(1)(2)(3)(4)(5)(6)(7)
HAL Id: jpa-00223227
https://hal.archives-ouvertes.fr/jpa-00223227
Submitted on 1 Jan 1983
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
IN VIVO PHOTOACOUSTIC SPECTROSCOPY OF THE SKIN
P. Poulet, J. Chambron
To cite this version:
P. Poulet, J. Chambron. IN VIVO PHOTOACOUSTIC SPECTROSCOPY OF THE SKIN. Journal
de Physique Colloques, 1983, 44 (C6), pp.C6-413-C6-418. �10.1051/jphyscol:1983668�. �jpa-00223227�
I N V I V O P H O T O A C O U S T I C SPECTROSCOPY O F THE S K I N P. P o u l e t and J . Chambron
I n s t i t u t de Physique BioZogique, FacuZte' de Me'decine, 4 , rue KirschZeger,
6 7 0 8 5 Strasbourg Cedex, France
Resume - Un spectrometre photoacoustique u t i l i s a n t un d e t e c t e u r d i f f e r e n t i e l permet de mesurer i n v i v o l e s spectres d ' a b s o r p t i o n o p t i q u e de l a peau humai- ne. Sa conception e t ses c a r a c t e r i s t i q u e s sont d e c r i t e s ; le s premiers r e s u l - t a t s experimentaux sont presentes.
A b s t r a c t - b!e describe the conception and t h e c h a r a c t e r i s t i c s of an ooen- ended photoacoustic d e t e c t o r developed f o r doing i n v i v o measurements o f s k i n o p t i c a l absorption. P r e l i m i n a r y r e s u l t s a r e presented.
As soon as he introduced the modern photoacoustic spectroscopy, Rosencwaig attempted t o demonstrate the usefulness of t h i s new method i n medical sciences such as derma- t o l o g y . He soon mentionned t h e f e a s i b i l i t y 06 performing i n v i v o measurements on hu- man skin, by the use of an open-ended c e l l ' .
I n s p i t e of t h i s , almost a l l the cutaneous a p p l i c a t i o n s o f photoacoustic d e t e c t i o n have been done on excised epidermal samples. These s t u d i e s have shown t h a t t h e pho- t o a c o u s t i c spectrum r e v e a l s t h e a b s o r p t i o n band o f p r o t e i n s a t about 280 nanometres and depends on t h e h y d r a t i o n o f t h e sample through i t s thermal p r o p e r t i e s . The pho- t o a c o u s t i c s i g n a l s produced by drugs o r sunscreens g i v e t h e i r o ~ t i c a l absorption i n s i t u , t h a t i s on and i n the epidermis and t h e i r d i f f u s i o n c o e f f i c i e n t i n stratum corneum' .
At t h e time being, o n l y Campbell e t a12 and Pines and Cunningham3 r e p o r t e d i n v i v o measurements. The major d i f f i c u l t y i n doing i n v i v o photoacoustic measurements 1 ie s i n t h e f a c t t h a t the microphone i n s e n s i t i v e t o the body's movements. We undertook t o study t h e r e a l n o s s i b i l i t i e s o f such i n v i v o cutaneous photoacoustic spectroscopy.
A f t e r v a r i o u s approaches, t h i s l e d us t o c o n s t r u c t a photoacoustic d e t e c t o r u s i n g a d i f f e r e n t i a l microphone between two i d e n t i c a l c e l l s . I t s c h a r a c t e r i s t i c s a l l o w us t o measure d i f f e r e n t i n v i v o s i g n a l s w i t h good r e p r o d u c i b i l i t y and s i g n a l t o n o i s e r a t i o .
I - EXPERIFIENTAL SET-UP
The o r i g i n a l i t y of t h e spectrometer r e s i d e s i n t h e conception o f t h e photoacoustic c e l l which must be considered so as t o o p t i m i z e t h e s i g n a l t o n o i s e r a t i o . One o f t h e important features o f t h e conception o f t h i s c e l l i s t o maximizf i t s s e n s i t i v i - t y by t h e o p t i m i z a t i o n o f i t s dimensions. According t o Aamodt e t a1 and t o Quimby and Yen5, the optimal dimensions o f t h e c e l l a t 20 Hz modulation frequency and i n standard temperature and pressure c o n d i t i o n s a r e 1.6 mm l o n g and 7 mm l a r g e . This 20 Hz t h r e s h o l d was chosen because t h e v i b r a t i o n s o f t h e s k i n a r e t o o s u b s t a n t i a l a t any lower frequencies. This choice a l s o means t h a t i t i s p o s s i b l e t o use t h e o r e t i c a l models of t h e photoacoustic e f f e c t based on the Rosencwaig-Gersho theory6. The m i - crophone should be i n c o n t a c t w i t h t h e gaseous volume s p e c i f i e d above by means o f a small a c o u s t i c pipe.
Having defined t h e dimensions which o n t i m i z e t h e s e n s i t i v i t y o f t h e c e l l , we must now consider t h e second aspect o f i t s conception : t o minimize t h e noise and syn- chronous background s i g n a l produced by t h e microphone. The synchronous background i s u s u a l l y t h e most d i f f i c u l t t o reduce. I t can be generated by a b s o r p t i o n o f l i g h t w i t h i n t h e c e l l by a body o t h e r than t h e s t u d i e d sample, such as the s i d e w a l l s o r t h e microphone i t s e l f . Another synchronous background can be produced by t h e l i g h t modulator.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983668
C6-414 JOURNAL DE PHYSIQUE
Using an open c e l l r a i s e s problems o f damping e x t e r n a l sounds and v i b r a t i o n s . When t h e c e l l i s a p p l i e d t o t h e body under s c r u t i n y , the gaseous volume d e l i m i t e d by t h e c e l l and t h e surface being analysed must be p e r f e c t l y sealed w i t h r e g a r d t o t h e o u t - s i d e atmosphere i n o r d e r t o o b t a i n a p h o t o a c o u s t i c s i g n a l . As l o n g as t h i s hermetic seal can be achieved, i n s u l a t i o n a g a i n s t o u t s i d e sounds w i l l be sat.isfactory. On the o t h e r hand, i t i s imoossible t o i n s u l a t e the c e l l from t h e v i b r a t i o n s o f t h e body under s c r u t i n y , the s u r f a c e o f which i n e v i t a b l y forms one o f t h e i n t e r n a l w a l l s of t h e c e l l . It i s t h e r e f o r e e s s e n t i a l t o use a m o d u l a t i o n frequency a t which the v i - b r a t i o n s o f t h e s k i n a r e small, a l t h o u g h t i t must n o t be f o r g o t t e n t h a t as t h e mo- d u l a t i o n frequency r i s e s , t h e s i g n a l becomes weaker. I t i s a l s o advantageous t o seek o u t zones o f s k i n where v i b r a t i o n i s weakest. I n s p i t e o f a l l these precautions, t h e l e v e l o f noise detected on t h e s u r f a c e o f t h e s k i n made t h e f i r s t photoacoustic c e l l s v i r t u a l l y impossible t o use.
MICROPHONE
F i g . 1 - The photoacoustic d e t e c t o r f o r s k i n spectroscopy
I n o r d e r t o reduce the e f f e c t o f the s k i n v i b r a t i o n s on t h e measured s i g n a l , a d i f - f e r e n t i a l method was a p p l i e d , u s i n p a c l o s e - t a l k i n g microphone : t h e KnowlesBW1789.
This microphone i n c l u d e s two sounds p o r t s , one on each s i d e o f t h e diaphragm, and t h e s i g n a l i t d e l i v e r s depends on t h e p r e s s u r e d i f f e r e n c e between t h e t w o s i d e s of t h i s diaphragm. The microphone i s f i t t e d t o t h e cutaneous d e t e c t o r , i n between two i d e n t i c a l c e l l s , one o f which i s c l o s e d by t h e l i g h t guide, t h e o t h e r b y a volume- a d j u s t i n g screw. T h i s screw should enable t h e responses o f t h e two c e l l s t o be' equa- l i z e d , a t t h e frequency i n use, so as t o m i n i m i z e noise. The sketch o f t h e photoa- c o u s t i c d e t e c t o r i s shown i n F i g . 1 . I t s dimensions have been o p t i m i z e d according t o t h e c r i t e r i a s e t o u t above. It i s a t t a c h e d t o t h e s u r f a c e o f t h e s k i n b y a double -
sided adhesive tape i n which two h o l e s have been bored o p p o s i t e t h e c e l l s , and t h e
whole apparatus can be fastened t o t h e forearm by means o f an armband.
a p p l i c a t i o n s which a r e considered. The s e n s i t i v i t y o f t h e d e t e c t o r was measured on a t h i n black body, obtained by blackening a p l e x i g l a s s h o l d e r over a flame. A t 20 h e r t z , t h e s e n s i t i v i t y o f t h e c e l l i s 430 pascals per watt, and expresses t h e r a t i o between the r.m.s. values o f v a r i a t i o n s i n a c o u s t i c pressure and i n c i d e n t l i g h t po- wer. This s e n s i t i v i t y v a r i e s as a f u n c t i o n o f t h e modulation frequency. Thetheore- t i c a l decrease i n l / f i s v e r i f i e d a t frequencies h i g h e r than 25 h e r t z , as c o u l d be foreseen from t h e s i z e o f t h e c e l l . A t lower frequencies, t h e thermal losses occu- r i n g i n t h e w a l l s o f t h e c e l l weaken t h e s i g n a l . The spectrum o f t h e noise d e n s i t y d e l i v e r e d by t h e microphone i s estimated from t h e r.m.s. value o f t h e output v o l t a g e o f t h e l o c k i n a m p l i f i e r , w i t h a time constant o f 100 m i l l i s e c o n d s , which c o r r e s - ponds t o a bandwidth o f 1.25 h e r t z .
While the noise was being measured, a l l t h e instruments were operating, t h e c e l l sea- l e d a g a i n s t t h e s t u d i e d sample and t h e modulated l i g h t beam shut o f f by a b l i n d . Va- r i o u s tyoes o f n o i s e a r e detected on a v i b r a t i o n - f r e e sample : a wide-band n o i s e w i t h frequencies above 150 h e r t z , 50, 100 and 260 h e r t z p a r a s i t e s , a l / f noise w i t h a knee a t approximately 30 h e r t z , and a narrow-band n o i s e between 30 and 60 h e r t z . This spectrum i s the d i r e c t r e s u l t o f t h e d i f f e r e n t sources o f n o i s e r e f e r r e d t o a- bove, except f o r t h e synchronous background, which would appear t o be produced by t h e l i g h t s c a t t e r e d o r r e f l e c t e d by t h e sample i t s e l f , and which depends on t h e na- t u r e o f t h e l a t t e r .
W h i l s t the c e l l was attached t o the forearm o f a volunteer, t h e n o i s e l e v e l i s higher, whatever t h e frequency used, although a much more s u b s t a n t i a l l e v e l can be observed a t low frequencies (below 20 h e r t z ) , and t h i s i s due t o i n v o l u n t a r y movements of the forearm.
I I I I I
- 200- ,2000
- 1600
- 1200
W
