Stuart R. Pomerantz

Cranial CT with Adaptive Statistical Iterative Reconstruction: Improved Image Quality with Concomitant Radiation Dose Reduction

Most state-of-the-art CT scanners have some type of iterative reconstruction program that allows for lower patient radiation exposure. At my institution we use it, when available, for most neuroimaging studies. Six levels in 100 CT iterative reconstruction studies were compared with conventional CT obtained previously in the same patients. SNR and CNR were computed and the studies were blindly and qualitatively evaluated. The results showed that iterative reconstruction studies had lower image noise and increased low-contrast resolution while allowing lower radiation doses without affecting spatial resolution. BACKGROUND AND PURPOSE: To safeguard patient health, there is great interest in CT radiation-dose reduction. The purpose of this study was to evaluate the impact of an iterative-reconstruction algorithm, ASIR, on image-quality measures in reduced-dose head CT scans for adult patients. MATERIALS AND METHODS: Using a 64-section scanner, we analyzed 100 reduced-dose adult head CT scans at 6 predefined levels of ASIR blended with FBP reconstruction. These scans were compared with 50 CT scans previously obtained at a higher routine dose without ASIR reconstruction. SNR and CNR were computed from Hounsfield unit measurements of normal GM and WM of brain parenchyma. A blinded qualitative analysis was performed in 10 lower-dose CT datasets compared with higher-dose ones without ASIR. Phantom data analysis was also performed. RESULTS: Lower-dose scans without ASIR had significantly lower mean GM and WM SNR (P = .003) and similar GM-WM CNR values compared with higher routine-dose scans. However, at ASIR levels of 20%–40%, there was no statistically significant difference in SNR, and at ASIR levels of ≥60%, the SNR values of the reduced-dose scans were significantly higher (P < .01). CNR values were also significantly higher at ASIR levels of ≥40% (P < .01). Blinded qualitative review demonstrated significant improvements in perceived image noise, artifacts, and GM-WM differentiation at ASIR levels ≥60% (P < .01). CONCLUSIONS: These results demonstrate that the use of ASIR in adult head CT scans reduces image noise and increases low-contrast resolution, while allowing lower radiation doses without affecting spatial resolution.

Virtual Monochromatic Reconstruction of Dual-Energy Unenhanced Head CT at 65–75 keV Maximizes Image Quality Compared with Conventional Polychromatic CT

PURPOSE To determine the virtual monochromatic imaging (VMI) energy levels that maximize brain parenchymal image quality in dual-energy unenhanced head computed tomography (CT) and to assess the improvement with this technique compared with conventional polychromatic scanning. MATERIALS AND METHODS Institutional review board approval was obtained with no informed consent required for this HIPAA-compliant retrospective analysis. Twenty-five consecutive unenhanced head CT scans were acquired with a 64-section dual-energy scanner with fast tube voltage switching (80-140 kVp). Scans were retrospectively reconstructed at VMI energy levels from 40 to 140 keV in 5-keV increments and were analyzed by using four quality indexes: gray matter (GM) signal-to-noise ratio (SNR), white matter (WM) SNR, GM-WM contrast-to-noise ratio (CNR), and posterior fossa artifact index (PFAI). Optimal mean values for each parameter were compared with those from 50 consecutive scans obtained with the same scanner in 120-kVp single-energy mode. Repeated-measures analysis of variance and Dunnett post hoc t test were then used to determine significance. RESULTS Maximal GM SNR, WM SNR, and GM-WM CNR values were observed at 65 keV, and minimal PFAI was observed at 75 keV. These values were significantly better than those of conventional polychromatic CT (P < .01); quality index improvement ratios (corrected for radiation dose) ranged from 17% to 50%. CONCLUSION Virtual monochromatic reconstruction of dual-energy unenhanced head CT scans at 65-75 keV (optimal energy levels) maximizes image quality compared with scans obtained with conventional polychromatic CT.

CT Perfusion Mean Transit Time Maps Optimally Distinguish Benign Oligemia from True “At-Risk” Ischemic Penumbra, but Thresholds Vary by Postprocessing Technique

BACKGROUND AND PURPOSE: Various CTP parameters have been used to identify ischemic penumbra. The purpose of this study was to determine the optimal CTP parameter and threshold to distinguish true “at-risk” penumbra from benign oligemia in acute stroke patients without reperfusion. MATERIALS AND METHODS: Consecutive stroke patients were screened and 23 met the following criteria: 1) admission scanning within 9 hours of onset, 2) CTA confirmation of large vessel occlusion, 3) no late clinical or radiographic evidence of reperfusion, 4) no thrombolytic therapy, 5) DWI imaging within 3 hours of CTP, and 6) either CT or MR follow-up imaging. CTP was postprocessed with commercial software packages, using standard and delay-corrected deconvolution algorithms. Relative cerebral blood flow, volume, and mean transit time (rCBF, rCBV and rMTT) values were obtained by normalization to the uninvolved hemisphere. The admission DWI and final infarct were transposed onto the CTP maps and receiver operating characteristic curve analysis was performed to determine optimal thresholds for each perfusion parameter in defining penumbra destined to infarct. RESULTS: Relative and absolute MTT identified penumbra destined to infarct more accurately than CBF or CBV*CBF (P < .01). Absolute and relative MTT thresholds for defining penumbra were 12s and 249% for the standard and 13.5s and 150% for the delay-corrected algorithms, respectively. CONCLUSIONS: Appropriately thresholded absolute and relative MTT-CTP maps optimally distinguish “at-risk” penumbra from benign oligemia in acute stroke patients with large-vessel occlusion and no reperfusion. The precise threshold values may vary, however, depending on the postprocessing technique used for CTP map construction.

Functional Contrast-Enhanced CT for Evaluation of Acute Ischemic Stroke Does Not Increase the Risk of Contrast-Induced Nephropathy

BACKGROUND AND PURPOSE: Concerns have recently grown regarding the safety of iodinated contrast agents used for CTA and CTP imaging. We tested whether the incidence of AN, defined by a ≥25% increase in the post−contrast scan creatinine level, was higher among patients with ischemic stroke who underwent a functional contrast-enhanced CT protocol compared with those who had no iodinated contrast administration. MATERIALS AND METHODS: The contrast-exposed group consisted of 575 patients with acute ischemic stroke who underwent CTA (n = 313), CTA/CTP (n = 224), or CTA/CTP followed by conventional angiography (n = 38) within 24 hours of stroke onset and were consecutively enrolled in a prospective cohort study. The nonexposed group consisted of 343 patients with ischemic stroke, consecutively admitted to the same institution, who did not receive iodinated contrast material. Patients were stratified by baseline eGFR. In the primary analysis, the Fisher exact test was used to compare the incidence of AN between the contrast-exposed and the nonexposed patients at 24, 48, and 72 hours and on a cumulative basis. A secondary analysis compared the incidence of AN in patients who underwent conventional angiography following CTA/CTP versus patients who underwent CTA/CTP only. RESULTS: The incidence of AN was 5% in the exposed and 10% in the nonexposed group (P = .003). Patients who underwent conventional angiography after contrast CT were at no greater risk of AN than patients who underwent CTA/CTP alone (26 patients, 5%; and 2 patients, 5%, respectively; P = .7). CONCLUSIONS: Administration of a contrast-enhanced CT protocol involving CTA/CTP and conventional angiography in selected patients does not appear to increase the incidence of CIN.

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.

Value of Coronal Reformations in the CT Evaluation of Acute Head Trauma

BACKGROUND AND PURPOSE: Routine axial CT images may not be ideally suited for detecting ICH in transversely oriented locations such as the floor of the anterior and middle cranial fossas and vertex. This study was performed to evaluate whether coronal reformations improve detection of ICH in NCCT performed for head trauma. MATERIALS AND METHODS: All patients undergoing a first NCCT in the ED for evaluation of head trauma were included prospectively during a 6-month interval. NCCT images were reconstructed into standard 5-mm axial datasets and were also reformatted into coronal datasets of 5-mm sections and 2.5-mm intervals. Thirty-two of 213 (15%) scans were interpreted as showing traumatic ICH. These cases were interspersed with 30 studies without ICH. Cases were reviewed for the presence and location of ICH by 2 staff neuroradiologists. RESULTS: Of 213 patients, 32 NCCTs demonstrated ICH (a total of 104 foci). Fifteen of 104 (14%) ICHs (8 patients) were detected solely on coronal images. Locations included the floor of the anterior and middle cranial fossas, vertex, corpus callosum, falx, tentorium, and occipital convexity. Coronal reformations allowed exclusion of suspicious findings on axial images in 14 instances (7 patients). Coronal images aided interpretation in 29/104 (28%) findings. CONCLUSIONS: Coronal reformations improve the detection of ICH over axial images alone, especially for lesions that lie in the axial plane immediately adjacent to bony surfaces. The use of coronal reformations should be considered in the routine interpretation of head CT examinations performed for the evaluation of head trauma.

Clot Length Distribution and Predictors in Anterior Circulation Stroke Implications for Intra-Arterial Therapy

Background and Purpose— Thin-section noncontrast computed tomography images can be used to measure hyperdense clot length in acute ischemic stroke. Clots ≥8 mm have a very low probability of intravenous tissue-type plasminogen activator recanalization and hence may benefit from a bridging intra-arterial approach. To understand the prevalence of such clots, we sought to determine the distribution and predictors of clot lengths in consecutive anterior circulation proximal artery occlusions. Methods— Of 623 consecutive patients with acute ischemic stroke, 53 met inclusion criteria: presentation <8 hours from onset; intracranial internal carotid artery-terminus or proximal-middle cerebral artery occlusion; admission thin-slice noncontrast computed tomography (⩽2.5 mm); and no intravenous tissue-type plasminogen activator pretreatment. For each patient, hyperdense clot length was measured and recorded along with additional relevant imaging and clinical data. Results— Mean age was 70 years, and mean time to computed tomography was 213 minutes. Median baseline National Institutes of Health Stroke Scale was 16.5. Occlusions were located in the internal carotid artery-terminus (34% [18 of 53]), middle cerebral artery M1 (49% [26 of 53]) and M2 segments (17% [9 of 53]). Hyperdense thrombus was visible in 96%, with mean and median clot lengths (mm) of 18.5 (±14.2) and 16.1 (7.6–25.2), respectively. Occlusion location was the strongest predictor of clot length (multivariate, P=0.02). Clot length was ≥8 mm in 94%, 73%, and 22% of internal carotid artery-terminus, M1, and M2 occlusions, respectively. Conclusions— The majority of anterior circulation proximal occlusions are ≥8 mm long, helping to explain the low published rates of intravenous tissue-type plasminogen activator recanalization. Internal carotid artery-terminus occlusion is an excellent marker for clot length ≥8 mm; vessel-imaging status alone may be sufficient. Thin-section noncontrast computed tomography seems useful for patients with middle cerebral artery occlusion because of the wide variability of clot lengths.

Computed Tomography Angiography of the Carotid and Cerebral Circulation

As a result of the development of multidetector row computed tomography (CT) technology, multidetector CT angiography is rapidly becoming the preferred examination for the initial evaluation of an increasing number of clinical neurovascular applications such as carotid artery steno-occlusive disease, acute ischemic and hemorrhagic stroke, subarachnoid hemorrhage, and cerebral vasospasm. This article reviews the most recent literature on these topics, provides the reader with useful clinical tips for performing and interpreting these increasingly complex diagnostic examinations, presents illustrative cases, and looks at future developments in this vibrant area of neuroradiology research.

Rate of Contrast Extravasation on Computed Tomographic Angiography Predicts Hematoma Expansion and Mortality in Primary Intracerebral Hemorrhage

Background and Purpose— In primary intracerebral hemorrhage, the presence of contrast extravasation after computed tomographic angiography (CTA), termed the spot sign, predicts hematoma expansion and mortality. Because the biological underpinnings of the spot sign are not fully understood, we investigated whether the rate of contrast extravasation, which may reflect the rate of bleeding, predicts expansion and mortality beyond the simple presence of the spot sign. Methods— Consecutive intracerebral hemorrhage patients with first-pass CTA followed by a 90-second delayed postcontrast CT (delayed CTA) were included. CTAs were reviewed for spot sign presence by 2 blinded readers. Spot sign volumes on first-pass and delayed CTA and intracerebral hemorrhage volumes were measured using semiautomated software. Extravasation rates were calculated and tested for association with hematoma expansion and mortality using uni- and multivariable logistic regressions. Results— One hundred and sixty-two patients were included, 48 (30%) of whom had ≥1 spot sign. Median spot sign volume was 0.04 mL on first-pass CTA and 0.4 mL on delayed CTA. Median extravasation rate was 0.23 mL/min overall and 0.30 mL/min among expanders versus 0.07 mL/min in nonexpanders. Extravasation rates were also significantly higher in patients who died in hospital: 0.27 mL/min versus 0.04 mL/min. In multivariable analysis, the extravasation rate was independently associated with in-hospital mortality (odds ratio, 1.09 [95% confidence interval, 1.04–1.18], P=0.004), 90-day mortality (odds ratio, 1.15 [95% confidence interval, 1.08–1.27]; P=0.0004), and hematoma expansion (odds ratio, 1.03 [95% confidence interval, 1.01–1.08]; P=0.047). Conclusions— Contrast extravasation rate, or spot sign growth, further refines the ability to predict hematoma expansion and mortality. Our results support the hypothesis that the spot sign directly measures active bleeding in acute intracerebral hemorrhage.

Spot sign on 90-second delayed computed tomography angiography improves sensitivity for hematoma expansion and mortality: prospective study.

Background and Purpose— The computed tomography angiography (CTA) spot sign is a validated biomarker for poor outcome and hematoma expansion in intracerebral hemorrhage. The spot sign has proven to be a dynamic entity, with multimodal imaging proving to be of additional value. We investigated whether the addition of a 90-second delayed CTA acquisition would capture additional intracerebral hemorrhage patients with the spot sign and increase the sensitivity of the spot sign. Methods— We prospectively enrolled consecutive intracerebral hemorrhage patients undergoing first pass and 90-second delayed CTA for 18 months at a single academic center. Univariate and multivariate logistic regression were performed to assess clinical and neuroimaging covariates for relationship with hematoma expansion and mortality. Results— Sensitivity of the spot sign for hematoma expansion on first pass CTA was 55%, which increased to 64% if the spot sign was present on either CTA acquisition. In multivariate analysis the spot sign presence was associated with significant hematoma expansion: odds ratio, 17.7 (95% confidence interval, 3.7–84.2; P=0.0004), 8.3 (95% confidence interval, 2.0–33.4; P=0.004), and 12.0 (95% confidence interval, 2.9–50.5; P=0.0008) if present on first pass, delayed, or either CTA acquisition, respectively. Spot sign presence on either acquisitions was also significant for mortality. Conclusions— We demonstrate improved sensitivity for predicting hematoma expansion and poor outcome by adding a 90-second delayed CTA, which may enhance selection of patients who may benefit from hemostatic therapy.