In 1880 Alexander G. Bellheard “a pure musicaltone” from an enclosed gasthat had been illuminated by avery brief flash of sunlight. Thistone was a result of the optoacousticeffect,the conversionof light into sound. This occurswhen a pulse of light absorbedby a target is excited and emitsa sound wave. In recent years,advances in laser technology,electronics and materials andthe understanding how light andsound interact with the humanbody have created an opportunityto use this phenomenon inmedical imaging.
Opto–acoustic imaging hasemerged as a sensitive modalityfor visualizing and quantitativelycharacterizing malignanttumors. Opto–acoustic imagingcombines the most compellingfeatures of optical imaging withultrasound imaging to provideblood maps of the body. Thisnew imaging method providesthe image contrast of opticalimaging with the resolution ofultrasound imaging — a featureno other imaging technology canaccomplish.
The basic principles behindopto–acoustic imaging are thatvery short pulses of laser lightscatter through the part of thebody being imaged. The colors(wavelengths) of the light pulsesare chosen due to their ability tobe preferentially absorbed eitherby oxygenated blood or by deoxygenatedblood. The acousticwaves that result from the lightabsorption travel effectively withminimal distortion through thebody to the skin. Transducers(embedded in a probe) are incontact with the skin, receivethe sound waves and transmitthe information to an electronicsystem that converts the soundinto images of the objects thatabsorbed the light. By usingarrays of transducers it is possibleto reconstruct two–dimensionaland three–dimensional images.The illustration presented withinthis article depicts opto–acousticimaging used for diagnosticimaging of breast cancer.
Malignant solid tumorsdevelop an enhanced networkof micro vessels to supplynutrition and oxygen to multiplycancer cells. This phenomenonis known as angiogenesis. Asa result of rapid consumptionof oxygen by an aggressivelygrowing cancerous tumor andleakiness of its vessels, the bloodin and around the cancerousmass becomes de–oxygenated;the blood in and around abenign tumor is more highlyoxygenated. Blood stronglyabsorbs the two wavelengths oflight selected for opto–acousticimaging. Strong light absorbersproduce correspondinglystrong acoustic waves, resultingin a bright image. Therefore,blood vessels provide highopto–acoustic contrast. As aresult, optical contrast betweennormal and cancerous tissuesis substantially greater thanthe contrast obtained fromconventional imaging methods.Furthermore, functionalinformation about hemoglobinconcentration in blood and itslevel of oxygen saturation intumors can serve as a basis fornoninvasive diagnostics usingopto–acoustic imaging. Studiesindicate that opto–acousticimaging can provide acceptableimage contrast and resolution toa depth of up to 60mm.
Opto–acoustic technologyapplied to medical imagingcan provide high–resolution3D maps containing functionalinformation on bloodconcentration and its oxygensaturation, thereby providingvaluable diagnostic informationnot otherwise available.Clinical applications include(1) diagnostic imaging of breasttumors and potentially othertypes of malignant solid tumorsin soft tissues, (2) intravascularimaging and characterizationof atherosclerotic plaques, (3)detection of vascular occlusionsand (4) detection of brainhemorrhages. Recent studiesperformed in breast cancerpatients demonstrated that thefunctional imaging capability ofopto–acoustic imaging providesadditional medically relevantinformation regarding breasttumors, which results in bettersensitivity and specificity ofcancer detection without the useof injected contrast agents orX–rays.
A unique opportunity forfurther substantial enhancementof the opto–acoustic detectionsensitivity comes from mergingopto–acoustic imaging withnanotechnology. An optoacousticcontrast agent basedon gold nanorods may bedesigned and selectivelydelivered to cancer cells inorder to substantially increasecontrast of cancerous tumorson opto–acoustic images. Thesame contrast agent can servepotentially as a therapeutic agentfor treatment of early cancer.
Seno Medical Instruments,Inc., a San Antonio–basedmedical device developer iscommercializing the optoacoustictechnology for theearliest detection of cancer.Seno’s platform technology alsoaddresses medical applicationssuch as stroke, cardiovascularand inflammatory diseases.
