Iterative Reconstruction Improves Both Objective and Subjective Image Quality in Acute Stroke CTP

<div><p>Purpose</p><p>Computed tomography perfusion (CTP) imaging in acute ischemic stroke (AIS) suffers from measurement errors due to image noise. The purpose of this study was to investigate if iterative reconstruction (IR) algorithms can be used to improve the diagnostic value of standard-dose CTP in AIS.</p><p>Methods</p><p>Twenty-three patients with AIS underwent CTP with standardized protocol and dose. Raw data were reconstructed with filtered back projection (FBP) and IR with intensity levels 3, 4, 5. Image quality was objectively (quantitative perfusion values, signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR)) and subjectively (overall image quality) assessed. Ischemic core and perfusion mismatch were visually rated. Discriminative power for tissue outcome prediction was determined by the area under the receiver operating characteristic curve (AUC) resulting from the overlap between follow-up infarct lesions and stepwise thresholded CTP maps.</p><p>Results</p><p>With increasing levels of IR, objective image quality (SNR and CNR in white matter and gray matter, elimination of error voxels) and subjective image quality improved. Using IR, mean transit time (MTT) was higher in ischemic lesions, while there was no significant change of cerebral blood volume (CBV) and cerebral blood flow (CBF). Visual assessments of perfusion mismatch changed in 4 patients, while the ischemic core remained constant in all cases. Discriminative power for infarct prediction as represented by AUC was not significantly changed in CBV, but increased in CBF and MTT (mean (95% CI)): 0.72 (0.67–0.76) vs. 0.74 (0.70–0.78) and 0.65 (0.62–0.67) vs 0.67 (0.64–0.70).</p><p>Conclusion</p><p>In acute stroke patients, IR improves objective and subjective image quality when applied to standard-dose CTP. This adds to the overall confidence of CTP in acute stroke triage.</p></div

Baseline characteristics and therapy of 23 patients.

<p>Baseline characteristics and therapy of 23 patients.</p

Mean perfusion parameter values.

<p>Mean perfusion parameter values.</p

Power of perfusion maps to predict infarction represented by AUC.

<p>Predictive power for CBV, CBF and MTT map across three different IR levels and FPB. CBV showed no significant changes. CBF and MTT predictive power changed statistically significant between FBP and IR, with relative differences < 4%. (* = p<0.05).</p

MTT map, number of error voxels.

<p>The amount of error voxels (i.e. voxels without perfusion information) was significantly reduced between FBP and IR level 5. (* p = 0.004).</p

Example CTP images 2.

<p>In this case, the clinical decision differed from FBP (no relevant perfusion mismatch) to IR level 3–5 (visible relevant perfusion mismatch). While there was no change of the ischemic core visible in CBV, accentuated ischemic lesions became visible in CBF and MTT with increasing IR levels. In 4 of 23 cases, there was a change in the assessment of a perfusion mismatch.</p

Number of patients rated with a clinically relevant perfusion mismatch.

<p>Number of patients rated with a clinically relevant perfusion mismatch (> 20% mismatch between CBV and MTT lesion). Using FBP, 13 patients were rated positively. This number increased to 17 using IR level 5. The difference was not statistically significant.</p

Example CTP images 1.

<p>Note the improved image quality from FBP to IR levels 3–5 (rated 1.3, 2.0, 3.6 and 3.0, respectively). With rising levels of IR, error voxels (i.e. voxels without perfusion information, white) diminished in favor of voxels with perfusion information (colored). For this patient, as for 19 out of 23 patients, the assessment of a relevant perfusion mismatch did not depend on the reconstruction algorithm.</p

Feasibility of Quantification of Intracranial Aneurysm Pulsation with 4D CTA with Manual and Computer-Aided Post-Processing

<div><p>Background and Purpose</p><p>The analysis of the pulsation of unruptured intracranial aneurysms might improve the assessment of their stability and risk of rupture. Pulsations can easily be concealed due to the small movements of the aneurysm wall, making post-processing highly demanding. We hypothesized that the quantification of aneurysm pulsation is technically feasible and can be improved by computer-aided post-processing.</p><p>Materials and Methods</p><p>Images of 14 cerebral aneurysms were acquired with an ECG-triggered 4D CTA. Aneurysms were post-processed manually and computer-aided on a 3D model. Volume curves and random noise-curves were compared with the arterial pulse wave and volume curves were compared between both post-processing modalities.</p><p>Results</p><p>The aneurysm volume curves showed higher similarity with the pulse wave than the random curves (Hausdorff-distances 0.12 vs 0.25, p<0.01). Both post-processing methods did not differ in intra- (r = 0.45 vs r = 0.54, p>0.05) and inter-observer (r = 0.45 vs r = 0.54, p>0.05) reliability. Time needed for segmentation was significantly reduced in the computer-aided group (3.9 ± 1.8 min vs 20.8 ± 7.8 min, p<0.01).</p><p>Conclusion</p><p>Our results show pulsatile changes in a subset of the studied aneurysms with the final prove of underlying volume changes remaining unsettled. Semi-automatic post-processing significantly reduces post-processing time but cannot yet replace manual segmentation.</p></div

Boxplots of Hausdorff distances between the volume curves and random curves and the pulse wave on the right and left-hand side (0.12 vs 0.25, p<0.01).

<p>Boxplots of Hausdorff distances between the volume curves and random curves and the pulse wave on the right and left-hand side (0.12 vs 0.25, p<0.01).</p