Dosimetry of iodine-123 iomazenil in humans

Detection of Transient Increase of Cerebral Blood Flow and Reversible Neuronal Dysfunction by Iodine-123-Iomazenil Single Photon Emission Computed Tomography After Cerebral Hyperperfusion Syndrome After Revascularization Surgery for Moyamoya Disease

BACKGROUND

Early and late images of single photon emission computed tomography (SPECT) using ¹²³I-iomazenil (¹²³I-IMZ) can demonstrate cerebral blood flow and cortical neuronal viability. Hyperperfusion syndrome is one of the serious complications after revascularization surgery for moyamoya disease; therefore, the real-time observation of the hemodynamics and neuronal viability is important for the treatment after the revascularization. Here we report, a case of moyamoya disease where ¹²³I-IMZ SPECT had a significant efficacy to delineate the hemodynamics and transient neuronal dysfunction in hyperperfusion state after revascularization.

CASE DESCRIPTION

A 47-year-old woman presented with motor aphasia 3 days after superficial temporal artery-middle cerebral artery anastomosis with indirect revascularization. Magnetic resonance imaging (MRI) on the same day showed no new ischemic changes but high intensities along the left frontal sulci observed on fluid-attenuated inversion recovery images, and ¹²³I-IMZ SPECT demonstrated the increased uptake on the early images and the decreased uptake on the late images around the anastomosis site. The patient was completely recovered 1 month after surgery, and abnormal changes on MRI and ¹²³I-IMZ SPECT returned to normal along with the symptom withdrawal.

CONCLUSIONS

These findings indicate that ¹²³I-IMZ SPECT could be the index for the treatment of revascularization for obstructive vascular diseases such as moyamoya disease.

Population exposure to diagnostic use of ionizing radiation in The Netherlands

The use of ionizing radiation for diagnostic medical procedures and the exposure of the Dutch population to this radiation were assessed for 1998. The annual average effective dose from diagnostic medical exposures has increased by 26% to 0.59 mSv per capita since the last inventory of medical radiation exposure in the Netherlands a decade ago. The population-averaged effective dose comprises x-ray procedures in hospitals (87%), nuclear medicine examinations (11%), mammography screening (1.5%), and extramural dentistry (0.2%). The rise has resulted mainly from an increase in frequency and patient dose for CT examinations and from vascular radiology. The increase in the number of CT examinations leveled off in the mid-1990's. Medically exposed people were found to be significantly older than the general population. Based on age distribution alone, an "age reduction factor" for the risk coefficient of 0.64 was found to apply to the medically exposed group. More information on patient dose for the complete set of procedures should, according to this study, become available.

Dosimetry related to SPECT and PET applications

Nuclear medicine methods permit the in vivo measurement of physiological and metabolic functions. For this purpose metabolically active molecules are labelled with radionuclides. After the injection, these radiopharmaceuticals can be detected from outside the body with appropriate detectors such as conventional gamma cameras, single photon emission computerized tomographs (SPECT), or positron emission tomographs (PET). As with any application of ionizing radiation the benefits of nuclear medicine (the diagnosis and treatment of disease) must be balanced against the risks of the procedures. Risks in nuclear medicine are evaluated through dosimetry, the calculation of the radiation dose estimates, in which the energy deposited by the radiation in human tissues is quantitated. The first step of dosimetry is the determination of the biodistribution, i.e. the spatial and temporal distribution of the radiopharmaceutical within the body. When a new radiopharmaceutical is introduced such data are first derived from ex vivo experiments in animals. Further evaluations of the biokinetics and dosimetry are carried out via quantitative measurements in humans using the nuclear medicine imaging equipment, applying whole-body planar and tomographic methods. Based on the knowledge of the biodistribution and using the framework introduced by the MIRD committee as well as by the ICRP, the radiation dose to single organs and to the total body can be calculated. The purpose of this paper is to review methods for measuring radiopharmaceutical biodistributions in humans and for calculating the appropriate radiation dose quantities.

Similarity and robustness of PET and SPECT binding parameters for benzodiazepine receptors

The single photon emission computed tomography (SPECT) radiotracer [123I]iomazenil is used to assess benzodiazepine receptor binding parameters. These measurements are relative indices of benzodiazepine receptor concentration (B'max). To evaluate the ability of such indices in accurately accessing the B'max the authors compared them with absolute values of B'max, measured using positron emission tomography (PET). The authors performed SPECT, PET, and magnetic resonance imaging (MRI) studies on a group composed of seven subjects. For SPECT studies, the authors administered a single injection of [123I]iomazenil and estimated the total and specific distribution volumes (DV(T SPECT), DV(S SPECT)) and the binding potential (BP) using unconstrained (BP(SPECT)) and constrained (BP(C SPECT)) compartmental models. For PET studies, the authors used a multiinjection approach with [11C]flumazenil and unlabeled flumazenil to estimate absolute values of receptor concentration, B'max, and some other binding parameters. The authors studied the correlation of different binding parameters with B'max. To study the robustness of the binding parameter measurements at the pixel level, the authors applied a wavelet-based filter to improve signal-to-noise ratio of time-concentration curves, and the calculated kinetic parameters were used to build up parametric images. For PET data, the B'max and the DV(PET) were highly correlated (r = 0.988). This confirms that it is possible to use the DV(PET) to access benzodiazepine receptor density. For SPECT data, the correlation between DV(SPECT) estimated using a two- and three-compartment model was also high (r = 0.999). The DV(T SPECT) and BP(C SPECT) parameters estimated with a constrained three-compartment model or the DV(T''SPECT) parameter estimated with a two-compartment model were also highly correlated to the B'max parameter estimated with PET. Finally, the robustness of the binding parameters allowed the authors to build pixel-by-pixel parametric images using SPECT data.

Changes of benzodiazepine receptors during chronic benzodiazepine administration in humans

Changes of central type GABA(A)/benzodiazepine receptors during 24-day per-oral administration of alprazolam (2 mg/day) were measured with single photon emission computed tomography (SPECT) in nine healthy human subjects. Receptor densities were measured on days -4 (baseline), 3, 10, 17 and 24. Comparison of baseline and day 3 SPECT images was used to assess receptor occupancy; comparisons of the four scans on medication were used to assess alterations in receptor levels. Clinical effects were evaluated by subjective ratings of mood and the Hopkins verbal learning test. Alprazolam induced sedation associated with a 16% receptor occupancy. Unoccupied receptor levels decreased 10% from day 3 to day 10 but then normalized to baseline values by day 17. Clinical effects showed corresponding changes 1-2 weeks after the changes in the receptor. Thus, the decrease of benzodiazepine receptor densities may be one of the major mechanisms for tolerance development in humans.

Decreased benzodiazepine receptor binding in panic disorder measured by IOMAZENIL-SPECT. A preliminary report

Single photon emission tomography (SPECT) imaging of the central benzodiazepine receptor (BZr) became possible with the newly developed ligand 123I-IOMAZENIL. The BZr binding was investigated in ten patients with panic disorder (PP) compared to ten epileptic patients (EP). Panic patients had lower IOMAZENIL uptake rates in the frontal, occipital and temporal cortex than EP indicating the involvement of the BZr complex in panic disorder.