There are a number of different ways of imaging the living human body non-invasively. The two most common such methods used today are computed tomography (CT, also called CAT) scans and magnetic resonance imaging (MRI). These two imagining methods vary in a number of ways in which they compute an image of the internal organs of the body.
The CT scan is an advanced form of x-ray technology, the difference between CT and the traditional x-ray is that where an x-ray compresses three dimensional objects into a two dimensional image the CT scan retains the third dimension. The CT scanner consists of two main parts – an x-ray source and a radiation detector located on opposite sides of the patient’s body. These can be rotated in order to obtain images of any angle. X-rays are passed through the patient’s body where some are absorbed by body tissue and the rest are picked up by the detectors on the other side. The underlying principle of computed tomography is that biological material varies in its density. Areas of different density absorb different levels of radiation, so a high density material, such as bone, would absorb more radiation than a low density material such as blood. Thus, when viewing an image obtained by CT scanning what the image is really showing is the differential absorption of radiation by the body’s tissues. The resolution of a CT scan is approximately 0.5 to 1.0cm, thus if objects within the body are closer than five millimeters it is not possible to discriminate between them using a CT scan.
The magnetic properties of organic tissue form the basis for magnetic resonance imaging. Many atoms, such as hydrogen, are electrically imbalanced, that is, their number of protons and electrons is not equal, which causes the atom to spin about. Such atoms are called ions. In a MRI scan the patient is placed within a magnetic field which causes hydrogen ions to align themselves in a direction parallel to magnetic field. Radio waves which cause the hydrogen ions to rotate in a uniform fashion are then passed through the field. When the radio waves are turned off the hydrogen ions ‘snap back’ towards their former orientation which causes a local magnetic field which is able to be detected magnetic detectors within the scanning unit. The computing of MRI images by computer software is similar to that used by the CT scan and differences in density are able to be seen as different tissues in the body have different levels of hydrogen ions. The resolution of an MRI scan is much greater than that of a CT scan, down to approximately 0.1mm.
Thus it can be seen that CT and MRI scans work on very different principles and utilise different properties of biological material. The CT scan utilises the differential radiation absorption levels of different body while MRI exploits the electrical properties of biological material. MRI scans are able to achieve a much higher resolution than CT scans and thus they are more useful for imaging very small structures within the body. A further difference is the cost involved with CT scans being cheaper than MRI scans. CT scans are most commonly used for imagining the body’s organs and the larger structures of the brain while MRI scans are used when high detail is required in order to image small structures or where different tissues, such as the grey and white matter in the brain, absorb similar levels of radiation.
