiments. The optical intensity, I 0 , of the laser beam is defined as

I 0 = 1 S dE
dt , where S is the beam surface and E is the beam energy. When emitted on the surface of a medium and in the absence of reflection, the laser beam is absorbed with an exponential decay as a function of the medium depth z: I(z) = I 0 exp(-γz), where γ is the absorption coefficient of the medium. We have experimentally estimated γ by measuring the fraction of light transmitted through different slices of the medium (with thicknesses of 0, 30, 50, and 100 µm) with a laser beam energy-measurement device (QE50LP-S-MB-D0 energy detector, Gentec, Québec, QC, Canada). We found respective transmitted powers of 100%, 42%, 28%, and 11%, which gives γ -1 ≈ 40 µm in our sample as indicated by an exponential fit.

Absorption of the laser beam by the medium subsequently gives rise to an absorbed optical energy, γI. Assuming that all the optical energy is converted to heat, a local increase in temperature occurs. Temperature distribution, T , in the absence of convection and phase transition, can be computed using the following heat equation:

k∇ 2 T = ρC ∂T ∂t -γI ( 1
)
where k is the thermal conductivity, ρ is the density, C is the heat capacity and t is the time. Calculating the exact solution to this equation is beyond the scope of this article, but we can roughly approximate the first and second terms to be kT /γ 2 and ρCT /τ , respectively, during laser emission. Given that k = 0.6 W.m -1 .K -1 (water thermal conductivity), ρ = 1000 kg.m -3 (water density), C = 4180 J.kg -1 .m -3 (water heat capacity), γ -1 ≈ 40 µm and τ = 10 ns, the first term is negligible compared to the second one. Substituting low-energy experimental parameters (E = 10 mJ, S = 20 mm 2 ) leads to a maximum increase in temperature of 3 K, which produces a local dilatation of the medium. The induced displacements can then generate shear waves, which constitutes the thermoelastic regime.

To estimate the initial displacement amplitude in this regime, we assumed the medium as homogeneous and isotropic. As the depth of absorption (about 40 µm) is 100 times smaller than the beam diameter (5 mm), we discarded any boundary effects. The stress, σ zz , is the sum of the axial strain component and the thermal expansion component [START_REF] Scruby | Laser Ultrasonics Techniques and Applications[END_REF] :

σ zz = (λ + 2µ) ∂u z ∂z -3(λ + 2 3 µ)α E ρCSζ ( 2
)
where λ and µ are respectively the first and second Lamé's coefficients, α is the thermal dilatation coefficient, and ζ is the average depth of absorption. This equation can be simplified by the fact that in most soft media, including biological tissues, µ λ. Moreover, in the absence of external constraints normal to the surface, the stress across the surface must be zero, i.e. σ zz (z = 0) = 0. This allows the equation 2 to be integrated, giving at the surface a displacement u z = 3αE/(ρCS). Substituting the same experimental parameters used previously along with α = 70.10 -6 K -1 (water linear thermal dilatation coefficient), we obtain a displacement u z of 0.025 µm. This value is very close to the measured experimental displacement (about 0.02 µm). Note that both the experimental and theoretical central displacements are directed towards the outside of the medium (see white circle arrow in Figures 2-(A

) and 3-(A)).

To calculate the propagation of the displacements as shear waves, we must consider the transverse dilatation. Indeed, the illuminated zone is a disk of 5 mm in diameter with a thickness of about 40 µm (average depth of absorption along Z axis). The dilatation stress is consequently about two orders of magnitude stronger along the transverse direction than along the Z axis: to compute the displacements along time, we neglected the stress along the Z axis and only considered the stress in the transverse direction. We modeled thus the thermoelastic regime in 2D as two opposite forces directed along the Y axis with a depth of 40 µm and with an amplitude decreasing linearly respectively from 2.5 to 0 mm and from -2.5 to 0 mm. The magnitude of the force along space and time is stored in a matrix, H thermo y (y, z, t). Displacements along the Z axis are then equal to the convolution between H thermo y (y, z, t) and G yz 1 :

G yz = cos θ sin θ 4πρr ( 1 c 2 p δ P - 1 c 2 s δ S + 3 r 2 r/cs r/cp τ δ τ dτ ) (3)
where δ P = δ(t-r cp ), δ S = δ(t-r cs ), δ τ = δ(t-τ ), c p and c s are the compression and shear wave speed respectively, τ is the time, and δ is a Dirac distribution. The three terms of the equation correspond respectively to the farfield compression wave, the far-field shear wave, and the near-field component.

Using constants ρ = 1000 kg.m -3 , c p = 1500 m.s -1 and c s = 5.5 m.s -1 (similarly to experimental measurement), displacement maps along the Z axis were calculated 1.0, 1.5, 2.0, 2.5, and 3.0 ms after force application, as illustrated in Figure 3-(A). The normalized displacement maps present many similarities to the experimental results displayed in Figure 2-(A), with an initial central displacement directed towards the outside of the medium, and a propagation of three half cycles.

Next, we examine the physical characteristics of the other regime observed in our experiments. Solving the equation 1 with the same experimental parameters used previously, but with a laser energy of 200 mJ, we find a maximum increase in temperature of 60 K, i.e., a maximum medium temperature of about 360 K assuming a room temperature of about 298 K. While slightly below the vaporization point of our medium, supposed close to 373 K (water vaporization temperature), its proximity to the water vaporization temperature may be sufficient to vaporize the medium. Indeed, it has been demonstrated that graphite and other small particles can act as nucleation sites to facilitate the vaporization of the medium at temperature lower than the vaporization point 2 . A series of reactions then leads to displacements inside the medium, which generate shear waves; this constitutes the ablative regime.

To estimate the initial displacement amplitude in this regime, we again assume that the medium was homogeneous and isotropic, and we discard any boundary effect. The stress, σ zz , is now defined as the sum of the axial strain component and a term given by the second law of motion caused by the reaction of the particles ejected outside the medium upon reaching the vaporization point [START_REF] Ready | Effects of High Power Laser Radiation[END_REF] :

σ zz = (λ + 2µ) ∂u z ∂z - 1 ρ I 2 (L + C(T V -T 0 )) 2 (4)
where L is the latent heat required to vaporize the solid, and T V and T 0 are the vaporization and initial temperatures, respectively.

By assuming µ λ and a zero stress state at the medium surface, equation 4 leads to a displacement u z = ζI 2 /(ρλ(L + C(T V -T 0 )) 2 ). Using high-energy experimental parameters, ζ ≈ γ -1 = 40 µm (average depth of absorption), λ = 2 GPa (first Lamé's coefficient of water), L = 2.2 MJ.kg -1 (vaporization latent heat of water), and T V -T 0 = 373-298 = 75 K, we obtain a displacement u z of 2.9 µm. Again, this value is in agreement with the experimentally obtained displacement (2.5 µm). Both theoretical and experimental displacements are directed towards the inside of the medium (see arrow in the white circle in Figures 2-(B) and 3-(B)).

To calculate the propagation of the displacement as a function of space and time, we modeled the ablative regime as a point force directed along the Z axis with a depth of 40 µm and a constant value from -2.5 to 2.5 mm. The magnitude of the force was stored in a matrix, H abla z (y, z, t). Displacements along the Z axis are again Using the physical quantity values previously defined, displacement maps along the Z axis were calculated 1.0, 1.5, 2.0, 2.5, and 3.0 ms after force application, as illustrated in Figure 3-(B). The displacement maps present many similarities to the experimental results of Figure 2-(B), with initial displacement directed towards the inside of the medium, and propagation of only two half cycles.

Finally, the dependence of the shear wave amplitude versus laser energy was quantitatively investigated by increasing the laser beam energy from 10 to 200 mJ. Amplitudes were averaged over four experiments for each energy level and successive energy levels were randomly chosen to avoid any time-related bias. In addition, impact location was changed after each laser emission to avoid any potential local degradation of the medium. Shear wave amplitude was measured as the mean square amplitude at t = 1 ms of the displacement estimated from ultrasound images between 0 and 10 mm of the medium surface, which was an arbitrary location where shear waves demonstrated high amplitudes. Resulting measurements are illustrated in Figure 4. Two linear dependencies are observed: one from 10 to 40 mJ, with a slope of 1.05 (R 2 = 0.87), and one from 30 to 200 mJ, with a slope of 2.18 (R 2 = 0.97). This is in accordance with the theory that displacement is linearly dependent on energy at low energies, i.e. in the thermoelastic regime, but quadratically dependent at high energies, i.e. in the ablative regime. The threshold around 30-40 mJ is specific to our material characteristics, notably its absorption coefficient.

In the context of shear wave elastography, the thermoelastic regime is a priori preferred over the ablative regime, because it is not destructive. Even if first shear wave elastography experiments assumed that a displacement of a few hundred nanometers would be sufficient 16 , displacements of the order of a few micrometers are usually required in practice for ultrasound or magnetic resonance elastography in biological tissues 17 , 13 . This amplitude is higher than the displacement we observed at 10 mJ (thermoelastic regime), but along the same order of magnitude of the displacement observed at 200 mJ (ablative regime). For application in human body, the lowest fluence (500 J/m 2 , corresponding to a 10 mJ, 5 mm-diameter laser beam) that was used in these experiments is incidentally 2.5 times above the maximum permissible exposure for skin (200 J/m 2 ) given by the Z136.1-2007 standard of the American National Standard Institute. To overcome this issue, different strategies could be adopted, including using other types of laser, with higher maximum permissible exposure; emission of the laser beam onto a protective absorbing layer, such as a black sheet covering the patient's skin 10 ; or using a higher resolution imaging technique able to track smaller displacements, such as high frequency (>100 MHz) ultrasound imaging or optical coherence tomography -in this last case however, low penetration depth may lead to the observation of surface acoustic waves instead of shear waves. Combination with optical coherence tomography may even lead to a realtime, fully remote, small-scale laser-based technique to assess a soft solid stiffness 11 , 22 .

Figure 1 .

 1 Figure 1. Experimental setup. A laser beam is emitted on a soft medium. This generates shear waves following (a) thermoelastic and/or (b) ablative regimes. The medium is observed with an ultrasound probe. A speckle-tracking algorithm calculates displacements from the ultrasound images.

Figure 2 .

 2 Figure 2. Experimental displacement amplitude maps along the Z axis with a (A)-10 mJ or (B) 200-mJ laser beam, 1.0, 1.5, 2.0, 2.5 and 3.0 ms after laser emission in the tissuemimicking phantom. In both cases, direction of the initial central displacements is indicated in the white circle.

Figure 3 .

 3 Figure 3. Normalized displacement maps along the Z axis 1.0, 1.5, 2.0, 2.5, 3.0 ms after laser emission as produced by simulation in a 5 mm disk diameter of (A) two opposite forces along the Y and (B) a force along Z axis. Applied forces are illustrated by the violet arrows on the most left end side, and direction of the initial central displacement by the arrows in the white circles.

  with the same notations as presented in equation 3.

Figure 4 .

 4 Figure 4. Experimentally measured amplitude of the shear wave versus laser beam power (log-log scale). Two linear dependencies are observed: one from 10 to 40 mJ (slope of 1.05), and one from 30 to 200 mJ (slope of 2.18). Four displacement maps at t = 2.0 ms are illustrated, for laser beam energies of 10, 30, 80 and 200 mJ. At low energy, a third half cycle is observed, which disappears when energy increases.


Yvon Lemaître

Nicolas Pauwels

Christophe Le Gac

Christophe Le

Micro-rain radar en bande X pour la climatologie des propriétés microphysiques des systèmes précipitants : ROXI (Rain Observation with an X-band Instrument)

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