By: Richard Semelka, MD
Considerable attention has been focused of late on screening imaging studies for the detection of early disease, including a recent article that appeared in Money magazine.[1] Computed tomographic (CT) scanning has often been cited as the modality of choice for screening examinations. This article discusses magnetic resonance imaging (MRI) as the emerging modality of choice for whole body screening for disease.
To Screen or Not to Screen
In a medical sense, what is meant by the term screening? Screening is the systematic examination of the body to detect unsuspected disease. By definition, the optimal candidate for a screening study is without signs (physical finding) or symptoms (feelings) that suggest a particular disease.
Screening methods include blood tests, urine tests, and imaging studies. The purpose of screening is to detect disease in its earliest stages while effective treatment is still possible. Another term related to the detection of possible disease is surveillance, which refers to searching for a disease process in an individual who is healthy but at a high risk for a particular disease. For example, MRI is used worldwide as a surveillance imaging technique to identify liver cancer (hepatocellular carcinoma) in individuals who are at increased risk for this disease because of underlying chronic liver problems, including viral hepatitis and cirrhosis. Both screening and surveillance must be distinguished from diagnostic imaging studies, which are performed because the patient is suspected of having a certain disease due to certain signs or symptoms. In summary, screening is done on normal healthy people; surveillance studies are performed on people who have a propensity for a certain disease; and diagnostic studies are done specifically to investigate a strong suspicion that a certain disease is present.
The rationale for imaging screening of the whole body is obvious. In the event that an individual harbored some horrendous disease, it would greatly affect how long that person would live if the disease was discovered at a very early stage and small size. To illustrate this point, in a patient with no suspicion of kidney cancer, a small, treatable kidney cancer is detected (see Figures 1 and 2 of kidney cancer) that can be removed for a complete patient cure, and the patient will live a normal life expectancy. If instead this cancer grew to the size that the patient had become aware something was wrong, in my experience the life expectancy of patients in this circumstance is a matter of months to 1 year. In theory, screening ensures early detection and maximizes one’s chances of successful treatment and cure while minimizing suffering. Statistically, a low number of actual diseases will be found screening people without signs or symptoms. This highlights the fact that a good screening test must have high sensitivity, so that the test does not miss the few cases of disease that are present, and high specificity to reduce the number of people with false-positive results (an incorrect description of disease by the screening study when no disease is present). A screening test that has low sensitivity will give people a false sense of good security or health when disease is present but undetected. A screening test that has low specificity will promote a sense of vulnerability or sickness when disease is absent but incorrectly detected. Other factors that must be considered in imaging screening are the cost of the study and safety.
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Figure 1. Assessment of kidney cancer: noncontrast MRI. Notice that it is not seen without contrast. The inability to see this tumor on noncontrast MRI illustrates that it would be virtually impossible to see it on a screening noncontrast CT study.
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Figure 2. Assessment of kidney cancer: intravenous gadolinium contrast-enhanced MRI images. This patient underwent MRI to look for disease of the liver. By chance, a small kidney cancer was found. It is well seen (arrow) on the postcontrast image. This tumor was detected in a screening-type setting (kidney cancer was not being looked for). Cutting this small cancer out at this point will result in a normal life span for this patient. If an imaging study was performed some time later when the patient was starting to feel that this tumor was present, the tumor would have been large and the patient may have had only 1 year left to live, because the cancer was so advanced. The inability to see this tumor on noncontrast MRI illustrates that it would be virtually impossible to see it on a screening noncontrast CT study.
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To use an imaging study to screen for disease, it seems obvious that it must be able to detect disease accurately and reliably while avoiding describing normal parts of the body as diseased. Because whole body CT and MRI screening are relatively new concepts, most occurring on a private pay or self-referral basis, limited data have been collected comparing the 2 methods. However, there is growing concern in the medical community that whole body CT screening leads to a large number of questionable findings, requiring additional procedures, including surgery, creating added risks and costs for the individual.[2] Most of the information comparing the performance of CT and MRI has been gathered through diagnostic studies, that is, imaging studies ordered for the detection of suspected disease. Overall, MRI has been found to discover more lesions and correctly characterize disease, whether benign or malignant, than CT. In these capacities, MRI has been shown to be superior to CT for examining specific regions of the body, for example, the head, abdomen, and pelvis.[3-9] In the past, the sole exception to this rule has been imaging of the lungs, performed better by CT than MRI, although MRI is rapidly advancing.[10] CT remains the optimal imaging technique for visualizing small arteries, such as the vessels that supply blood to the heart. On this score, I predict MRI will demonstrate comparable results in about 3 more years.
Cost of the imaging study is also an important consideration, especially if it will be society that bears the burden of cost for the procedure. This is somewhat less of an issue if the individual is paying for it independently. However, if one is paying for the study oneself, then one is particularly interested to know whether they are getting the value they expect (early disease detection) for the money they spend. Because of the nature of the complexity of the imaging system and intrinsic maintenance costs, MRI is unavoidably a more expensive test than CT. The machinery has many more components, and the requirement for liquid helium and liquid nitrogen to be continuously replenished renders it intrinsically more expensive. The real cost of MRI is greater than that of CT; an approximate estimate is that studies are about 20% more expensive.
How Safe Is Safe?
Regarding safety, MRI is a safer modality than CT, both the imaging system itself and the intravenous contrast agent used.[11-15] The powerful magnetic field and radiofrequency energy of MRI have not been shown to cause cancer or fetal abnormalities, unlike the ionizing radiation (x-rays) used in CT that is a known cause of cancer and fetal anomalies. It is important to note that although x-rays are known to cause cancer, the exact risk of cancer from receiving CT scans, and even repeat CT exams, is unknown. The reader is directed to a recent article published in the major medical journal The Lancet for more discussion on that topic.[16] The intravenous contrast agents used routinely in MRI, gadolinium-containing agents, are also considerably safer than the analogous intravenous agents used in contrast-enhanced CT, which are iodine-based agents. There is a much lesser association with causing kidney injury with the MRI contrast agents, and much lesser association with allergic reactions, including severe allergic reactions that can lead to death.[17-19] That is why in general individuals who are undergoing diagnostic imaging studies will have an MRI test rather than CT if they have poor kidney function or a history of allergies. Another very compelling aspect of the use of intravenous contrast agents with MRI is that the diameter of the needle inserted is much smaller with MRI; the volume of contrast material that is injected intravenously is 10 times less than with CT (the rapid injection of the large volume of contrast with CT can create a strong sense of nausea); and the injection rate is slower with MRI than with CT. Additionally, the chance of the injected contrast agent not going into the vein but into the surrounding tissues is also greater with CT, which is a problem compounded by the large volume of the fluid injected.
Rationale for MRI as a Primary Screening Tool
For this reason, if MRI is superior to CT for finding small-volume disease, correctly classifying the disease and demonstrating that normal structures are intact and not diseased, with better safety measures and less pain (smaller needlestick with MRI), then why is MRI still not considered the method of choice for whole body screening? Historically, the primary reason for this is that MRI is much slower to perform than CT, and imaging of the lungs was suboptimal. Over the recent few months, the MR systems that are manufactured have imaging capability that allows them to image the entire body much faster, while maintaining high image quality that even machines made 1 year ago did not possess. Basically, new designs of transmit-receiver coils, easier movement of the imaging table, and new data-acquisition techniques have allowed rapid imaging of the entire body as well as acceptable image quality of the lungs. High-quality, whole body MRI imaging that would take 2 hours even 1 year ago now can be done in 10-15 minutes, making the technique very suitable for rapid, highly accurate whole body imaging in an easily tolerable time frame. Other reasons for using CT include cost and the number of available systems (greater with CT). Also, MRI studies are more difficult to perform (because there are more different types of data acquired); the interpretation of MR images is more complex; and fewer radiologists are trained to interpret MRI than CT.
An important consideration when performing a screening study is what to look for. Unfortunately, even with whole body imaging, every single disease process cannot be looked for in great detail. This is due to the fact that very detailed imaging of every part of the body would take an extremely long time even on the newest MR machines. Therefore, studies should be tailored to look for the most common lethal diseases that afflict the general population in the part of the world the study is performed and tailored to the types of disease that the person in particular is at a risk of acquiring. The most prevalent diseases in North America include stroke, heart attack, and the common deadly cancers — lung, breast (in women), colon, prostate (in men), pancreas, lymphoma, and liver. Screening for common diseases in the general public should be supplemented with screening for disease that the particular person undergoing the screening study is at a risk for (for example, kidney cancer in patients with kidney failure). I foresee the day, in the not too distant future, when doctors will know what diseases we have a propensity to acquire on the basis of a computer-generated reading of our genetic material, obtained from a blood sample, and then the MRI scan will be performed specifically looking for those diseases.
MRI Expertise: An Evolving Art
Although at present the reliability and accuracy of MRI at detecting diseases of certain organs, including liver, brain, spine, pancreas, and kidneys, are extremely high and imaging of the lungs is reasonable, optimal imaging of the heart, breast, and colon still requires further development to achieve consistent image quality and consistent accurate display of small-volume disease. Nonetheless, imaging of all the latter mentioned organs is currently at a diagnostically acceptable level.
At the time of writing this article, the currently manufactured MR systems have evolved to the point that the equipment can image through the entire body with good image quality and sensitivity to detect disease within a 15-minute period. Whole body, high-quality MRI screening is now feasible. It remains to be seen if the public will demand screening with whole body MRI because of its superiority of disease detection and higher safety than that offered by CT.
quoted from:http://www.medscape.com
