Medical imaging is the process by which physicians evaluate an area of the subject's body that is not normally visible. Medical imaging may be "clinical", seeking to diagnose and examine disease in specific human patients (see pathology). Alternatively, it may be research-motivated, attempting to understand processes in humans or animal models. Many of the techniques developed for medical imaging also have scientific and industrial applications.
Mathematically speaking, medical imaging usually involves the solution of inverse problems. This means that we infer cause (in this case properties of living tissue) from effect. The effect in this case is the response to being probed by various means. In the case of ultrasonography the probe is ultrasound; in the case of radiography, the probe is X-ray radiation.
In its most primitive form, imaging can refer to the physician simply feeling an area of the body in order to visualize the condition of internal organs. This was used historically to diagnose aortic aneurysms, fractures, enlarged internal organs, and many other conditions. It remains an important step today in making initial assessments of potential problems, although additional steps are often used to confirm a diagnosis. The primary drawback of this approach is that findings are subject to interpretation, and while a recorded image can be produced manually, in practice this is often not done.
Modern imaging technology
Main article: Radiography
Radiographs, more commonly known as x-rays, are often used to determine the type and extent of a fracture. With the use of radioactive dyes, such as barium, they can also be used to visualize the structure of the intestines - this can help diagnose certain types of colon cancer.
Main article: Computed axial tomography
A CT scan, also known as a CAT scan (Computed Axial Tomography scan), traditionally produces a 2D image of the stuctures in a small section of the body. It uses radiation, just like radiographs, and thus repeat scans are not recommended for children.
Magnetic Resonance Imaging
Main article: Magnetic resonance imaging
An MRI uses powerful magnets to excite hydrogen nuclei in water molecules in human tissue, producing a detectable signal. Like a CT scan, an MRI traditionally creates a 2D image of a small "slice" of the body. As an MRI does not use radiation, it is the preferred imaging method for children and pregnant women.
Main article: Medical ultrasonography
Medical ultrasonography uses high frequency sound waves of between 3.5 to 7.0 megahertz that are reflected by tissue at varying rates to produce a 2D image, traditionally on a TV monitor. This is often used to visualize the fetus in pregnant women. It has a low resolution compared to CT, MRI, and radiographs.
Creation of three-dimensional images
Recently, techniques have been developed the enable CT, MRI and Ultrasound scans to produce 3D images for the physician. Traditionally CT and MRI scans produced static output on film. To produce 3D images, many scans are made, then combined by computers to produce a 3D model, which can then be manipulated by the physician. 3D ultrasounds are produced using a somewhat similar technique.
With the ability to visualize important structures in great detail, 3D visualization methods are a valuable resource for the diagnosis and surgical treatment of many pathologies. It was a key resource (and also the cause of failure) for the famous, but ultimately unsuccessful attempt by Singaporean surgeons to separate Iranian twins Ladan and Laleh Bijani in 2003. The 3D equipment was used previously for similar operations with great success.
Other imaging techniques
Other proposed or developed medical imaging techniques (often termed modalities) include:
- diffuse optical tomography
- electrical impedance tomography
- nuclear medicine
- opto acoustic imaging
- positron emission tomography
Some of these techniques are still at a research stage and not yet used in clinical routines.