What is an ideal medical imaging instrument? Cost, conve­nience, and safety are important considerations. However, the most important issue is whether the imager can provide de­finitive information about the medical condition of the pa­tient. To have a better appreciation of these issues, it is neces­sary to review the conditions under which medical imaging is performed.

Medical imaging may be used as part of therapeutic proce­dures, to detect disease where no symptoms are present (screening), or to monitor the process of healing. The most common use of medical imaging is for differential diagnosis, where a patient exhibits symptoms that can be caused by one

Table 1. Modalities, Subdivisions, and Annual Case Load in a Large Teaching Hospital




X ray

Chest, bone, gastrointestinal,


Computer tomography

genitourinary, neurological, neurosurgical, angiography, mammography Body, neurological (head)


Nuclear medicine


Magnetic resonance





or more diseases, and there is a need to ascertain the specific condition so that proper treatment can be applied. The physi­cian orders a specific imaging procedure, which may require patient preparation (for example, ingestion of a contrast ma­terial), the use of specific imaging instruments and settings (X rays of a certain voltage, intensifying screens, and films), and proper positioning of the patient. Most procedures are performed by properly trained technicians, but some very dangerous or invasive procedures, such as injecting contrast agents directly into the heart, are performed with direct par­ticipation of a medical doctor. The majority of imaging proce­dures are administered by a radiology department of a hospi­tal, and images are interpreted (read) by radiologists, medical doctors who undergo post-MD training (residency) in this spe­cialty. The choice of procedure for a given medical problem is based on medical knowledge and on guidelines promulgated by medical and/or health insurance organizations. We see that most medical imaging tasks are highly specialized and are performed in a very structured setting.

Table 1 shows the divisions of a radiology department of a large teaching hospital. The divisions have been grouped by modality. It also shows the number of studies performed in one year in each modality. It is clear that X-ray imaging is still the predominant technique. Other modalities are grow­ing more rapidly, but none are likely to displace X rays in the near future.

Medical imaging procedures and modalities exist in an ex­tremely competitive environment. For example, heart disease may be diagnosed either with angiography or nuclear medi­cine. Each procedure, to survive, must fit an ecological niche: it must satisfy a need where it has advantages over other methods. The same issue arises when new image processing techniques are introduced: images produced with these proce­dures must have an advantage over other imaging proce­dures. The advantage will probably be application dependent. An image processing technique that is effective for mammog­raphy may not be of much value for cardiology, and vice versa.

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