Ahmed E. Othman

Radiation dose reduction in perfusion CT imaging of the brain: A review of the literature

Perfusion CT (PCT) of the brain is widely used in the settings of acute ischemic stroke and vasospasm monitoring. The high radiation dose associated with PCT is a central topic and has been a focus of interest for many researchers. Many studies have examined the effect of radiation dose reduction in PCT using different approaches. Reduction of tube current and tube voltage can be efficient and lead to a remarkable reduction of effective radiation dose while preserving acceptable image quality. The use of novel noise reduction techniques such as iterative reconstruction or spatiotemporal smoothing can produce sufficient image quality from low-dose perfusion protocols. Reduction of sampling frequency of perfusion images has only little potential to reduce radiation dose. In the present article we aimed to summarize the available data on radiation dose reduction in PCT imaging of the brain.

Effect of Noise-Optimized Monoenergetic Postprocessing on Diagnostic Accuracy for Detecting Incidental Pulmonary Embolism in Portal-Venous Phase Dual-Energy Computed Tomography.

Objectives The aim of this study was to evaluate the diagnostic accuracy of virtual monoenergetic images (MEI+) at low kiloelectronvolt levels for the detection of incidental pulmonary embolism (PE) in oncological follow-up portal-venous phase dual-energy (DE-CTpv) staging. Materials and Methods Twenty-six patients with incidental PE in DE-CTpv staging, which was confirmed by computed tomography pulmonary angiography (CTPA), were included in this study. In addition, 26 matched control patients who were negative for PE in both DE-CTpv and CTPA were included. All examinations were performed on a third-generation DE-CT system in single-energy (CTPA) and dual-energy mode (DE-CTpv). Subsequently, 2 series of MEI+ data sets were reconstructed at 40 and 55 keV from the DE-CTpv data and compared with CTPA and the linearly blended CTpv images. Diagnostic accuracy and diagnostic confidence were calculated and compared for MEI+ reconstructions and for CTpv images regarding the detection of PE with CTPA as standard of reference. Results In 3 patients, PE was only detectable in CTPA and in the 40-kV and 55-kV MEI+ reconstructions but not in CTpv images. Diagnostic accuracy increased significantly for both MEI+ series (40 keV; area under the curve [AUC], 0.928; 95% confidence interval [CI], 0.879–0.978; 55 keV; AUC, 0.960; 95% CI, 0.922–0.998) as compared with CTpv (AUC, 0.814; 95% CI, 0.741–0.887; P ⩽ 0.004). Diagnostic confidence was rated highest in CTPA (median, 5; range, 3–5) followed by 55-keV MEI+ and 40-keV MEI+ (median, 5; range, 2–5 and median, 5; range, 2–5, respectively) and was lowest for CTpv (median, 4; range, 1–5), with significant differences to CTPA and MEI+ reconstructions (P ⩽ 0.001). Conclusions Monoenergetic image reconstructions from DE-CT data sets at low kiloelectronvolt levels improve diagnostic accuracy for the detection of incidental PE in oncological follow-up DE-CTpv staging, with the highest subjective diagnostic confidence in MEI+ at 55 keV.

Optimized Fast Dynamic Contrast-Enhanced Magnetic Resonance Imaging of the Prostate: Effect of Sampling Duration on Pharmacokinetic Parameters.

ObjectiveThe aim of this study was to evaluate the effect of sampling duration on pharmacokinetic parameters from dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) and their diagnostic accuracy regarding the detection of potentially malignant prostate lesions. Materials and MethodsSixty-six consecutive male patients (mean [SD] age, 65.4 [10.8] years) with clinically suspected prostate cancer were included. All patients underwent multiparametric MRI of the prostate (T2-weighted imaging, diffusion-weighted imaging, and DCE-MRI) on a 3 T MRI scanner. Patients were divided into 2 groups depending on the prostate imaging reporting and data system (PI-RADS) score of the detected lesions (group A: PI-RADS ⩽3, n = 32; group B: PI-RADS >3, n = 34). In all patients, DCE-MRI was performed using a CAIPIRINHA-Dixon-TWIST volume interpolated breath-hold examination sequence (spatial resolution, 3 × 1.2 × 1.2 mm; temporal resolution, 5 seconds; total sampling duration, 4:10 minutes [250 seconds]) with body weight–adapted administration of contrast agent (gadobutrol, Bayer Healthcare, Berlin, Germany). Five DCE-MRI series with different acquisition durations ranging from 50 to 250 seconds were retrospectively generated from the original data sets. Pharmacokinetic parameters (ie, Ktrans, Kep, Ve, and iAUC60) were calculated for the different sampling durations using the Tofts model. Both lesion groups and all 5 DCE-MRI series were compared regarding pharmacokinetic parameters. Diagnostic accuracy for the detection of potentially malignant lesions was calculated for all 5 series using receiver operating characteristic analysis. ResultsFor all 5 series, Ktrans, Kep, and iAUC60 in patient group B were significantly higher than the respective parameters in patient group A (all P ⩽ 0.008). In both groups, Ktrans, Kep, and iAUC60 remained constant at 200 and 150 seconds acquisition duration and did not significantly differ from parameters estimated from the original data sets (250 seconds; all P ≥ 0.310). Ve did not differ significantly between the 2 groups (P ≥ 0.337) and acquisition time did not have a significant effect on this parameter (P ≥ 0.275). Receiver operating characteristic analyses showed consistent diagnostic accuracy for the different series; only diagnostic accuracy of Kep decreased with lowered sampling duration, showing lowest accuracy for the 50-second series (0.682; 95% confidence interval, 0.553–0.811). ConclusionsUsing fast optimized DCE-MRI of the prostate, a minimum sampling duration of 150 seconds is required for sufficient pharmacokinetic parameter estimates, providing a high diagnostic accuracy regarding the discrimination between benign and potentially malignant lesions.

Effect of Temporal Resolution on Diagnostic Performance of Dynamic Contrast-Enhanced Magnetic Resonance Imaging of the Prostate.

ObjectiveThe aim of this study was to assess the effect of temporal resolution on semiquantitative and pharmacokinetic parameters from dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) and their diagnostic accuracy regarding the detection of potentially malignant prostate lesions. Materials and MethodsSixty consecutive male patients (age, 64.5 ± 7.0 years) with clinically suspected prostate cancer were included. All patients underwent multiparametric MRI of the prostate (T2-weighted, diffusion-weighted imaging, and DCE-MRI) on a 3 T MRI scanner. Patients were divided into 2 groups depending on Prostate Imaging Reporting and Data System (PI-RADS) score of the detected lesions (group A: PI-RADS score ⩽3, n = 30; group B: PI-RADS score >3, n = 30). In all patients, DCE-MRI was performed using a CAIPIRINHA-Dixon-TWIST Volume-Interpolated Breath-Hold Examination sequence (spatial resolution, 3 × 1.2 × 1.2 mm; temporal resolution, 5 seconds; total sampling duration, 4:10 minutes [250 seconds]) with body weight–adapted administration of contrast agent (gadobutrol, Gadovist; Bayer Healthcare, Berlin, Germany). Six DCE-MRI series with different temporal resolutions ranging from 5 to 30 seconds per time point were retrospectively generated from the original data sets. Semiquantitative parameters (ie, wash-in, wash-out, and time-to-peak [TTP]) as well as pharmacokinetic parameters (ie, Ktrans, Kep, and ve) were calculated for the different temporal resolutions. Both lesion groups and all 6 DCE-MRI series were compared regarding semiquantitative and pharmacokinetic parameters. Diagnostic accuracy for the detection of potentially malignant lesions was calculated for all 6 series using ROC analysis. ResultsA significant effect of temporal resolution was found on wash-in (P < 0.001). Series with temporal resolution lower than 10 s/time point showed significantly lower wash-in values with more pronounced effects in group B compared with group A. For 30-second series, the differences between both groups diminished reaching insignificant levels (P = 0.052), resulting in a significant decrease of the diagnostic accuracy of wash-in (area under the curve, 0.609; 95% confidence interval, 0.451–0.766; P < 0.015). No significant effects were detected on wash-out. For TTP, a significant effect of temporal resolution was detected (P < 0.001) with significantly increasing TTP levels for all down-sampled series compared with the original 5-second series. These effects did not impact the diagnostic accuracy of TTP. No significant effects of temporal resolution were detected on pharmacokinetic parameters (P < 0.112). ConclusionsIn DCE-MRI of the prostate, temporal resolution affects the diagnostic performance of semiquantitative parameters. For a sufficient detection of malignant prostate lesions on DCE-MRI, a temporal resolution of at least 10 s/time point or higher is recommended.

Comparison of different population-averaged arterial-input-functions in dynamic contrast-enhanced MRI of the prostate: Effects on pharmacokinetic parameters and their diagnostic performance.

PURPOSE To assess the effect of different population-averaged arterial-input-functions (pAIF) on pharmacokinetic parameters from dynamic contrast-enhanced MRI (DCE-MRI) and their diagnostic accuracy regarding the detection of potentially malignant prostate lesions. MATERIALS AND METHODS 66 male patients (age 65.4±10.8y) with suspected prostate cancer underwent multiparametric MRI of the prostate including T2-w, DWI-w and DCE-MRI sequences at a 3T MRI scanner. All detected lesions were categorized based on ACR PI-RADS version 2 and divided into 2 groups (A: PI-RADS ≤3, n=32; B: PI-RADS >3, n=34). In each DCE-MRI dataset, pharmacokinetic parameters (Ktrans, Kep and ve) and goodness of fit (chi(2)) were generated using the Tofts model with 3 different pAIFs (fast, intermediate, slow) as provided by a commercially available postprocessing software. Pharmacokinetic parameters, their diagnostic accuracies and model fits were compared for the 3 pAIFs. RESULTS Ktrans, Kep and ve differed significantly among the 3 pAIFs (all p<.001). Ktrans and Kep were significantly higher in group B compared to group A (all p<.001). For chi(2), lowest results (representing highest goodness of fit) were found for intermediate pAIF (chi(2) 0.073). ROC analyses revealed comparable diagnostic accuracies for the different pAIFs, which were high for Ktrans and Kep and low for ve. CONCLUSION Choosing various pAIF types causes a high variability in pharmacokinetic parameter estimates. Therefore, it is of great importance to consider this as potential artifact and thus keep AIF type selection constant in DCE-MRI studies.

Metal artifact reduction for flat panel detector intravenous CT angiography in patients with intracranial metallic implants after endovascular and surgical treatment

Background Flat panel detector CT angiography with intravenous contrast agent injection (IV CTA) allows high-resolution imaging of cerebrovascular structures. Artifacts caused by metallic implants like platinum coils or clips lead to degradation of image quality and are a significant problem. Objective To evaluate the influence of a prototype metal artifact reduction (MAR) algorithm on image quality in patients with intracranial metallic implants. Methods Flat panel detector CT after intravenous application of 80 mL contrast agent was performed with an angiography system (Artis zee; Siemens, Forchheim, Germany) using a 20 s rotation protocol (200° rotation angle, 20 s acquisition time, 496 projections). The data before and after MAR of 26 patients with a total of 34 implants (coils, clips, stents) were independently evaluated by two blinded neuroradiologists. Results MAR improved the assessability of the brain parenchyma and small vessels (diameter <1 mm) in the neighborhood of metallic implants and at a distance of 6 cm (p<0.001 each, Wilcoxon test). Furthermore, MAR significantly improved the assessability of parent vessel patency and potential aneurysm remnants (p<0.005 each, McNemar test). MAR, however, did not improve assessability of stented vessels. Conclusions When an intravenous contrast protocol is used, MAR significantly ameliorates the assessability of brain parenchyma, vessels, and treated aneurysms in patients with intracranial coils or clips.

CT imaging of bone and bone marrow infiltration in malignant melanoma—Challenges and limitations for clinical staging in comparison to 18FDG-PET/CT

Rationale of this study was the evaluation of the diagnostic value of computed tomography (CT) in the detection of bone marrow infiltration in comparison to PET/CT. Fifty patients (age 61 ± 15.12 years) with metastatic malignant melanoma underwent 18F-FDG-PET/CT, including contrast-enhanced CT. 2 readers evaluated the CT images in consensus for bone and bone marrow lesions focusing on lesion location, type and size. PET/CT was used as reference standard to estimate sensitivity, specificity, negative and positive predictive value. Moreover, the bone marrow density was estimated in the long bones and the sacral bone. Serum hamoglobin, thrombocyte level and S100 protein were correlated with the presence or absence of bone and bone marrow lesions. According to PET/CT as standard of reference, of 594 bone and medullary lesions 495 were considered malignant. Of these 77.8% were medullary, 20.4% lytic, 1% sclerotic and 0.8% mixed lytic/sclerotic. Contrast-enhanced CT yielded a lesion-based sensitivity of 36.8% and a specificity of 87.9% (PPV 93.8%; NPV 21.8%). Patient-based sensitivity and specificity were 78.8% and 82.4%, respectively. Of the missed lesions, most were medullary (95.8%). A disseminated bone marrow involvement (defined as >10 bone marrow lesions or diffuse infiltration of a whole body segment) was described in 11 cases, in 6 cases the disseminated involvement was underestimated or missed on CT. In cases with disseminated bone marrow involvement the bone marrow density was significantly higher in the humerus (p=0.04), but not in the femur or sacral bone (p=0.06). Multivariate analysis revealed no isolated effect of bone metastases on S100 serum and hemoglobin level, but both were significantly altered in patients with disseminated bone marrow involvement (p<0.05). In conclusion, the diagnostic value of computed tomography for the detection of bone marrow metastases in patients with melanoma, is limited. Especially in cases with disseminated bone marrow involvement about 50% of the cases were missed or underestimated.

Clinical Impact of Ventilation Duration in Patients with Stroke Undergoing Interventional Treatment under General Anesthesia: The Shorter the Better?

The authors investigated the impact of early extubation and ventilation duration in a cohort of 103 patients that underwent thrombectomy under general anesthesia. Prolonged ventilation was associated with pneumonia during hospitalization and unfavorable functional outcome (mRS greater than or equal to 3) and death at follow-up. According to ROC analysis, a cutoff after 24 hours predicted unfavorable functional outcome with a sensitivity and specificity of 60% and 78%, respectively. The authors conclude that short ventilation times are associated with a lower pneumonia rate and more favorable clinical outcome. BACKGROUND AND PURPOSE: Whether general anesthesia for neurothrombectomy in patients with ischemic stroke has a negative impact on clinical outcome is currently under discussion. We investigated the impact of early extubation and ventilation duration in a cohort that underwent thrombectomy under general anesthesia. MATERIALS AND METHODS: We analyzed 103 consecutive patients from a prospective stroke registry. They met the following criteria: CTA-proved large-vessel occlusion in the anterior circulation, ASPECTS above 6 on presenting cranial CT, revascularization by thrombectomy with the patient under general anesthesia within 6 hours after onset of symptoms, and available functional outcome (mRS) 90 days after onset. RESULTS: The mean ventilation time was 128.07 ± 265.51 hours (median, 18.5 hours; range, 1–1244.7 hours). Prolonged ventilation was associated with pneumonia during hospitalization and unfavorable functional outcome (mRS ≥3) and death at follow-up (Mann-Whitney U test; P ≤ .001). According to receiver operating characteristic analysis, a cutoff after 24 hours predicted unfavorable functional outcome with a sensitivity and specificity of 60% and 78%, respectively. Our results imply that delayed extubation was not associated with a less favorable clinical outcome compared with immediate extubation after the procedure. CONCLUSIONS: Short ventilation times are associated with a lower pneumonia rate and more favorable clinical outcome. Cautious interpretation of our data implies that whether patients are extubated immediately after the procedure is irrelevant for clinical outcome as long as ventilation does not exceed 24 hours.

Feasibility of CAIPIRINHA-Dixon-TWIST-VIBE for dynamic contrast-enhanced MRI of the prostate.

PURPOSE To evaluate the feasibility of a CAIPIRINHA-Dixon-TWIST (CDT)-VIBE sequence for improving image quality and temporal resolution in dynamic contrast-enhanced MRI (DCE-MRI) of the prostate. MATERIAL AND METHODS 44 male patients (age 63.9 ± 8.9 years) with clinically suspected prostate cancer underwent DCE-MRI at a 3T MRI scanner (Magnetom Skyra, Siemens Healthcare, Erlangen, Germany) using a CDT-VIBE sequence (spatial resolution=3×1.2×1.2mm(3), temporal resolution=5s, total scan duration=4:10 min) with body-weight-adapted administration of contrast agent (Gadobutrol, Bayer Healthcare, Berlin, Germany). To investigate effects on image quality, the same sequence was acquired three times per patient during the late phase: 1. with the same protocol as in the arterial phase (VS5), 2. without view-sharing (no view-sharing, NVS) using a 2-fold CAIPIRINHA acceleration R=2 (temporal resolution=15s, NVS15) and 3. NVS using a 6-fold CAIPIRINHA acceleration R=6 (temporal resolution=5s, NVS5). SNR and CNR were evaluated with the subtraction method. Image quality of the three sequences (VS5, NVS15, NVS5) was subjectively assessed by 2 blinded radiologists using a 5-point Likert scale (5 being excellent). Perfusion profiles of visually normal prostate and of malignant lesions as characterized by Wash-In, Wash-Out, time-to-peak (TTP) and initial area under the curve (iAUC) from the original datasets (temporal resolution=5s) and from datasets with a downsampled temporal resolution (15s) were compared. RESULTS In 20 of 44 included patients, potentially malignant lesions were identified in which 16 had histologically confirmed prostate cancer. SNR was highest for VS5 and NVS15, and lowest for NVS5 (p <.001). Concordantly, subjective image quality was comparable for VS5 and NVS15 (sum score 23.20 ± 1.03 vs 23.53 ± 1.34) and significantly lower for NVS5 (sum score 9.83 ± 2.32; p<.001). Perfusion parameters of macroscopically normal prostate tissue and suspect lesions differed significantly between original datasets and datasets with simulated lower temporal resolution, with the latter showing higher Wash-In (p=.002), lower Wash-Out (p=.003), higher Time-to-Peak (p<.001) and lower iAUC (p<.001). CONCLUSION CDT-VIBE can be readily exploited for DCE-MRI of the prostate preserving the diagnostic image quality while providing high temporal resolution for quantitative diagnostic assessment of enhancement curves in malignant lesions.

Monoenergetic Dual-energy Computed Tomographic Imaging: Cardiothoracic Applications

Monoenergetic imaging is an increasingly used reconstruction technique in postprocessing of dual-energy computed tomography (DECT). The main advantage of this technique is the ability to substantially increase image contrast of structures with uptake of iodinated contrast material. Although monoenergetic imaging was mainly used in oncological DECT applications, recent research has further demonstrated its role in vascular imaging. Using this dedicated postprocessing algorithm, image contrast of vascular structures in the thorax can be increased, a drastic reduction of contrast material is feasible, and even beam-hardening artifacts can be reduced. The aim of this review article is to explain the technical background of this technique, showcase its relevance in cardiothoracic DECT, and provide an outlook on the clinical impact of this technique beyond solely improvements in image quality.