Catálogo de publicaciones - libros

For the nuclear imaging technologist, success in obtaining a highquality imaging study in children is both challenging and rewarding. Imaging children for general nuclear medicine (NM) procedures requires versatile strategies that can be applied successfully to positron emission tomography (PET) imaging. This chapter discusses from the technologist’s perspective the strategies for general NM imaging, the special considerations and requirements for PET imaging, and the appropriate use of sedation in the pediatric patient.

Imaging pediatric patients provides many interesting challenges, not the least of which is keeping them motionless for the studies. Almost any imaging study is degraded, to a greater or lesser degree, by patient motion, and this can be particularly true for nuclear medicine studies where spatial resolution is an issue. Unfortunately, even the most cooperative child often cannot hold still for an imaging study of any length. This means that adequate sedation is one of the most important factors in performing high-quality imaging studies in children. This chapter discusses some of the basic concepts in pediatric sedation and suggests an approach to providing safe and effective sedation of children who are referred for PET scanning.

Few topics engender more vigorous debate than the biologic effects of low-level radiation and selection of a mathematical model to predict the incidence of cancer. A recent review on radiation risk stated (1): The A-bomb survivors represent the best source of data for risk estimates of radiation-induced cancer. It is clear that children are ten times more sensitive than adults to the induction of cancer. There are no assumptions, and no extrapolation indicated.

Radiopharmaceuticals are widely used for diagnostic imaging and radiation therapy. Although radiation therapy uses damage to living tissue to the advantage of the patient, this damage, however, is a limitation for the diagnostic application. Radiation dosages for specific indications are optimized based on thorough studies performed on animals and through clinical trials on human subjects prior to approval for clinical applications. Proper dosages are derived through careful study of pharmacokinetics, the physical characteristics of the radionuclide, metabolism of the subject, and the pharmacodynamics of the radiopharmaceutical in animal and human subjects. The chemical- and radiotoxicities and adverse reactions are well understood before an optimal and safe dosage is recommended. Doses are typically scaled by weight, or total body surface area, and reduced for children. The recommended dosage for a specific indication and route of administration are stated by the drug manufacturers in the package insert, and are readily available online.

Good intentions are necessary, but not sufficient, to conduct pediatric positron emission tomography (PET) research. This chapter provides direction to guide the process of conducting PET research in children.

The lack of reliable information on the use of medications for children has been addressed in the United States through two legislative initiatives: the Best Pharmaceuticals for Children Act (BPCA) of 2002 (1) and the Pediatric Research Equity Act (PREA) of 2003 (2). These two initiatives have stimulated pediatric pharmaceutical research, resulting in valuable information to guide the appropriate use of many medications (3). In addition, the National Institutes of Health now requires (as of 1998) that children be included in research unless there are scientific and ethical reasons not to include them (4). The resulting increase in pediatric research has led to concerns that the regulations governing pediatric research provide insufficient protection. This chapter refers to only the Food and Drug Administration (FDA) regulations governing research with children (21 CFR 50 and 56), as the use of radiopharmaceuticals in PET scanning is regulated by the FDA. Comparable regulations are found in 45 CFR 46, subparts A and D.

Positron emission tomography (PET) technology offers clinical researchers the opportunity to gain unprecedented understanding of the neurobiologic correlates of pediatric illness. In contrast to other forms of functional neuroimaging, PET provides direct information on neurochemical activity, such as neurotransmitter function in the human brain (1). Such data may prove invaluable to the understanding of brain maturation and the development of novel pharmacologic treatments for children. However, because PET is a radionuclear medicine technique and children are classified as a vulnerable population requiring special safeguards, PET utilization in pediatric research is controversial. The involvement of healthy children in PET research is an especially contentious issue, and to date fewer than a dozen such studies have been conducted in the United States.

The radioactive decay of many radioisotopes generates penetrating photons capable of escaping outside the matter in which the isotopes are located. From this radiation it is possible to image the spatial distribution of such isotopes inside an object. However, by itself the detection of a single photon outside the body of a patient carries minimal information on the location of its origin, unless some device capable of connecting the detection with the emission location is used. These devices are the optics of the imaging instrument and they identify, in combination with a position sensitive radiation detector, a line in space (the line of response, LOR) along which the photon must have originated (Fig. 8.1A,B). The LOR data are manipulated in reconstruction software to produce three-dimensional (3D) images of the activity distribution. When imaging humans, it is necessary to use photons capable of escaping undeflected from a few centimeters of tissue. The energy of these photons is such that their path cannot be bent by reflection (mirrors), refraction (lenses), or diffraction as in visible light optics. Nuclear scintigraphy and single photon emission computed tomography (SPECT) instrumentation resort to absorptive collimation, in which photons are selectively passed or absorbed depending on their emission location and angle of incidence on the optics. The drawback of this approach is that the wide majority of photons are lost before image reconstruction. For example, typical parallel-hole collimators [low energy-technetium-99m (99mTc; 140keV); general purpose] pass on the order of 1 in 10,000 (10-4) photons, but sensitivity is even lower for high-resolution and high-energy collimators, which need lower acceptance angles and thicker septa, respectively. Although sensitivity can be recouped by trading off resolution (as with high-sensitivity collimators) or field-of-view (as with converging collimators), it is the concept of absorptive collimation itself that implies an inefficient use of emitted photons.

Positron emission tomography (PET)–computed tomography (CT), which merges functional and anatomic imaging, is likely to herald a new generation of imaging modalities. Despite increasing interest and expertise in PET–CT, incorporation of such new technology into any department can be a challenge. Each department has its individual needs, personality, strengths, and weaknesses. The organization and integration of such imaging equipment must reflect these individual institutional and departmental characteristics, plus available supporting resources and the characteristics of patient cohorts.

The concept of using gamma cameras to detect the 511keV g rays, arising from the annihilation of a positron, was first implemented by H.O. Anger in the late 1950s (1–4). These cameras were capable of acquiring planar images of low-energy single photons as well as acquiring tomographic images of 511-keV photons in the coincidence mode. In the absence of a collimator, the resolution and sensitivity of the images obtained using the coincidence mode was far superior to that obtained using a collimated single photon emission computed tomography (SPECT) system. However, due to the limited count rate capability, slow computational speed, and lack of algorithms to process the acquired coincidence data, coincidence imaging was not widely used for clinical applications. Further, the limited availability of the fast decaying positron emitting tracers, coupled with limited reimbursement from the insurance companies, temporarily halted the development of gamma camera-based positron emission tomography (GCPET) systems (5,6).