Spotlight 3: Optical Techniques in Medical Imaging - Institute of ...

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Optical imaging techniques allowdoctors to see more clearlyEarly detection ofbreast cancer has beeninstrumental in reducingthe number of deathsfrom the disease. Opticalmammography - whichreveals tumours by detectingthe increased blood supplythey need in order to grow- used in conjunction withthe existing X-ray screeningprogramme would improvethe accuracy of diagnosisfor pre-menopausal womenby providing better imagecontrast, and allow morefrequent imaging for existingpatients. In the opticalmammogram shown here(uppermost image), thelarge yellow area on theleft reveals a tumour in onebreast of a patient, whichshows up as a white regionin the MRI scan below. Withfurther development, opticalmammography could alsodetermine whether tumoursare cancerous or not.03Most of us will undergo some form ofmedical scan in our lifetimes, even if it isjust a dental X-ray. Of course much moresophisticated scanning equipment such asultrasound and MRI is also commonplace inthe UK, bringing many benefits to patientsincluding early diagnosis of disease andaccurate monitoring of treatments. Onthe negative side however, some of theseimaging procedures expose the patientto unwanted radiation and so cannot beperformed repeatedly. In addition manytechniques require bulky, expensiveequipment that may not be available atevery hospital.To help combat these problems physicistshave been developing a range of newoptical imaging techniques that onlyinvolve exposing the patient to light, andin some cases are portable enough to beused by a bedside. These include OpticalMammography systems for monitoringthe effectiveness of treatments such aslumpectomy and regular screening for earlysigns of breast cancer, and Diffuse OpticalTomography and Photoacoustic Imagingwhich are both designed to monitor bloodvolume, oxygenation and flow in tissue.As unusual patterns in blood oxygenationand flow reveal the early stages of diseasein soft tissues, as well as blockages inarteries, these techniques are likely to proveparticularly valuable for patients such asstroke victims and premature babies.Other emerging optical techniques includea Scanning Laser Ophthalmoscope forimaging damage and disease inside eyes,and Optical Coherence Tomography whichcan differentiate between cancerousand normal tissue for a range of cancersincluding skin cancer, and can be used tomonitor microsurgery.Diffuse Optical TomographyIn diffuse optical tomography, an arrayof optical fibre sources is used to shineinfrared light - with a range of wavelengthsjust beyond visible red light - into whicheverregion of the body needs to be lookedat. Boundaries between different parts oforgans, different tissue layers, cell walls,and sub-structures within individual cells,Optical Techniques in Medical Imagingreflect and scatter (or ‘diffuse’) this light inall directions. Any light that happens to bescattered towards a corresponding arrayof optical fibre detectors is recorded, andspecially designed image reconstructionsoftware processes this data to produce a3D image of the area being studied.Particular wavelengths of infrared lightare absorbed by certain tissues andsubstances. For example the absorptionof haemoglobin (the protein in blood thatcarries oxygen around the body) is differentwhen it is carrying oxygen from when it isde-oyxgenated. So to determine detailsabout oxygen levels in the blood andtissues, and to discern different tissue types,light at various wavelengths is shone in turninto a patient. A composite image is thenbuilt up that displays the absorption of eachwavelength as a different colour, showingthe amount and location of the absorptionby the intensity of a particular colour ina given region. An optical mammogramproduced in this way is shown on the left.Diffuse optical tomography opens up thepossibility of more effective treatments forpatients with head injuries or strokes - andpremature babies - as all of these patientsneed quick, accurate diagnoses of any brainregions lacking in oxygen. If spotted fastenough, additional oxygen can be suppliedto the starved area before the tissue dies.The pictures on the back page show anexperimental diffuse optical tomographysystem devised at University CollegeLondon for premature babies. A variation onthis set-up can be used in sports medicineto monitor changes in muscle oxygenationduring exercise.Measuring the amount of oxygen in bloodcan also help reveal the presence ofcancerous tumours, as rapidly growingtissue needs lots of oxygen and anincreased blood supply, and many largetumours have a dead region inside. Diffuseoptical tomography can generally ‘see’ 2-4centimetres into the body, but this is deepenough to image the premature infanthead or the grey matter on the surface ofthe adult brain, and breasts up to 12cmin diameter can be fully imaged as breasttissue is relatively transparent.
Each of these images showsthe retina (inner lining of theeye containing the lightsensitivecells that allow usto see) of a patient with eyedisease. The large regionin the centre of the imagesis the optic disc, which isthe starting point for theoptic nerve. In the left-handimage, which was taken witha Scanning LaserOphthalmoscope usingthree different colouredlasers simultaneously, whitedeposits - caused by thetissue degenerating - canbe clearly seen within theoptic disc. It is much harderto make these deposits outin the right-hand image,which was taken using aconventional fundus camera.Photoacoustic ImagingEven more detailed images of bloodoxygenation levels can be provided byphotoacoustic imaging. This involves shininga very short pulse (a few tens of nanosecondslong) of infrared light into a patient usuallyvia an optical fibre. Wherever this light isabsorbed, the energy from the light warmsthat region of blood or tissue up by abouta thousandth of a degree. This warmingcauses the absorbing region to expand bya tiny amount, and because this happensso quickly, a pressure wave is set up whichtravels through the tissue at the speed ofsound before being picked up by a standardultrasound detector.The more a section of tissue or region ofblood absorbs, the hotter it becomes andthe greater the ultrasound signal it creates.So unlike a standard ultrasound image,which shows the mechanical structure of thetissue, the photoacoustic image will revealthe different optical absorption properties ofthe tissue or blood. As with diffuse opticaltomography, different wavelengths of light areused to reveal varying levels of oxygenation inthe blood as well as different tissue types.Optical Coherence TomographyThe optical equivalent of ultrasound isprovided by Optical Coherence Tomography(OCT). This uses a beam of infrared from alaser that is shone into the body. Boundariesbetween the tissues reflect a small amountback, and this reflected signal is picked upand processed by an ultrafast detector. Tobuild up an image from the reflected signals,the laser beam (usually delivered by anoptical fibre) is moved around the region ofinterest - which can either be accessed fromoutside the body, or internally via a catheter.The resolution of OCT images is greaterthan ultrasound because the wavelengthsof light are shorter, and so can be reflectedby smaller objects that would simply bewashed over by the longer wavelengths ofsound waves. In fact with an ability to revealfeatures about a millionth of a metre in size,the images are so accurate that they couldremove the need for biopsy in many cases,and so provide a totally non-invasivemethod of detecting early signs of cancer.On the downside, OCT can only image2-3 millimetres into tissue, but since manycancers occur on the surface of organs, thisis not a severe limitation.Imaging the eyeAs with the other optical imagingtechniques, particular wavelengths of lightshow up certain features in the eye well.Green light, for instance, is particularlygood at imaging microaneurysms, whichare bulges that form on weakened areasof the eyes’ blood vessels prior to thembreaking. At present, the “fundus camera”- a photographic camera designed to imagethe retina at the back of the eye - is themain instrument used to detect many signsof early eye disease. Unfortunately thisrequires a large amount of light to be shoneinto the pupil to produce enough reflectedsignal to create an image. This not onlymeans eye drops are needed to dilate thepupil, but when several images need to betaken in close succession, it becomes veryuncomfortable for the patient. However,a Scanning Laser Ophthalmoscope(SLO) currently under development atthe University of Aberdeen could removethese problems, and may even allow thediagnosis of a much wider range of eyediseases at an even earlier stage thancurrently possible.To obtain a full colour image, the AberdeenSLO shines three low-power lasers - onered, one blue and one green - in turn ata succession of points at the back of theeye. The reflections from each spot areelectronically detected and combined,enabling an image to be built up over a fewmilliseconds as the SLO scans the retinain the same sort of ‘raster’ pattern used tocreate TV pictures. As shown above, thisproduces clearer images than a funduscamera and because the scan is so fast,the eye - which is constantly in motion - haslittle time to move. Also, since the eye isilluminated by a narrow concentrated beamof light, the amount of light needed to createan image is very low and there is no needfor eye drops to dilate the pupil.By measuring the amount of light of aparticular colour reflected from the retina,it may be possible in the future to analyseretinal tissue and see, for instance, if itsblood flow is low. The SLO could also beused to image different types of blood cellsmoving through the eye, by labelling eachtype of cell with a fluorescent dye that onlyglows under light of a certain colour. If thisenabled scientists to learn exactly how whiteblood cells react to inflammation in the eye,more effective treatments could be produced.
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