Radionuclide Imaging

PET and SPECT are techniques that image the uptake of radioactively labeled compounds into tissue; these labeled compounds are called radiotracers and are usually injected intravenously. The subsequent tissue uptake of the radiotracer is measured over time and used to make a series of images. Although PET and SPECT rely on similar principles to produce their images, important differences in instrumentation and, especially, clinical use necessitate separate discussion.

FIGURE 11.4. Patient being slid into the scanner; the Snow White machine is in place. (With permission from M. D. Abramoff, "Objective Measurement of Motion in the Orbit" [Ph.D. thesis] Transatlantic Publishing, New York, 2001.)

motion. The arrowheads on the perimeter of the circle indicate motion of 0.3 mm/deg directed anteriorly and left, respectively. Blobs that move in the orientation indicated by the white arrows (posteriorly and left-anteriorly) at 0.3 mm/deg are included as examples. (With permission from M. D. Abramoff, "Objective Measurement of Motion in the Orbit" [Ph.D. thesis] Transatlantic Publishing, New York, 2001.)

FIGURE 11.5. Normal orbital soft tissue motion imaged by MR dynamic color mapping. (A) Static MR image from the sequence. MRI images are as viewed from below. (mrm, medial rectus muscle; ON, optic nerve.) (B) MR dynamic color map. The subject gazes from left to right. Wherever the flow is zero or cannot be measured reliably, the original MR image is visible. (C) The circle serves as an aid to relate a particular color to orientation and motion. The arrowheads on the perimeter of the circle indicate motion of 0.3 mm/deg directed anteriorly and left, respectively. Blobs that move in the orientation indicated by the white arrows (posteriorly and left-anteriorly) at 0.3 mm/deg are included as examples. (With permission from M. D. Abramoff, "Objective Measurement of Motion in the Orbit" [Ph.D. thesis] Transatlantic Publishing, New York, 2001.)

Positron Emission Tomography

A PET scanner is similar to a CT scanner in appearance. Instead of imaging the transparency of tissues to x-rays, however, PET scanners measure the emission of positrons (photons) from the radiotracer that has been injected intravenously. Because of the amount of radioactivity thus introduced into the body, individuals are limited to approximately five scans per year. After injection, multiple scans are made over several hours. Among the most commonly used positron-emitting nuclides are 11-carbon (11C) and 18-fluorine (18F). These nuclides replace the normal, nonemitting atoms in medically interesting compounds to obtain a labeled compound that is taken up into the tissue of interest. Thus, 18-fluorine replaces normal fluorine in fluoridated glucose, resulting in the labeled compound [2-18F]fluoro-2-deoxy-d-glucose, usually known as FDG.

FDG PET takes advantage of the increased gly-colytic activity associated with neoplastic disease. FDG PET has been shown to be superior to MR in the detection of squamous cell carcinoma head/neck metastases in a prospective study, although neck dissection and biopsy of lymph nodes remained neces-sary.41 FDG PET has also been found to be superior to gallium scanning in the staging of lymphoma (Hodgkin's, indolent non-Hodgkin's, or aggressive non-Hodgkin's), with four patients reclassified to a higher stage, necessitating a change in treatment.42

When 11-carbon replaces 12-carbon, the resulting radiotracer compound is referred to as 11C PK11195 (1-[2-chlorphenyl]-N-methyl-N-[1-methyl-propyl]-3-isoquinoline carboxamide). This compound binds specifically to peripheral benzodiazepine receptors on macrophages, and, in the absence of blood-borne inflammatory cells, on the surface of activated mi-croglia.43 Activated microglia proliferation is a marker

FIGURE 11.6. (A) Static transversal MR scan of the face of a patient who had persistent pain after enucleation with implant, optic nerve attached. (B) MR dynamic color mapping with patient gazing from left to right. (C) Same with patient gazing from right to left. The implant on the left shows decreased motion compared to

the healthy right orbit (0.14 mm/deg) but moves concurrently with the stump. Shear is absent, and the optic nerve is continuous with the implant. (With permission from M. D. Abramoff, "Objective Measurement of Motion in the Orbit" [Ph.D. thesis] Transatlantic Publishing, New York, 2001.)

FIGURE 11.6. (A) Static transversal MR scan of the face of a patient who had persistent pain after enucleation with implant, optic nerve attached. (B) MR dynamic color mapping with patient gazing from left to right. (C) Same with patient gazing from right to left. The implant on the left shows decreased motion compared to the healthy right orbit (0.14 mm/deg) but moves concurrently with the stump. Shear is absent, and the optic nerve is continuous with the implant. (With permission from M. D. Abramoff, "Objective Measurement of Motion in the Orbit" [Ph.D. thesis] Transatlantic Publishing, New York, 2001.)

FIGURE 11.7. Histological section of surgically removed implant of patient who had persistent pain after enucleation and abnormal MR dynamic color mapping. The scleral cover on the right is connected to the stump of the optic nerve on the left by a mass of collagen fibers forming a pseudodisk (black arrow). Inset: Macroscopic aspect of the removed implant with the optic nerve including the remnant of the central retinal artery attached to it (hematoxylin-eosin, original magnification X 0.8. (With permission from M. D. Abramoff, "Objective Measurement of Motion in the Orbit" [Ph.D. thesis] Transatlantic Publishing, New York,

FIGURE 11.7. Histological section of surgically removed implant of patient who had persistent pain after enucleation and abnormal MR dynamic color mapping. The scleral cover on the right is connected to the stump of the optic nerve on the left by a mass of collagen fibers forming a pseudodisk (black arrow). Inset: Macroscopic aspect of the removed implant with the optic nerve including the remnant of the central retinal artery attached to it (hematoxylin-eosin, original magnification X 0.8. (With permission from M. D. Abramoff, "Objective Measurement of Motion in the Orbit" [Ph.D. thesis] Transatlantic Publishing, New York, of inflammation of and damage to neural tissue. Binding of UC PK11195 is minimal in normal nerve tissues and increases significantly in cases of neuronal damage or inflammation; the compound has been used to visualize activation of microglia in patients with stroke, multiple sclerosis, facial nerve lesion, and Rasmussen's encephalitis.44 The suitability of UC PK11195 PET to detect inflammation in the orbit and along the optical tract is under investigation by the author.

Single Photon Emission Computed Tomography

In SPECT, a scanner is used to image the number of y-rays coming from the tissue being studied. Therefore, different radioactive labels that emit y-rays are needed, such as 123-iodine (123I), the long-lived nuclide 99m-technetium (99mTc), and indium-111 (111In). Just as in PET, these y-ray-emitting labels can be incorporated into compounds that target the tissue of interest, such as methoxyisobutylisonitrile labeled with 99m-technetium (99mTc-MIBI), N-isopropyl-p-[123I] iodoamphetamine (123I-IMP), and [111In]-DTPA-octreotide (111INOCT). 123I-IMP SPECT has been found to be sensitive and specific in differentiating uveal melanomas from nevus, as determined by his-topathology of the melanoma specimen and follow-up of nevus eyes by ultrasonography and funduscopy.45 111INOCT SPECT can differentiate the origin of mucosa-associated lymphoid tissue (MALT) lymphoma, including lymphomas in the orbit, as gastric or extragastric, as shown in a prospective study that used SPECT imaging to compare the histopathology of biopsy specimens.46

The radiotracer 99mTc-MIBI, probably a marker for mitochondrial activity, was found to be successful in differentiating malignant from benign orbital masses.47 Although not directly related to tumor imaging, SPECT imaging of Graves orbitopathy activity has been found to be effective: mINOCT SPECT was able to predict the response of Graves orbitopathy to im-munosuppressive therapy and radiotherapy in 22 patients in a prospective study.48

The radionuclide imaging techniques are meanwhile well established in oncology. However, because of the issues of ionizing radiation exposure and cumbersome instrumentation needs, among others, they are still not available in many clinics, precluding their widespread use in orbital imaging.

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