George J. Hunter

Guidelines and Recommendations for Perfusion Imaging in Cerebral Ischemia A Scientific Statement for Healthcare Professionals by the Writing Group on Perfusion Imaging, From the Council on Cardiovascular Radiology of the American Heart Association

A number of techniques have been developed during the past four decades to evaluate cerebral perfusion. The oldest used 133Xe, a lipophilic radioactive tracer that easily diffuses through the blood-brain barrier (BBB). It was either injected or inhaled, and probes placed over the scalp were used to measure perfusion to the cerebral cortex.1,2 In the mid-1970s, the development of a scanner to detect the emission of positrons led to positron emission tomography (PET) in humans.3 Using a number of radioisotopes, this technology can measure cerebral blood flow (CBF) and various metabolic processes, but until recently it has been primarily used as a research tool. Stable (“cold”) xenon was found to attenuate x-rays in a manner similar to iodine, and there were a number of projects in the 1970s to use this gas as a contrast agent for the rapidly emerging technology of computed tomography (CT), particularly as a perfusion tracer.4 This resulted in the development of the xenon-enhanced CT (XeCT) technique to calculate CBF in patients.5 With improvements in single photon emission CT (SPECT) during the 1980s, a number of compounds that are metabolized in the central nervous system (CNS) were found to be appropriate for perfusion imaging.6,7 Perfusion-weighted and diffusion-weighted magnetic resonance (MR) imaging (PWI and DWI) were developed in the late 1980s,8,9 and that technology has continued to improve. Finally, with the evolution of helical and spiral multislice CT technology, CT perfusion (CTP) imaging is becoming a potentially important clinical technique.10 Although the development of these technologies has been fascinating, their role in evaluating a variety of diseases of the CNS is controversial. It might seem obvious that a disorder of blood flow, such as acute stroke or chronic vascular occlusive disease, should be studied with a perfusion imaging technique. …

CT angiography in the rapid triage of patients with hyperacute stroke to intraarterial thrombolysis: accuracy in the detection of large vessel thrombus.

Purpose The purpose of this work was to evaluate the accuracy of CT angiography (CTA) for the detection of large vessel intracranial thrombus in clinically suspected hyperacute (<6 h) stroke patients. Method Forty-four consecutive intraarterial thrombolysis candidates underwent noncontrast CT followed immediately by CTA. Axial source and two-dimensional collapsed maximum intensity projection reformatted CTA images were rated for the presence or absence of large vessel occlusion. Five hundred seventy-two circle-of-Willis vessels were reviewed; arteriographic correlation was available for 224 of these. Results Sensitivity and specificity for the detection of large vessel occlusion were 98.4 and 98.1%; accuracy, calculated using receiver operating characteristic analysis, was 99%. Mean time for acquisition, reconstruction, and analysis of CTA images was approximately 15 min. Conclusion CTA is highly accurate for the detection and exclusion of large vessel intracranial occlusion and may therefore be valuable in the rapid triage of hyperacute stroke patients to intraarterial thrombolytic treatment.

Measurement of cerebrovascular changes in cats after transient ischemia using dynamic magnetic resonance imaging.

Background and Purpose Hemodynamic changes associated with acute ischemia cannot be measured with conventional nuclear magnetic resonance imaging. In this study, we used dynamic susceptibility-contrast magnetic resonance imaging to measure the changes in vascular transit time and relative cerebral blood volume in a feline occlusion-reperfusion model. Methods Dynamic susceptibility-contrast measurements were obtained before and during 10 minutes of global cerebral ischemia and for up to 3 hours after the onset of reperfusion. A cerebral blood flow index was calculated from the vascular transit time and relative cerebral blood volume measurements. Functional maps were constructed to demonstrate the regional hemodynamic differences resulting from the induced ischemia. Results During the early phase after reperfusion, both the relative cerebral blood volume and blood flow index rose sharply, followed by a fall to near-basal levels at 45 minutes (1 x control and 1.3 x control, respectively). Thereafter, the volume rose slowly, whereas the flow index continued to drop. At 3 hours, cerebral blood volume had reached 1.6 times its control value, whereas the flow index had returned to its base value. Conclusions The hemodynamic behavior we observed in our model reflects the independent responses of the cerebral blood volume and flow index to ischemic insult. Measurements acquired by our method were consistent with the temporal behavior reported in previous radionuclide studies. Susceptibility-contrast nuclear magnetic resonance tomographic imaging proved to be valuable in detecting and quantifying both immediate and subsequent changes in the hemodynamic state of the ischemic and hyperemic feline brain.

Accuracy of Four-dimensional CT for the Localization of Abnormal Parathyroid Glands in Patients with Primary Hyperparathyroidism

PURPOSE To investigate multiphase multidetector four-dimensional computed tomography (CT) as a technique to correctly localize abnormal parathyroid glands in patients with primary hyperparathyroidism. MATERIALS AND METHODS Informed consent was waived by the institutional review body for this retrospective, chart review study. Radiology reports from four-dimensional CT and surgical notes were reviewed in 143 patients with primary hyperparathyroidism (35 men, 108 women; median ages, 58 and 60 years, respectively) who underwent parathyroid surgery between August 2004 and January 2007 and in whom four-dimensional CT predicted a single lesion. Accuracy of four-dimensional CT was stratified by patient and was determined separately for localization to the correct side and quadrant (upper and lower for each side), with surgical findings serving as standard of reference. RESULTS In 143 patients, 148 abnormal parathyroid glands were found at surgery; 137 (93%) of these were weighed, with mean and median weights of 757 and 417 mg, respectively. Four-dimensional CT lateralized the abnormal glands with 93.7% accuracy (134 of 143). For localization according to quadrant, the accuracy was 86.6% (116 of 134). CONCLUSION Four-dimensional CT has sufficiently high accuracy in presurgical localization to allow confident performance of unilateral parathyroidectomy in patients with sporadic primary hyperparathyroidism. The superior accuracy compared with that of ultrasonography and technetium 99m sestamibi scanning may be sufficient to allow four-dimensional CT to be used as the sole presurgical localization method.

Accuracy of 4-dimensional computed tomography in poorly localized patients with primary hyperparathyroidism

BACKGROUND Four-dimensional computed tomography (4D-CT) utilizes multiplanar images and perfusion characteristics to identify abnormal parathyroid glands. We assessed the role of 4D-CT in patients with inconclusive preoperative ultrasound and sestamibi localization studies. METHODS Adult patients with primary hyperparathyroidism with negative or discordant standard imaging who underwent both localization with 4D-CT and operative intervention for curative intent were included. Patient characteristics, 4D-CT scan results compared with operative findings, and curative proportion were assessed. RESULTS Of the 60 patients, 4D-CT accurately lateralized 73% and localized 60% of abnormal glands found at operation. Single candidate lesions (46/60) were confirmed at operation in 70%. When multiple lesions were identified on 4D-CT (14/60), accuracy dropped to 29% (P = .03). The accuracy of 4D-CT was not different between primary and reoperative cases (P = .79). Of the 8 patients with multigland disease diagnosed perioperatively, 5 had multiple candidate lesions noted on 4D-CT. In 94% (48/51) of patients, a >50% drop in intraoperative parathormone (IOPTH) level was achieved after resection and 87% (48/55) had long-term cure with a median follow-up of 221 days. CONCLUSION 4D-CT identifies the more than half of abnormal parathyroids missed by traditional imaging and should be considered in cases with negative or discordant sestamibi and ultrasound. Bilateral exploration is warranted when multiple candidate lesions are reported on 4D-CT. Multigland disease remains a challenging entity.

Increased cerebral blood volume in HIV‐positive patients detected by functional MRI

Objective To study changes in cerebral hemodynamics related to HIV infection. Background Cerebral injury is a well-known manifestation of HIV infection. Physiologic changes in the HIV brain may precede structural changes and may be detected by functional MRI (fMRI). Methods Dynamic contrast fMRI was used to measure the cerebral blood volume (CBV) in 13 patients infected with HIV and in 7 healthy control subjects. Results Significant increases in dynamic CBV were found in the deep (p < 0.001) and cortical gray matter (p < 0.05) of HIV-positive (HIV+) patients. Patients with definite cognitive impairment showed significantly greater increases in CBV in the deep gray matter (DGM) compared with those without impairment. In one patient with rapidly progressive cognitive impairment, these abnormalities reversed and paralleled clinical improvement after initiation of zidovudine monotherapy. Conclusions This study supports the hypothesis that HIV infection is associated with significant cerebral hemodynamic changes, particularly in the DGM, that may contribute to cognitive dysfunction in AIDS. Functional MRI may be useful for early detection of cerebral injury and for the assessment of novel therapies.

The Massachusetts General Hospital acute stroke imaging algorithm: an experience and evidence based approach

The Massachusetts General Hospital Neuroradiology Division employed an experience and evidence based approach to develop a neuroimaging algorithm to best select patients with severe ischemic strokes caused by anterior circulation occlusions (ACOs) for intravenous tissue plasminogen activator and endovascular treatment. Methods found to be of value included the National Institutes of Health Stroke Scale (NIHSS), non-contrast CT, CT angiography (CTA) and diffusion MRI. Perfusion imaging by CT and MRI were found to be unnecessary for safe and effective triage of patients with severe ACOs. An algorithm was adopted that includes: non-contrast CT to identify hemorrhage and large hypodensity followed by CTA to identify the ACO; diffusion MRI to estimate the core infarct; and NIHSS in conjunction with diffusion data to estimate the clinical penumbra.

Increasing Contrast Agent Concentration Improves Enhancement in First-Pass CT Perfusion

BACKGROUND AND PURPOSE: Our aim was to evaluate whether increasing iodine concentration, at a constant total iodine dose, resulted in better brain tissue opacification in patients with acute stroke symptoms during their evaluation by first-pass CT perfusion (CTP). MATERIALS AND METHODS: One hundred two patients presenting to the emergency department within 3 hours of onset of acute stroke symptoms underwent CTP scanning. Three different concentrations of iodinated nonionic contrast material were used (300, 350, or 400 mg/mL). Total iodine dose (15 g) and injection rate (7 mL/s) were kept constant. There were 25, 53, and 19 patients in the different concentration groups, respectively; 5 patients were excluded due to uncorrectable motion artifacts. CTP scanning was performed at the level of the putamen, and data were analyzed by determining peak opacification for normal gray and white matter, arterial input, and venous output. Mean and SD values were calculated, and 3 concentration groups, stratified by region-of-interest location, were compared by using a single-tailed unpaired t test. RESULTS: Monotonic increasing peak opacification was observed in all region-of-interest locations. Statistically significant differences were observed between the 300 and 350 mg/mL, 300 and 400 mg/mL, as well as the 350 and 400 mg/mL groups (P < .01) in white matter, gray matter, and the arterial input. Statistical significance was seen in the venous output group between the 300 and 400 mg/mL (P < .005) and 350 and 400 mg/mL (P < .007) groups, but not between the 300 and 350 mg/mL (P = .2) groups. CONCLUSION: Increasing contrast concentration improves peak opacification of tissue, suggesting that CTP evaluation of patients with acute stroke is better performed with the highest available concentration contrast agent.

CT perfusion identifies increased salvage of tissue in patients receiving intravenous recombinant tissue plasminogen activator within 3 hours of stroke onset.

BACKGROUND AND PURPOSE: In spite of the advent of thrombolytic therapy, CT-perfusion imaging is currently not fully used for clinical decision-making and not included in published clinical guidelines for management of ischemic stroke. We investigated whether lesion volumes on cerebral blood volume (CBV), cerebral blood flow (CBF), and mean transit time (MTT) maps predict final infarct volume and whether all these parameters are needed for triage to intravenous recombinant tissue plasminogen activator (rtPA). We also investigated the effect of intravenous rtPA on affected brain by measuring salvaged tissue volume in patients receiving intravenous rtPA and in controls. MATERIALS AND METHODS: Forty-four patients receiving intravenous rtPA and 19 controls underwent CT perfusion (CTP) studies in the emergency department within 3 hours of stroke onset. Lesion volumes were measured on MTT, CBV, and CBF maps by region-of-interest analysis and were compared with follow-up CT volumes by correlation and regression analysis. The volume of salvaged tissue was determined as the difference between the initial MTT and follow-up CT lesion volumes and was compared between intravenous rtPA-treated patients and controls. RESULTS: No significant difference between the groups was observed in lesion volume assessed from the CTP maps (P > .08). Coefficients of determination for MTT, CBF, and CBV versus follow-up CT lesion volumes were 0.3, 0.3, 0.47, with intravenous rtPA; and 0.53, 0.55, and 0.81 without intravenous rtPA. Regression of MTT on CBF lesion volumes showed codependence (R2 = 0.98, P < .0001). Mean salvaged tissue volumes with intravenous rtPA were 21.8 ± 17.1 and 13.2 ± 13.5 mL in controls; these were significantly different by using nonparametric (P < .03) and Fisher exact tests (P < .04). CONCLUSIONS: Within 3 hours of stroke onset, CBV lesion volume does not necessarily represent dead tissue. MTT lesion volume alone can be used to identify the upper limit of the size of abnormally perfused brain. More brain is salvaged in patients with intravenous rtPA than in controls.

4D-CT for Preoperative Localization of Abnormal Parathyroid Glands in Patients with Hyperparathyroidism: Accuracy and Ability to Stratify Patients by Unilateral versus Bilateral Disease in Surgery-Naive and Re-Exploration Patients

BACKGROUND AND PURPOSE: 4D-CT is an emerging technique that uses high-resolution images, multiplanar reformats, and perfusion characteristics to identify abnormal parathyroid glands in patients with hyperparathyroidism. This study evaluates the accuracy of 4D-CT for localization and lateralization of abnormal parathyroid glands in preoperative planning for minimally invasive parathyroidectomy vs bilateral neck exploration at a tertiary referral center. MATERIALS AND METHODS: Radiology, pathology, and operative reports were retrospectively reviewed for 208 patients with hyperparathyroidism who underwent 4D-CT and parathyroid surgery between May 2008 and January 2012. 4D-CT performance in localizing side and site was determined by use of surgical and pathologic findings as a reference. RESULTS: Of 208 patients, 155 underwent initial surgery and 53 underwent re-exploration parathyroid surgery. No lesions were found in 8 patients (3.8%). A total of 284 lesions were found in 200 patients; 233 were correctly localized by 4D-CT (82.0%). Of the 200 patients with parathyroid lesions, 146 underwent unilateral and 54 bilateral neck exploration. 4D-CT correctly identified unilateral vs bilateral disease in 179 (89.5%) of 200. 4D-CT correctly localized parathyroid lesions in 126 of the unilateral cases (86.3%). In the re-exploration cohort, 4D-CT correctly identified unilateral vs bilateral disease in 46 (95.8%) of 48. There was no statistically significant difference in subgroups stratified by surgery type (primary or subsequent) and number of scan phases (3 or 4) (P > .56). CONCLUSIONS: 4D-CT leverages modern high-resolution CT scanning and dynamic contrast enhancement to localize abnormal parathyroid glands in patients with hyperparathyroidism of any cause and can be used for planning minimally invasive parathyroidectomy vs bilateral neck exploration.