B

Gamma Camera Diagram

Figure 7. A diagrammatic representation of the gamma camera imaging chain. Those gamma rays passing through the collimator cause the sodium iodide crystal to scintillate. The light is detected by photomultiplier tubes and analyzed in sum for the energy of the interaction and differentially for its position. The sum of many such interactions form the image projected on an oscilloscope or stored electronically.

Figure 7. A diagrammatic representation of the gamma camera imaging chain. Those gamma rays passing through the collimator cause the sodium iodide crystal to scintillate. The light is detected by photomultiplier tubes and analyzed in sum for the energy of the interaction and differentially for its position. The sum of many such interactions form the image projected on an oscilloscope or stored electronically.

all combine detectors with electronic amplification and analysis of the signal. The detectors in gamma cameras (Fig. 7) are scintillators made of sodium iodide crystals, whereas in positron emission detection bismuth germinate is often used but other materials are being explored for both applications including semi-conductor detection systems. A discussion of the relative advantages and disadvantages of these materials is beyond the scope of this chapter.

The electronic processing firstly involves pulse height analysis. This determines if the parcel of energy associated with each detected y-ray of electromagnetic radiation corresponds with the energy anticipated knowing the radionuclide injected. If it does not, and it might, for example, come from cosmic radiation or the naturally occurring radioactive tracer potassium-40 present in each of us, then that signal is rejected as having the potential to degrade the image. Secondly the electronic processing involves giving the signal an "address" which describes the coordinates of the interaction in the crystal. The collimator in the imaging system makes the image coherent much as the lens in a camera focuses light. The vertical perforations in the collimator between the patient being studied and the crystal make the radiation reaching the crystal reflect the distribution of the nuclide in the patient's body. In this way an image of that distribution can be built up.

It is important to note that since the data acquired are inherently digital (the positional address is computed) then nuclear medicine readily lends itself to quantitative methods. Several specific detection methods exist.

Static Imaging

The gamma camera is used to image a single organ within its field-of-view, such as heart or lung, after injection of an appropriate tracer. An example is a map of lung perfusion obtained after injection of 99mTc-protein aggregates which trap in lung capillaries.

Dynamic Imaging

This technique is often combined with static imaging. The arrival and uptake of tracer in an organ as well as its washout may be imaged or analyzed from repeated images taken over a span of time.

Gated Imaging

To "arrest" body motion during the time it takes to make an image, and thus reduce image blurring, the image may be gated by linking image acquisition to particular times in the cardiac or respiratory cycles. Images from the same segment of many such cycles are thus combined into what is in effect a ciné loop of the organ in motion.

Whole-Body Imaging

Among other imaging techniques involving exposure to ionizing radiation, such as computed radiography, the radiation exposure to the patient increases with each additional image made. This is not the case in nuclear medicine. In nuclear imaging the radiation exposure is determined by the injection of the tracer and additional images take time but do not otherwise expose the patient to risk. Radionuclide imaging is a particularly powerful method to search the whole body for disease, the distribution of which is unknown. Examples are bone scans done to detect metastases, tumor scans (for example with 18F-fluoro-deoxyglucose [FDG]) and scans with labeled white cells to detect occult infections.

Region-of-Interest (ROI) Analysis

Because, as we have seen, the data acquired in nuclear medicine images are inherently digital, it is easy to obtain quantitative information about organ function. From the gamma camera image the organ, or a part of it, is defined by a computergenerated outline—the "region-of-interest"—and the activity within this area measured, either as total uptake or rate-of-uptake in an activity-time plot. At present much research is being done to ensure that these measurements are accurately corrected for scattered radiations and attenuation of activity because of the depth of the organ in the body.

Single-Photon Emission (Computed) Tomography (SPECT or SPET)

The burgeoning of imaging methods in the second half of the twentieth century owes much to computer developments. These have made it possible to reconstruct sectional body images in CT, MRI, SPECT and PET. Sectional images avoid the superimposition of structures and reveal inner structure just as the slices of a loaf of bread reveal structure not apparent from merely looking at the loaf. Applied to nuclear medicine the sectional imaging method is called SPECT and is used almost invariably in brain and cardiac imaging and often in bone and tumor imaging. The detector system in SPECT, unlike PET in which a ring of crystal detectors is used, usually consists of two or three rotating gamma-camera heads. When not used for SPECT these are then available for other imaging methods (whole-body imaging, static imaging, etc.).

Positron-Emission Tomography (PET)

This technique images the radiations resulting from radionuclides decaying by positron emission (Fig. 6). The positrons (positive electrons) interact with negative electrons to yield two photons in opposite directions. Since this radiation is directional and simultaneous (the two rays arriving virtually simultaneously at the detector ring are identified as a single "coincidence" event) they must originate from a point on a line joining the sites of detection. In this way the use of a collimator may be avoided. PET is powerful in research and practice because it images the distribution in the body of compounds labeled with such biologically important atoms as carbon, nitrogen and oxygen.

As radiological methods have developed and multiplied in the last half century it sometimes has seemed that nuclear medicine might be overtaken by other technologies. That this has not occurred is an eloquent comment on the power of the tracer concept and the technology that supports it. Molecular biology and its offshoots such as the human genome project are poised to change medicine more in the next three decades than the previous thirty centuries. Indeed as medicine moves increasingly from descriptive science into an era of fundamental understanding, the molecular biological revolution, like the communications one, will present enormous opportunities for the promise of nuclear medicine to be fully realized.

Clinical Practice

While nuclear medicine is a clinical specialty practiced by physicians with specialty education, the development and practice of nuclear medicine owes a great deal to scientists of many disciplines:

Physics, Engineering and Computing Science

Not only have the imaging tools in nuclear medicine been developed by basic scientists in physics and engineering but these scientists continue to refine the use of gamma cameras by developing increasingly sophisticated techniques for image

The science of nuclear- or radio-pharmacy has provided a series of very effective tracers for diagnostic purposes. Many of these have been labeled with technetium-

analysis.

Radiopharmacy d99m. No stable form of this element is found on earth and it has no role in human metabolism. However, it is cheaply available from a generator, has advantageous physical properties as noted above, and has proved to be the work-horse for nuclear medicine for the past three decades. This fact owes a great deal to the innovations by radiopharmacists in finding ways to label biologically interesting molecules with technetium-99m.

A Perspective on the Future

Diagnostic imaging, by any method, does not exist in isolation but must respond to the changing context in which medicine is practiced. Several trends in healthcare stand out as the twentieth century gives way to the twenty-first:

• The high cost of care, and societal imperatives to contain such costs, and realign spending for other social purposes;

• The focus on the patient (sometimes called the client in this context) as a partner in health promotion rather than as the passive recipient of care;

• The public's increasing interest in and use of "complementary" medicine -either traditional methods such as acupuncture, or new techniques such as Bach flower therapy. By implication, this interest seems to be a measure of public skepticism about allopathic medicine and certainly represents a use of resources that might otherwise be used in conventional care;

• A further change in the traditional relationships between physicians and patients dictated by the accessibility of information, good and bad, about health from sources such as the world-wide-web;

• Increasing emphasis on the public health and the social determinants of illness as distinct from the "medical" view of illness;

• A requirement that any medical interventions be evidence-based. Again modern information technology will impact on this as decision support tools are developed and on-line records become a tool for audit and outcomes analyses; and

• Against this pattern of social change in the context in which medicine is practiced, medicine itself is also poised on the threshold of revolution. The insights afforded by molecular biology and the unfolding of the human genome project are about to change forever the human view of disease and the ability to treat it.

• Nuclear medicine methods are, by the standards of technology-intensive medicine, relatively low-cost, safe and minimally invasive as well as often able to be done for people as out-patients. This makes them likely to be important to the future of care.

• Nuclear medicine clearly will not and should not be expected to respond to every change in the social context in which medicine is practiced. Nevertheless it is capable of being made user-friendly. At the same time images are intrinsically an effective way to communicate with patients and might be used more often for this purpose.

• In the longer view the movement to an evidentiary basis for practice will serve nuclear medicine well given its rich tradition of intellectual inquiry. A considerable literature has already emerged, for example, to show that positron emission tomography with 18F-fluoro-deoxyglucose is, while expensive, both a cost-effective technique in cancer diagnosis and staging and one which positively influences patient outcomes.

Additional Reading

The following articles supply more information on the roots of nuclear medicine:

1. Brucer M. A Chronology of Nuclear Medicine. St. Louis: Heritage Publications Inc., 1990:piii.

2. Brucer M. Nuclear Medicine Begins with a Boa Constrictor. New York: Society of Nuclear Medicine, 1979:v-xxvi.

3. Cohen M. Ernest Rutherford at McGill University. In: Aldrich JE, Lentle BC, eds. A New Kind of Ray: The Radiological Sciences in Canada. 1895 - 1995. Montreal: The Canadian Association of Radiologists, 1995.

4. Levi H. George Hevesy and his concept of radioactive indicators—In retrospect. Eur J Nucl Med 1976; (1):3-7.

5. Röntgen WC. On a new kind of rays (English translation). Nature 1896; 53:274-276.

6. Mould RF. Discovery of radioactivity and radium. In: A Century of X-rays and Radioactivity in Medicine. London: The Institute of Physics Publishing, 1993:10.

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