Radionuclide imaging

X-ray examinations and nuclear medicine have in common the use of ionising radiation. All X -ray examinations (including CT) are based upon the detection of radiation having passed through the patient, i.e., transmitted radiation. Radionuclide imaging, on the other hand, involves detection of radiation emitted from a radioactive tracer inside the patient.

Radioactive tracers, termed radiopharmaceuticals, may be used for either diagnostic or therapeutic purposes. They all contain radionuclides, which are unstable atoms that decay spontaneously with the emission of energy. This radioactive part of the radiopharmaceutical is often coupled to a carrier molecule which determines the distribution in the body. The ideal radiopharmaceutical is distributed only to the organs or structures to be imaged. The distribution may be determined by e.g. metabolic processes (the carrier molecule may be part of the metabolic process), or by local perfusion or blood flow. Recording of radioactivity may then give important functional information. The ability to show physiological function, is the major advantage of radionuclide imaging as compared to alternative radiological modalities. A relative disadvantage is the low spatial resolution of the technique.

The radionuclide itself should ideally have a half-life in the order of one to few times the examination time, which may range from ten minutes to several hours. This would ensure sufficient radioactivity throughout the examination, without undue radiation to the patient after completion of the study. The radioactive decay process may yield emission of alpha, beta or gamma rays. For imaging purposes, radionuclides emitting gamma photons (high-energy electromagnetic radiation) are preferred. Alpha particles (helium nuclei) and beta particles (electrons) are unsuited for imaging due to poor tissue penetration. Similar to X-rays, the penetration of gamma rays increases with increasing photon energy. The energy should not be too high, however, or the photons may penetrate the detector without being absorbed. In radionuclide imaging, gamma photon energies in the range of 50 to 300 keV are preferred; the ideal energy is about 150 keV. (The most commonly used radionuclide, 99mTc, emits photons with an energy of 140 keV.)

The gamma camera

The detector used in the majority of radionuclide imaging procedures is the gamma camera (scintillation camera) (Fig. 9). The main component is a large, disk-shaped scintillation crystal (often made from sodium iodine, maximum diameter about 60 cm) (Fig. 10). Placed in front of the crystal, facing the patient, is a special lead shielding device, a so-called

Figure 9. Patient preparation prior to radioisotope scanning. The gamma camera is placed in dose proximity to the region (here brain) to be imaged. (Phototechnical Department, Rikshospitalet, Oslo.)

Figure 10. Schematic sectional drawing of gamma camera with parallel hole collimator (see text).

collimator. The collimator may have various designs, and it determines the projection of the emitted radiation onto the crystal. The parallel hole collimator shown in Figure 10, is a thick lead plate pierced with multiple parallel holes. Due to the parallel arrangement of the holes, the two-dimensional distribution of the radiopharmaceutical in the body is projected on the crystal surface in a 1:1 ratio.

Gamma photons absorbed by the scintillation crystal, result in the release of light. The light is distributed through an optically coupled light pipe to numerous photomultiplier tubes, where electrical signals are generated. The amplitudes of the electric al signals are proportional to the amounts of light received. The light from each scintillation is shared by all the photomultiplier tubes, but the light intensity is maximum in the photomultiplier tube located directly above the position of the scintillation. By simultaneous analysis of all the photomultiplier signals, the intensity and location of each scintillation are determined. From these data, a two-dimensional projection image of the radiopharmaceutical distribution is reconstructed. The final image may be displayed in an analogue format directly on a cathode ray tube (CRT) or on a photographic film. However, most gamma cameras also provide digital images by digitising the analogue electric al output signals from the photomultiplier tubes. Digital technology is a prerequisite for several radionuclide imaging procedures, e.g. various dynamic studies, ECG-gated cardiac studies, and tomographic studies.

The practical procedure

In radionuclide imaging (also referred to as radioisotope scanning), the vast majority of procedures are performed with intravenous administration of the radiopharmaceutical (one exception is inhalation of radioactive aerosols or gases, e.g. xenon, in pulmonary ventilation studies). As a radiation protection measure, a lead-shielded syringe is used for injection. The time lapse from injection to gamma camera detection of the emitted radiation, varies greatly with the type of study.

Pulmonary perfusion studies are examples of immediate post-injection imaging. The indication for this study is suspected pulmonary embolism. The most commonly used radiopharmaceuticals are 99mTc labelled serum albumin macroaggregates or microspheres with a particle size of 20-100 m. The particles are trapped in the smallest pulmonary arterioles, but only 0.2% of the arterioles are blocked at the same time. Immediate imaging of the lungs is performed in several projections by placing the gamma camera next to the part of the lungs to be imaged. A frontal view of the anterior parts of both lungs is obtained by placing the gamma camera in front of the lungs, and a posterior view is obtained with the gamma camera behind the lungs. Lateral and oblique views may be acquired as well. During exposure, the scintillations are counted, and the exposure is terminated when a predetermined number of counts have been acquired (e.g. 500,000). In pulmonary perfusion studies, this may take 2-3 minutes.

An example of moderately delayed post-injection imaging is bone scanning (skeletal imaging), where the detection with the gamma camera is performed 2-4 hours after injection of 99mTc labelled diphosphonates (distributed to metabolically active bone tissue). In the search for occult tumours or inflammation with gallium scanning, the post-injection delay is very long; scanning is not performed until at least 2 days after the injection of 67Ga citrate.

Emission computed tomography

Similar to X -ray computed tomography (CT), radionuclide imaging also has its tomographic techniques. Two major tomographic methods have evolved: l) single photon emission computed tomography (SPECT), and 2) positron emission tomography (PET).

1) SPECT

The least sophisticated versions of SPECT are simply based on an ordinary gamma camera made to rotate around the patient. By recording the radioactivity at numerous angles, sectional images may be reconstructed. SPECT is a widely used technique, especially in cardiac and brain studies.

2) PET

This tomographic technique involves the use of positron emitting radionuclides. The mass of positrons and electrons are identical, but positrons are positively charged. The emitted positron reacts quickly with a nearby electron; the reaction is termed annihilation and involves the formation of two 511 keV gamma photons that radiate in diametrical opposite directions. Collinearly opposed special detectors are used to detect the coincident annihilation photons; the photon energy (511 keV) is too high to employ ordinary gamma cameras.

PET allows quantitative estimations of radionuclide concentrations and has a great potential in the study of metabolic processes at various disease states. Several elements that take part in important biochemical processes have positron emitting isotopes, e.g. 11C, 13N, 150. Other important metabolites may be labelled with positron emitting isotopes; as an example, 18p labelled deoxyglucose may be used to study cerebral glucose metabolism. The major disadvantages of positron emitting radionuclides are their need for production by expensive cyclotrons, and their short half-lives (the half-lives of 150 and 18p are 2 minutes, and 110 minutes, respectively). The rapid decay in radioactivity requires a cyclotron very close to the laboratory, a requirement that has contributed to a slow distribution of PET units.

Hans-Jørgen Smith