Metabolic and kinetic considerations in the use of [125I]HIPDM for quantitative measurement of regional cerebral blood flow

Neuro-SPECT: On the development and function of brain emission tomography in the Copenhagen area

This review describes the development of single-photon emission tomography (SPECT) in the Copenhagen area under the leadership of the internationally renown scientist, Niels A. Lassen, and the history leading up to construction of the tomograph. Measurements of global cerebral blood flow (CBF) in the 1940s and 1950s were performed by Kety & Schmidt and Lassen & Munck. Determination of regional cerebral blood flow (rCBF) by intra-arterial injection of ¹³³ Xe and measurement with a 254-multicrystal scintillation detector and a computer system was a major step forward in the study of physiology and pathophysiology of cortical cerebral blood flow. Tomography with radioisotope ligands, including non-invasive administration, was advanced in different centres during the 1970s. An emission tomograph, the Tomomatic 64, was developed as a result of a multidisciplinary Danish and international collaboration. It was the first emission tomograph to provide dynamic data that could produce cross-sectional rCBF images. The present description of the construction and function of the Tomomatic 64 includes comparison with other contemporary and later brain-dedicated SPECT systems. Basic and clinical application of the Tomomatic 64 in Copenhagen resulted in several hundred important scientific publications and improved diagnostics for patients with a variety of neurological disorders. It is concluded that the development of the Tomomatic 64 was a major step forward in the study and examination of rCBF and brain function related to several brain disorders, in addition to vascular diseases.

First-pass extraction fraction of iodine-123 labeled perfusion tracers in living primate brain

The cerebral extraction and retention of three radioiodinatéd SPECT perfusion tracers were measured using residue detection in a baboon. A permeability-surface area product PS' with special relevance to SPECT was calculated from the retention of tracer in the brain after 10 min. PS' differs from the traditional PS value, which is calculated from the tracer clearance curve at 2 min. The PS' values ranged from 50 to 95 mL/min/100 g, decreased in the order [123I]IMP > [123I]iodoperidol approximately [123I]HIPDM, and did not differ for specific activities of 10 MBq/mmol to 74 TBq/mmol. These radioiodinated compounds exhibited extraction characteristics superior to those of [99mTc]HMPAO but underestimated cerebral blood flow when flows were above 20-30 mL/min/100 g, underscoring the need for development of a more ideal SPECT perfusion tracer.

Quantitative in vivo receptor binding. IV: Detection of muscarinic receptor down-regulation by equilibrium and by tracer kinetic methods

Newly-developed methods for estimation of in vivo binding to neurotransmitter receptors should enable the detection and quantification of physiologic or pathologic changes in receptor numbers. In the present study, both equilibrium and kinetic experimental strategies for in vivo muscarinic receptor determination were applied to the detection of receptor changes induced by chronic inhibition of acetylcholinesterase in the rat. Following one week of treatment, in vitro receptor autoradiography utilizing [3H]scopolamine revealed significant losses of muscarinic binding in the cerebral cortex, hippocampus, striatum and in cranial nerve motor nuclei. The in vivo distribution of [3H]scopolamine, following infusion to approach equilibrium binding in the brain, revealed reductions in binding which paralleled the pattern and magnitude of changes detected in vitro. A simplified tracer kinetic estimation following bolus injection of the ligand also detected substantial reductions in forebrain muscarinic receptor binding. These results indicate the feasibility of detecting receptor changes underlying neuropathologic conditions in vivo, and suggest that either equilibrium or kinetic experimental approaches may be extended to clinical research applications with the use of positron or single-photon emission tomography.

In vivo dissociation kinetics of [3H]quinuclidinyl benzilate: relationship to muscarinic receptor concentration and in vitro kinetics

The in vivo washout kinetics of [3H]quinuclidinyl benzilate ([3H]QNB) varies significantly in various structures in the rat brain. The slowest washout rates are from the hippocampus, corpus striatum, and cortex, intermediate rates are exhibited from the thalamus and colliculi, while the fastest washout rate is from the cerebellum. We have also demonstrated a difference in the in vitro dissociation rates (k-1) of [3H]QNB from various structures. The k-1 for the hippocampus, corpus striatum and cortex, is two-fold slower than that observed in the thalamus, colliculi, and cerebellum. The differences in the in vitro dissociation kinetics are not, however, sufficient to explain the differences in the in vivo washout kinetics. We have developed a theoretical formulation which describes conditions under which the washout kinetics are a function of the concentration of receptor in a structure. Furthermore, we present a graphical method in which a plot of the reciprocal of the observed washout rate constant, 1/k(obs), vs receptor concentration is linear. Analysis of the washout kinetics of [3H]QNB from various structures of the CNS of rat were well described by this theory when the differences in in vitro k-1 are included.

Overview of radiopharmaceuticals for diagnosis of central nervous disorders

There are two types of imaging instruments, single-photon emission computed tomography (SPECT) and positron emission tomography (PET) that use radiopharmaceuticals for the diagnosis of brain disorders. Brain perfusion imaging agents, labeled either with 123I or 99mTc, are useful in detecting various cerebral vascular abnormalities, such as stroke and transient ischemia with SPECT. The management of other neurological disorders (i.e., in Alzheimer's, epilepsy, schizophrenia, and head trauma patients) may also be benefitted by these agents. The exact trapping mechanisms and their relationships with potential clinical applications still remain to be elucidated. Imaging studies using 18F fluorodeoxyglucose with PET is currently the most promising diagnostic tool for the evaluation of local glucose metabolism related to various disease states, such as Alzheimer's disease, brain tumor, and epilepsy. In the past few years significant progress has been made in the design and characterization of new CNS neuronal and postsynaptic receptor imaging agents for PET and SPECT. The new diagnostic agents are aimed at measurements of localization and changes of neuronal function. It is likely that these types of agents have potential for clinical application, especially in the diagnosis of psychiatric disorders that do not involve morphological changes.

Recent developments in 99mTc and 123I-radiopharmaceuticals for SPECT imaging

Availability of 123I of high radionuclidic purity has encouraged the development of 123I-based radiopharmaceuticals for the assessment of myocardial fatty acid metabolism, myocardial neuronal activity, and for receptor and antibody imaging. Advances in the chemistry of technetium have resulted in the development of novel agents for myocardial and cerebral perfusion and renal function studies. Monoclonal antibodies labeled with 99mTc show promise for imaging neoplastic lesions, myocardial infarcts, and thrombus localization. Recent developments in 123I and 99mTc agents for myocardial and brain imaging studies are discussed.

In vivo metabolism and kinetics of 99mTc-HMPAO

The cerebral distribution of 99mTc-labeled d, l, hexamethyl-propylene-amine-oxime (99mTc-HMPAO) as a function of rCBF and time was examined in rats and in man. The results of this study confirm that 99mTc-HMPAO is distributed in brain in proportion to rCBF. However, the rapid systemic breakdown of the tracer in blood results in considerable difficulties in the assessment of the arterial concentration of the parent compound; incomplete extraction of 99mTc-HMPAO from blood to brain and significant efflux from brain represent further limitations in the use of this tracer for quantification of rCFB. Despite these limitations 99mTc-HMPAO is of potential interest for a qualitative assessment of rCBF in specific clinical conditions.

Retention of [99mTc]-d,l-HM-PAO in rat brain: an autoradiographic study

The regional cerebral distribution pattern of [99mTc]-d,l-HM-PAO in rat brain was studied by autoradiography. The regional cerebral uptake of this tracer is related to regional cerebral blood flow (rCBF); however, the ratio of retained radioactivity (determined by digital imaging techniques) in gray matter compared to white matter is lower than that reported for the blood flow ratio. Considerable inhomogeneity is observed in cortical gray matter for at least 60 min postinjection, demonstrating that minimal, if any, cerebral redistribution of this agent occurs.

Linearization correction of 99mTc-labeled hexamethyl-propylene amine oxime (HM-PAO) image in terms of regional CBF distribution: comparison to C15O2 inhalation steady-state method measured by positron emission tomography

The radioisotope distribution following intravenous injection of 99mTc-labeled hexamethylpropyleneamine oxime (HM-PAO) in the brain was measured by single photon emission computed tomography (SPECT) and corrected for the nonlinearity caused by differences in net extraction. The "linearization" correction was based on a three compartment model, and it required a region of reference to normalize the SPECT image in terms of regional cerebral blood flow distribution. Two different regions of reference, the cerebellum and the whole brain, were tested. The uncorrected and corrected HM-PAO images were compared with cerebral blood flow (CBF) image measured by the C15O2 inhalation steady state method and positron emission tomography (PET). The relationship between uncorrected HM-PAO and PET-CBF showed a correlation coefficient of 0.85 but tended to saturate at high CBF values, whereas it was improved to 0.93 after the "linearization" correction. The whole-brain normalization worked just as well as normalization using the cerebellum. This study constitutes a validation of the "linearization" correction and it suggests that after linearization the HM-PAO image may be scaled to absolute CBF by employing a global hemispheric CBF value as measured by the nontomographic 133Xe clearance method.

Single photon emission computed tomography in seizure disorders

Fourteen children with various seizure disorders were studied using a cerebral blood flow tracer, 123I iodoamphetamine (0.05 mCi/kg), and single photon emission computed tomography (SPECT). In the five patients with radiological lesions, SPECT showed congruent or more extensive abnormalities. Five of the nine children with a normal scan on computed tomography had abnormal SPECT studies consisting of focal hypoperfusion, diffuse hemispheric hypoperfusion, multifocal and bilateral hypoperfusion, or focal hyperperfusion. A focal lesion seen on SPECT has been found in children with tonic-clonic seizures suggesting secondarily generalised seizures. Moreover the pattern seen on SPECT seemed to be related to the clinical status. An extensive impairment found on SPECT was associated with a poor evolution in terms of intellectual performance and seizure frequency. Conversely all children with a normal result on SPECT had less than two seizures per year and normal neurological and intellectual development.

Recovery from aphasia and neglect after subcortical stroke: neuropsychological and cerebral perfusion study

Cortical regional cerebral perfusion was assessed by N, N, N1-trimethyl-N1-(2)-hydroxy-3-methyl-5-(I-123) iodobenzyl-1, 3-propanediamine 2 HCl I-123 (HIPDM) and single photon emission computerised tomography (SPECT) in six aphasic and two neglect patients with unilateral subcortical vascular lesions. Assessments were carried out both in the acute phase and after a period ranging from 1 to 6 months after stroke onset. In all patients an almost complete spontaneous recovery occurred and was associated with a significant improvement of cortical perfusion. A relationship between severity of aphasia and degree of cortical hypoperfusion was found, in both the acute and the follow up assessments, in the aphasic subgroup.

Remote effects of subcortical cerebrovascular lesions: a SPECT cerebral perfusion study

The remote effects of small unilateral cerebrovascular lesions confined to subcortical structures were evaluated by single photon emission computerized tomography (SPECT) and a CBF tracer, I-123 HIPDM. A CBF study was performed in 34 patients presenting with subcortical stroke either in the acute or in the chronic stages. Twenty-one of the 34 patients showed areas of cortical hypoperfusion ipsilateral to the subcortical lesion. In 14 patients, asymmetry of perfusion was also observed at the cerebellar level, perfusion being significantly reduced in the cerebellar hemisphere contralateral to the lesion. There was no correlation between the degree and extension of these remote effects and the type of stroke (ischemic or hemorrhagic), the patency of cerebral arteries, or the size and site of the lesion by transmissive computerized tomography (TCT). Subcortical hematomas showed a correlation between occurrence of remote effects and time interval from the onset of stroke, occurring more frequently in the acute phase. A correlation was observed between cortical and cerebellar remote effects and the severity of clinical presentation. The causes of remote effects are still unclear and have been extensively debated. Our data indicate that there is a relationship of remote effect to the neurological status. It is possible to show, by noninvasive, low-cost methods, remote CBF effects after stroke that may contribute to the assessment of brain functional impairment.

Positron emission tomographic measurement of cerebral blood flow and permeability-surface area product of water using [15O]water and [11C]butanol

We have previously adapted Kety's tissue autoradiographic method for measuring regional CBF in laboratory animals to the measurement of CBF in humans with positron emission tomography (PET) and H2(15)O. Because this model assumes diffusion equilibrium between tissue and venous blood, the use of a diffusion-limited tracer, such as H2(15)O, may lead to an underestimation of CBF. We therefore validated the use of [11C]butanol as an alternative freely diffusible tracer for PET. We then used it in humans to determine the underestimation of CBF that occurs with H2(15)O, and thereby were able to calculate the extraction Ew and permeability-surface area product PSw of H2(15)O. Measurements of the permeability of rhesus monkey brain to [11C]butanol, obtained by means of an intracarotid injection, external detection technique, demonstrated that this tracer is freely diffusible up to a CBF of at least 170 ml/min-100 g. CBF measured in baboons with the PET autoradiographic method and [11C]butanol was then compared with CBF measured in the same animals with a standard residue detection method. An excellent correspondence was obtained between both of these measurements. Finally, paired PET measurements of CBF were made with both H2(15)O and [11C]butanol in 17 normal human subjects. Average global CBF was significantly greater when measured with [11C]butanol (53.1 ml/min-100 g) than with H2(15)O (44.4 ml/min-100 g). Average global Ew was 0.84 and global PSw was 104 ml/min-100 g. Regional measurements showed a linear relationship between local PSw and CBF, while Ew was relatively uniform throughout the brain. Simulations were used to determine the potential error associated with the use of an incorrect value for the brain-blood partition coefficient for [11C]butanol and to calculate the effect of tissue heterogeneity and errors in flow measurement on the calculation of PSw.

Long-term noninvasive single photon emission computed tomography monitoring of perfusional changes after EC-IC bypass surgery

The rCBF was evaluated using I-123 HIPDM and single photon emission computed tomography (SPECT) on 14 patients undergoing extracranial-intracranial (EC-IC) bypass surgery because of internal carotid artery (ICA) occlusion. Before surgery, all patients showed cortical areas of hypoperfusion over the affected cerebral hemisphere. Shortly after EC-IC bypass a rCBF increase was observed in six patients. However, at the 6 and 12 month follow-ups, with angiographic control of bypass patency, rCBF studies did not show any significant rCBF change. Long-term noninvasive tomographic monitoring of perfusion changes occurring after EC-IC bypass surgery failed to show a long-lasting improvement in perfusion.

Tracer disposition kinetics in the determination of local cerebral blood flow by a venous equilibrium model, tube model, and distributed model

Tracer distribution kinetics in the determination of local cerebral blood flow (LCBF) were examined by using three models, i.e., venous equilibrium, tube, and distributed models. The technique most commonly used for measuring LCBF is the tissue uptake method, which was first developed and applied by Kety (1951). The measurement of LCBF with the 14C-iodoantipyrine (IAP) method is calculated by using an equation derived by Kety based on the Fick's principle and a two-compartment model of blood-tissue exchange and tissue concentration at a single data point (Sakurada et al., 1978). The procedure, in which the tissue is to be in equilibrium with venous blood, will be referred to as the tissue equilibration model. In this article, effects of the concentration gradient of tracer along the length of the capillary (tube model) and the transverse heterogeneity in the capillary transit time (distributed model) on the determination of LCBF were theoretically analyzed for the tissue sampling method. Similarities and differences among these models are explored. The rank order of the LCBF calculated by using arterial blood concentration time courses and the tissue concentration of tracer based on each model were tube model (model II) less than distributed model (model III) less than venous equilibrium model (model I). Data on 14C-IAP kinetics reported by Ohno et al. (1979) were employed. The LCBFs calculated based on model I were 45-260% larger than those in models II or III. To discriminate among three models, we propose to examine the effect of altering the venous infusion time of tracer on the apparent tissue-to-blood concentration ratio (lambda app). A range of the ratio of the predicted lambda app in models II or III to that in model I was from 0.6 to 1.3. In the future, there may be a need to determine which model should be used to calculate the LCBF based on this discriminator and to develop another discriminator by using multiple data points based on positron emission tomography.

Functional imaging of the brain

The radionuclide tracer method is unique amongst all other imaging methodologies in its ability to trace organ or tissue function and metabolism. It derives this advantage from the nature of the signal used for image generation, and its single interaction with the organ or system under examination. Physical processes such as electron or proton density assessment or resonance, edge identification, electrical or ultrasonic impedence, do not pertain to the image generation process in nuclear medicine, and if so, only in a rather secondary manner. The nuclear medicine imaging study is primarily a study of the chemical nature, distribution and interaction of the tracer/radiopharmaceutical utilised with the cellular system which requires investigation: the thyroid cells with sodium iodide, the recticular endothelial cells with colloidal particles, the adrenal medulla cells with metaiodobenzylguanidine, and so on. In the two most recent areas of nuclear medicine expansion, oncology (with labelled monoclonal antibodies) and neurology and psychiatry (with a whole new series of lipid soluble radiopharmaceuticals), specific cell systems can also be targeted and hence imaged and investigated. The study of structure as masterly performed by Virchow and all his successors over more than a century, is now definitely the prerogative of such imaging systems which excel with spatial and contrast resolution (x-ray computed transmission tomography, nuclear magnetic resonance imaging, diagnostic ultrasound). However the investigation of function and metabolism (as performed by Claude Bernard, Georg von Hevesy, and so many others), has clearly passed from the laboratory animal protocol and experiment to the direct investigation in man, this being the achievement of the radionuclide tracer methodology. In this article, we review present interest and developments in that part of nuclear medicine activity which is aimed at the study of the neurological or psychiatric patient.

Remote effects of caudate hemorrhage: a clinical and functional study

Clinical, anatomical and functional findings in two cases of previous caudate nucleus hemorrhage, with good recovery, are presented. CBF data obtained with SPECT and 123 I-HIPDM well correlate with functional residual deficit, despite normal TCT examinations. These observations are suggestive of a partial "wallerian" diaschisis.

Effects of insulin on local cerebral glucose utilization in the rat

The effects of hyperinsulinemia on local cerebral glucose utilization were studied by the quantitative autoradiographic 2-[14C]deoxyglucose method in normal conscious rats under steady-state normoglycemic conditions. Hyperinsulinemia and a steady state of normoglycemia were achieved and maintained during the experimental period by a continuous intravenous (i.v.) infusion of insulin given simultaneously with a programmed i.v. infusion of D-glucose. Hyperinsulinemia under normoglycemic conditions did not change the average rate of glucose utilization in the brain as a whole, but significant increases in local glucose utilization were found selectively in the ventromedial, dorsomedial, and anterior hypothalamic nuclei. The results suggest that a known anatomical pathway linking the dorsomedial and anterior nuclei with the ventromedial nucleus of the hypothalamus may be physiologically activated in response to hyperinsulinemia.

HIPDM single photon emission computed tomography brain imaging in partial onset secondarily generalized tonic-clonic seizures

HIPDM-Single photon emission computed tomography brain imaging was performed during interictal and ictal stages in three patients with complex partial seizures and secondarily generalized tonic-clonic seizures. In all three patients, interictal studies demonstrated decreased regional cerebral perfusion (rCP) and ictal studies showed increased rCP in the epileptogenic region. The demonstration of focal hyperperfusion by SPECT performed during secondarily generalized tonic-clonic seizures suggests that rCP in the epileptic focus remains higher than in other cerebral regions during immediate postictal stages, even in secondarily generalized seizures.

Cerebellar diaschisis in pontine ischemia. A case report with single-photon emission computerized tomography

Regional cerebral and cerebellar blood flows were studied by N,N,N'-trimethyl-N'-(2-hydroxy-3-methyl-5-[123I]iodobenzyl)-1,3- propanediamine 2 HCl (I-123 HIPDM) and single-photon emission computerized tomography (SPECT) in a patient with an ischemic lesion of the pons. An asymmetry of perfusion of the cerebellar hemispheres, normal on transmission computerized tomography scan, was demonstrated by SPECT studies in the early acute phase and confirmed 15 days after. This finding may be related to the interruption of the corticopontocerebellar pathways.

Sequential assessment of regional cerebral blood flow, regional cerebral blood volume, and blood-brain barrier in focal cerebral ischemia: a case report

Regional CBF (rCBF) and regional cerebral blood volume (rCBV) were evaluated by N,N,N'-trimethyl-N'-(2)-hydroxy-3-methyl-5-[123I]iodobenzyl-1, 3-propanediamine-2 HCl- and 99mTC-labeled red blood cells, respectively, and single-photon emission computerized tomography (SPECT) in a patient with focal cerebral ischemia. Sequential transmission computerized tomography (TCT) and SPECT functional data were compared with clinical findings to monitor the pathophysiological events occurring in stroke. A lack of correlation between rCBF-rCBV distributions and blood-brain barrier (BBB) breakdown was found in the acute phase. In the face of more prolonged alteration of BBB, as seen by TCT enhancement, a rapid evolution of transient phenomena such as luxury perfusion was shown by SPECT studies. Follow-up of the patient demonstrated a correlation between the neurological recovery and a parallel relative improvement of the cerebral perfusion.