Computer-assisted tomography Photoacoustic imaging Thermography tomography imaging
News in ophthalmology : Computer-assisted tomography Photoacoustic imaging Thermography tomography imaging
From Wikipedia, the free encyclopedia
Main article: Photoacoustic imaging in biomedicine
Photoacoustic imaging is a recently developed hybrid biomedical imaging modality based on the photoacoustic effect. It combines the advantages of optical absorption contrast with ultrasonic spatial resolution for deep imaging in (optical) diffusive or quasi-diffusive regime. Recent studies have shown that photoacoustic imaging can be used in vivo for tumor angiogenesis monitoring, blood oxygenation mapping, functional brain imaging, and skin melanoma detection, etc.
Primarily used for breast imaging. There are three approaches: tele-thermography, contact thermography and dynamic angiothermography. These digital infrared imaging thermographic techniques are based on the principle that metabolic activity and vascular circulation in both pre-cancerous tissue and the area surrounding a developing breast cancer is almost always higher than in normal breast tissue. Cancerous tumors require an ever-increasing supply of nutrients and therefore increase circulation to their cells by holding open existing blood vessels, opening dormant vessels, and creating new ones (neo-angiogenesis theory).
Tele-thermography and contact thermography supporters claim this process results in an increase in regional surface temperatures of the breast, however there is little evidence that thermography is an accurate means of identifying breast tumours. Thermography is not approved for breast cancer screening in the United States or Canada, and medical authorities have issued warnings against thermography in both countries.
Dynamic angiothermography utilizes thermal imaging but with important differences with the tele-thermography and contact thermography, that impact detection performance. First, the probes are improved over the previous liquid crystal plates; they include better spatial resolution, contrastive performance, and the image is formed more quickly. The more significant difference lies in identifying the thermal changes due to changes in vascular network to support the growth of the tumor/lesion. Instead of just recording the change in heat generated by the tumor, the image is now able to identify changes due to the vascularization of the mammary gland. It is currently used in combination with other techniques for diagnosis of breast cancer. This diagnostic method is a low cost one compared with other techniques. The angiothermography is not a test that substitutes for other tests, but stands in relation to them as a technique that gives additional information to clarify the clinical picture and improve the quality of diagnosis.
Tomography is the method of imaging a single plane, or slice, of an object resulting in a tomogram. There are two principal methods of obtaining such images, conventional and computer-assisted tomography. Conventional tomography uses mechanical means to record an image directly onto X-ray film, while in computer-assisted tomography, a computer processes information fed to it from detectors then constructs a virtual image which can be stored in digital format and can be displayed on a screen, or printed on paper or film.
In conventional tomography, mechanical movement of an X-ray source and film in unison generates a tomogram using the principles of projective geometry. Synchronizing the movement of the radiation source and detector which are situated in the opposite direction from each other causes structures which are not in the focal plane being studied to blur out. This was the main method of obtaining tomographic images until the late-1970s. It is now considered obsolete (except for certain dental applications), having been replaced with computer-assisted tomographic techniques. Historically, there have been various techniques involved in conventional tomography:
Linear tomography: This is the most basic form of conventional tomography. The X-ray tube moved from point "A" to point "B" above the patient, while the cassette holder (or "bucky") moves simultaneously under the patient from point "B" to point "A." The fulcrum, or pivot point, is set to the area of interest. In this manner, the points above and below the focal plane are blurred out, just as the background is blurred when panning a camera during exposure. Rarely used, and has largely been replaced by computed tomography (CT).
Poly tomography: This was achieved using a more advanced X-ray apparatus that allows for more sophisticated and continuous movements of the X-ray tube and film. With this technique, a number of complex synchronous geometrical movements could be programmed, such as hypocycloidic, circular, figure 8, and elliptical. Philips Medical Systems for example produced one such device called the 'Polytome'. This pluridirectional unit was still in use into the 1990s, as its resulting images for small or difficult physiology, such as the inner ear, was still difficult to image with CTs at that time. As the resolution of CTs got better, this procedure was taken over by CT.
Zonography: This is a variant of linear tomography, where a limited arc of movement is used. It is still used in some centres for visualising the kidney during an intravenous urogram (IVU), though it too is being supplanted by CT.
Panoramic radiograph: The only common tomographic examination still in use. This makes use of a complex movement to allow the radiographic examination of the mandible, as if it were a flat bone. It is commonly performed in dental practices and is often referred to as a "Panorex", but this is incorrect, as it is a trademark of a specific company.
In computer-assisted tomography, a computer processes data received from radiation detectors and computationally constructs an image of the structures being scanned. Imaging techniques using this method are far superior to conventional tomography as they can readily image both soft and hard tissues (while conventional tomography is quite poor at imaging soft tissues). The following techniques exist:
X-ray computed tomography (CT), or Computed Axial Tomography (CAT) scan, is a helical tomography technique (latest generation), which traditionally produces a 2D image of the structures in a thin section of the body. In CT, a beam of X-rays spins around an object being examined and is picked up by sensitive radiation detectors after having penetrated the object from multiple angles.
A computer then analyses the information received from the scanner's detectors and constructs a detailed image of the object and its contents using the mathematical principles laid out in the Radon transform. It has a greater ionizing radiation dose burden than projection radiography; repeated scans must be limited to avoid health effects. CT is based on the same principles as X-Ray projections but in this case, the patient is enclosed in a surrounding ring of detectors assigned with 500-1000 scintillation detectors (fourth-generation X-Ray CT scanner geometry). Previously in older generation scanners, the X-Ray beam was paired by a translating source and detector.
Positron emission tomography (PET) also used in conjunction with computed tomography, PET-CT, and magnetic resonance imaging PET-MRI.
Magnetic resonance imaging (MRI) commonly produces tomographic images of cross-sections of the body. (See separate MRI section in this article.)