Letter from the Editors

Article Outline

• Copyright

This second part of the Hybrid Imaging Anniversary Issue rounds out our review of the contribution of computed tomography (CT) to single-photon emission computed tomography (SPECT) and positron emission tomography (PET) imaging. In addition, it directs us toward the future and the potential role of PET/magnetic resonance imaging (MRI) as another example of the benefits and complimentary roles of metabolic and imaging devices. MRI provides an even more complex potential because it is a molecular imaging method which, as part of a hybrid device, would allow us to look at two different aspects of metabolism and an aspect of anatomy during the same patient examination. For the present, as Dr Thakur has pointed out, there are several needs, challenges, and potential solutions to improve hybrid imaging, using standard anatomical devices such as CT.

Early on, after the introduction of nuclear medicine into clinical medicine, it became apparent that its limited anatomical detail detracted from the exquisite metabolic information that the modality provides. The very important and unique contributions that nuclear medicine makes possible have led to its growth despite the lack of anatomical resolution. Growth of the field has been inhibited because physicians are trained early on in anatomy and deal with metabolism only as an abstract concept. It is much easier for a physician to integrate an anatomical picture rather than a scan that abstractly represents some metabolic function.

Nuclear medicine first began with simple probe studies, such as the thyroid uptake and kidney function. The interpretation of the uptake values was reasonably straightforward. As rectilinear scanning was introduced and became more popular, the problem of identifying hot or cold regions and conveying their meaning to physicians became difficult. The earlier studies, such as gold-198 and then technetium sulfur colloid imaging of the liver and spleen, were virtually anatomical studies because they showed the structure of these organs, and the metabolic uptake representing phagocytosis was relatively straightforward. As movement progressed to more abstract tracers, such as gallium, it became useful early on to draw an outline on the rectilinear scan of the body part being imaged. By so doing, it allowed the referring physician to appreciate approximately where this uptake is occurring. The word approximate should be emphasized.

Perfusion lung scanning soon became an important modality in the diagnosis of pulmonary embolism and was performed in conjunction with chest x-rays that became an integral and important part of the examination. This made the interpretation of the lung scan itself more accurate. The introduction in recent years of hybrid devices as reviewed in this issue has further enhanced this progress to a point where the impact it has made on medical imaging is indeed great. Particularly for PET studies, it is extremely important to be able to provide anatomical correlation, both to help recognize insignificant uptake and to orient the referring physician. When PET was first introduced in clinical institutions, many required that a CT scan be available for anatomical correlation. The problem noted was that because of changes in patient positioning and variation in technique, the correlation between the metabolic PET image and the anatomical CT image often was inaccurate. Although some of us believe that with further work, coregistration of a CT and PET done at different times could have been perfected, this approach was abandoned early with the introduction of combined PET/CT imaging. There is no question that this approach has advanced greatly. Problems in attenuation correction, using x-rays instead of γ-ray sources, have been greatly resolved to the point that the CT scan now provides an invaluable device for both attenuation correction and anatomical correlation.

However, although the first hybrid devices were SPECT/CT, progress in that area has been slow. This is possibly because of our familiarity of looking at SPECT images without the anatomical correlation and possibly because many of the agents currently used provide images that are relatively easy to recognize anatomically. However, in the last few years, the great success of PET/CT has stimulated renewed interest in the tremendous potential of SPECT/CT. At present, virtually all the PET units being sold in the United States are PET/CT devices. It is likely that the number of SPECT/CT devices will continue to grow during the next decade and stand along SPECT, as we know, will begin a lesser role in our daily practice.

These 2 issues review one of the most important advances in nuclear medicine imaging in the last decade. Clearly most nuclear medicine physicians would agree that the introduction of PET has revitalized the field tremendously and is perhaps the single greatest clinical innovation during this decade. Hybrid imaging represents the next natural step. The introduction of new radiopharmaceuticals and our ability to image and analyze a greater variety of organs with greater specificity remains an extremely important potential in nuclear medicine. The next decade will see increasing refinement of hybrid imaging, perhaps the introduction of MRI into hybrid devices clinically, and a greater variety of organ-specific and disease-specific radiopharmaceuticals. All of these advances will further enhance our diagnostic capability and ever growing contribution to clinical medicine and patient care.