X-ray imaging is perhaps the most familiar type of imaging. Images produced by X-rays are due to the different absorption rates of different tissues. Calcium in bones absorbs X-rays the most, so bones look white on a film recording of the X-ray image, called a radiograph. Fat and other soft tissues absorb less, and look gray. Air absorbs least, so lungs look black on a radiograph. The most familiar use of X-rays is checking for broken bones, but X-rays are also used in cancer diagnosis. For example, chest radiographs (fig:a)and mammograms are often used for early cancer detection or to see if cancer has spread to the lungs or other areas in the chest. Mammograms use X-rays to look for tumors or suspicious areas in the breasts.
Fig: a shows the typical x-ray radiograph
Conventional mammography uses X-rays to look for tumors or suspicious areas in the breasts. Digital mammography also uses X-rays, but the data is collected on computer instead of on a piece of film. This means that the image can be computer-enhanced, or areas can be magnified. Eventually, a computer could in certain appropriate situations, screen digital mammograms, theoretically detecting suspicious areas that human error might miss.
A computed tomography scan (CT scan, also called a CAT scan) uses computer-controlled X-rays to create images of the body. However a radiograph and a CT scan show different types of information. Although an experienced radiologist can get a sense for the approximate three-dimensional location of a tumor from a radiograph, in general, a plain radiograph is two-dimensional.
An arm or chest radiograph looks all the way through a body without being able to tell how deep anything is. A CT scan is three-dimensional. By imaging and looking at several three-dimensional slices of a body (like slices of bread) a doctor could not only tell if a tumor is present, but roughly how deep it is in the body. A CT scan can be three dimensional because the information about how much of the X-rays are passing through a body is collected not just on a flat piece of film, but on a computer.
The data from a CT scan can be enhanced to be more vivid than a plain radiograph. For both plain radiographs and CT scans, the patient may be given a contrast agent to drink and/or by injection to more clearly show the boundaries between organs or between organs and tumors.
Fig:b (Left) A 1 mm spiral CT "slice" through the mid-chest region, showing both lungs. The white spot in the right lung is a suspicious nodule that could be biopsied to see if it is cancerous. (Right) A close-up view of 8 "slices" focused on a lung nodule. Compared with a traditional X-ray, a series of CT images gives the radiologist a much better sense of nodule size and its potential threat. Images courtesy of A. P. Reeves, Cornell University.
Magnetic Resonance Imaging (MRI):
Magnetic Resonance Imaging (MRI) uses radio waves in the presence of a strong magnetic field that surrounds the opening of the MRI machine where the patient lies to get tissues to emit radio waves of their own.
Different tissues (including tumors) emit a more or less intense signal based on their chemical makeup, so a picture of the body organs can be displayed on a computer screen. Much like CT scans, MRI can produce three-dimensional images of sections of the body, but MRI is sometimes more sensitive than CT scans for distinguishing soft tissues.
Fig:c (Top) MRI scan without contrast showing possible tumor in the liver. (Bottom) MRI scan of the same patient using contrast. Images courtesy of Dr. Peter Choyke, Clinical Center, NIH.
1.Mammography is used for imaging the,
2. What is the principle behind in the x-ray image formation?
b) Differential absorption
b) Differential absorption
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