I.C. van der Schaaf

Level set based cerebral vasculature segmentation and diameter quantification in CT angiography

A level set based method is presented for cerebral vascular tree segmentation from computed tomography angiography (CTA) data. The method starts with bone masking by registering a contrast enhanced scan with a low-dose mask scan in which the bone has been segmented. Then an estimate of the background and vessel intensity distributions is made based on the intensity histogram which is used to steer the level set to capture the vessel boundaries. The relevant parameters of the level set evolution are optimized using a training set. The method is validated by a diameter quantification study which is carried out on phantom data, representing ground truth, and 10 patient data sets. The results are compared to manually obtained measurements by two expert observers. In the phantom study, the method achieves similar accuracy as the observers, but is unbiased whereas the observers are biased, i.e., the results are 0.00+/-0.23 vs. -0.32+/-0.23 mm. Also, the methods reproducibility is slightly better than the inter-and intra-observer variability. In the patient study, the method is in agreement with the observers and also, the methods reproducibility -0.04+/-0.17 mm is similar to the inter-observer variability 0.06+/-0.17 mm. Since the method achieves comparable accuracy and reproducibility as the observers, and since the method achieves better performance than the observers with respect to ground truth, we conclude that the level set based vessel segmentation is a promising method for automated and accurate CTA diameter quantification.

CT after subarachnoid hemorrhage Relation of cerebral perfusion to delayed cerebral ischemia

Background: Delayed cerebral ischemia (DCI) after subarachnoid hemorrhage (SAH) is difficult to predict. The authors studied the relation between several parameters of brain perfusion at admission and development of DCI. Methods: The authors analyzed the admission CT perfusion (CTP) scans of 46 patients scanned within 72 hours after SAH. They assessed cerebral blood volume (CBV) and flow (CBF), mean transit time (MTT), and time to peak (TTP) for eight predefined regions of interest. For patients with and without DCI, the authors compared perfusion quantitatively and semiquantitatively. With receiver-operator characteristic (ROC) curves, the authors assessed the relationship between DCI and perfusion parameters. To assess the potential prognostic value, they calculated sensitivity and specificity of optimal threshold values for the semiquantitative data. Results: DCI was not significantly related with quantitative perfusion values. For the semiquantitative data, patients with DCI had significantly more asymmetry in perfusion, and ROC curves indicated a good relation (0.75 to 0.81). Optimal threshold values distinguishing between patients with and without DCI were 0.77 for CBV and 0.72 for CBF ratios, and 0.87 seconds for MTT and 1.0 second for TTP differences. The corresponding sensitivity was 0.75 for all parameters; the specificity was 0.70 for CBV, 0.93 for CBF, 0.70 for MTT, and 0.90 for TTP. Conclusions: Delayed cerebral ischemia (DCI) is related to perfusion asymmetry on admission CT perfusion (CTP). The cerebral blood flow ratio (comparing contralateral regions of interest) seems the best prognosticator for development of DCI. Further studies are needed to investigate the additional value of CTP to other prognosticators for DCI and to validate the chosen threshold values.

New Detected Aneurysms on Follow-Up Screening in Patients With Previously Clipped Intracranial Aneurysms Comparison With DSA or CTA at the Time of SAH

Background and Purpose— Patients with a history of aneurysmal subarachnoid hemorrhage may have aneurysms on screening several years after the hemorrhage. For determining the benefits of follow-up screening, it is important to know whether these aneurysms have developed after the hemorrhage or are visible in retrospect, and if so, whether the size has increased. Methods— Aneurysms were categorized into de novo aneurysms and aneurysms visible in retrospect (already present) with increased or stable size. We studied aneurysm characteristics for these 3 categories: the relation between aneurysm development or enlargement and duration of follow up and the relation between enlargement and initial size of the aneurysm. Results— In 87 of 495 patients (17.6%), aneurysms were detected; for 51 of these patients with 62 aneurysms, the original catheter or computed tomographic angiogram was available for comparison. Of the 62 aneurysms, 19 were de novo and 43 were visible in retrospect, 10 with increased size and 33 with stable size. De novo aneurysms were mainly ≤5 mm (95%) and located at the middle cerebral artery (63%). For aneurysms visible in retrospect, the most frequent location was the posterior communicating artery (21%). There was no relation between the development of de novo aneurysms or enlargement and the duration of follow-up or between enlargement and the initial size of the aneurysm. Conclusions— Of aneurysms detected at screening, one third were de novo and two thirds were missed at the time of the initial hemorrhage. One quarter of initially small aneurysms had enlarged during follow-up.

Dynamic Perfusion CT Assessment of the Blood-Brain Barrier Permeability: First Pass versus Delayed Acquisition

BACKGROUND AND PURPOSE: The Patlak model has been applied to first-pass perfusion CT (PCT) data to extract information on blood-brain barrier permeability (BBBP) to predict hemorrhagic transformation in patients with acute stroke. However, the Patlak model was originally described for the delayed steady-state phase of contrast circulation. The goal of this study was to assess whether the first pass or the delayed phase of a contrast bolus injection better respects the assumptions of the Patlak model for the assessment of BBBP in patients with acute stroke by using PCT. MATERIALS AND METHODS: We retrospectively identified 125 consecutive patients (29 with acute hemispheric stroke and 96 without) who underwent a PCT study by using a prolonged acquisition time up to 3 minutes. The Patlak model was applied to calculate BBBP in ischemic and nonischemic brain tissue. Linear regression of the Patlak plot was performed separately for the first pass and for the delayed phase of the contrast bolus injection. Patlak linear regression models for the first pass and the delayed phase were compared in terms of their respective square root mean squared errors (√MSE) and correlation coefficients (R) by using generalized estimating equations with robust variance estimation. RESULTS: BBBP values calculated from the first pass were significantly higher than those from the delayed phase, both in nonischemic brain tissue (2.81 mL × 100 g−1 × min−1 for the first pass versus 1.05 mL × 100 g−1 × min−1 for the delayed phase, P < .001) and in ischemic tissue (7.63 mL × 100 g−1 × min−1 for the first pass versus 1.31 mL × 100 g−1 × min−1 for the delayed phase, P < .001). Compared with regression models from the first pass, Patlak regression models obtained from the delayed data were of better quality, showing significantly lower √MSE and higher R. CONCLUSION: Only the delayed phase of PCT acquisition respects the assumptions of linearity of the Patlak model in patients with and without stroke.

Timing-Invariant Reconstruction for Deriving High-Quality CT Angiographic Data from Cerebral CT Perfusion Data

PURPOSE To suggest a simple and robust technique used to reconstruct high-quality computed tomographic (CT) angiographic images from CT perfusion data and to compare it with currently used CT angiography techniques. MATERIALS AND METHODS Institutional review board approval was waived for this retrospective study, which included 25 consecutive patients who had had a stroke. Temporal maximum intensity projection (tMIP) CT angiographic images were created by using prior temporal filtering as a timing-insensitive technique to produce CT angiographic images from CT perfusion data. The temporal filter strength was optimized to gain maximal contrast-to-noise ratios (CNRs) in the circle of Willis. The resulting timing-invariant (TI) CT angiography was compared with standard helical CT angiography, the arterial phase of dynamic CT angiography, and nonfiltered tMIP CT angiography. Vascular contrast, image noise, and CNR were measured. Four experienced observers scored all images for vascular noise, vascular contour, detail of small and medium arteries, venous superimposition, and overall image quality in a blinded side-by-side comparison. Measurements were compared with a paired t test; P ≤ .05 indicated a significant difference. RESULTS On average, optimized temporal filtering in TI CT angiography increased CNR by 18% and decreased image noise by 18% at the expense of a decrease in vascular contrast of 3% when compared with nonfiltered tMIP CT angiography. CNR, image noise, vascular noise, vascular contour, detail visibility of small and medium arteries, and overall image quality of TI CT angiograms were superior to those of standard CT angiography, tMIP CT angiography, and the arterial phase of dynamic CT angiography at a vascular contrast that was similar to that of standard CT angiography. Venous superimposition was similar for all techniques. Image quality of the arterial phase of dynamic CT angiography was rated inferior to that of standard CT angiography. CONCLUSION TI CT angiographic images constructed by using temporally filtered tMIP CT angiographic data have excellent image quality that is superior to that achieved with currently used techniques, but they suffer from modest venous superimposition.

Psychosocial Impact of Screening for Intracranial Aneurysms in Relatives With Familial Subarachnoid Hemorrhage

Background and Purpose— In families with ≥2 relatives with intracranial aneurysms (IAs), screening for IAs in asymptomatic first-degree relatives is often recommended. We assessed the long-term psychosocial impact of such screening. Methods— We identified all persons with IA (screen-positives) and matched them for age and sex with 2 controls without IA (screen-negatives) from hospital-based registers of familial IA. Persons underwent telephone interviews using questionnaires that covered the areas of psychosocial impact of screening, health-related quality of life (HRQoL), and mood. Data were compared between screen-positives and screen-negatives, and with reference populations. Results— Overall, 105 persons from 33 families with IA were included, of whom 35 were screen-positive and 70 were screen-negative. Of the screen-positives, 12 (44%) had reduced their work and 23 (66%) had experienced changes in ≥1 area of independence, self-esteem, future outlook, or personal relationships. In contrast, only 1 (2%) screen-negative person had stopped working and 12 (17%) others had experienced changes in their self-esteem, future outlook, or relationships. Screen-positives had lower HRQoL compared with screen-negatives and the reference population, whereas both screen groups had higher mean depression scores than the reference population. Despite these effects, only 3 persons regretted participating in screening. Conclusion— Although screening for IA is an important preventative strategy in high-risk individuals, it is associated with considerable psychosocial effects, both positive and negative. Greater awareness of such outcomes, and appropriate intervention where necessary, would appear to be a necessary component of IA screening programs.

Yield of screening for new aneurysms after treatment for subarachnoid hemorrhage

Objective: Patients who have been successfully treated for subarachnoid hemorrhage (SAH) are at risk for new episodes. The authors studied the effect of screening with CT angiography (CTA) for new aneurysms. Methods: In a decision model, the authors compared the strategies “screening” and “no screening” after SAH. A literature review yielded the risks of aneurysm recurrence, complications of CTA, and re-treatment. The authors estimated the expected number of quality-adjusted life-years (QALYs), the number of SAH, and the mortality and disability rates for both strategies. They evaluated screening at intervals of 2, 5, and 10 years after SAH, using 10 years and remaining life expectancy as time horizon. Results: The expected number of QALYs 10 years after clipping was virtually the same for no screening (8.33), screening once after 5 years (8.28), and screening every 2 years (8.27). With screening every 2 years, the expected rate of new SAH decreased from 1.9 to 0.5%, and mortality decreased from 0.9 to 0.6%; however, the disability rate increased from 0.5 to 1.9%. Results were comparable with remaining life expectancy as time horizon and for screening after initial treatment with coils. The key estimates of the analyses were the incidence and rupture rate of new aneurysms, the risk of dying from recurrent SAH, the utility of disability, and the risk of complications from DSA and re-treatment. Conclusions: Presently, screening for new aneurysms after subarachnoid hemorrhage cannot be recommended. Screening may prevent new episodes of subarachnoid hemorrhage but with too high a cost in terms of complications from preventive treatment.

Acute Hydrocephalus and Cerebral Perfusion after Aneurysmal Subarachnoid Hemorrhage

BACKGROUND AND PURPOSE: Acute hydrocephalus after aneurysmal subarachnoid hemorrhage (SAH) may decrease cerebral perfusion by increasing intracranial pressure. We studied cerebral perfusion in patients with and without acute hydrocephalus after SAH. MATERIALS AND METHODS: We performed noncontrast CT scans, CT perfusion (CTP), and CT angiography on admission in all patients with aneurysmal SAH. Patients were dichotomized at a relative bicaudate index of 1 for the presence or absence of hydrocephalus. Cerebral perfusion was measured in the cortex, basal ganglia, and periventricular white matter. Mean CTP parameters were compared between patients with and without acute hydrocephalus (ie, within 3 days after SAH). RESULTS: We included 138 consecutive patients with successful CTP measurements, of whom 49 (36%) had acute hydrocephalus. Mean cerebral blood flow (CBF) was lower in patients with hydrocephalus than in those without in the basal ganglia (difference of means, 6.8; 95% CI, 1.6–11.0 mL/100 g/min) and periventricular white matter (difference of means, 3.8; 95% CI, 0.9–6.8 mL/100 g/min) but not in the cortex (difference of means, 1.8; 95% CI, −2.8 to 6.4 mL/100 g/min). In all regions studied, mean transit time (MTT) and time-to-peak (TTP) were statistically significantly longer in patients with hydrocephalus, but cerebral blood volume (CBV) values were similar. CONCLUSIONS: Acute hydrocephalus after SAH reduces CBF in the deep gray matter and periventricular white matter and delays MTT and TTP in all investigated brain areas. The negative effect of acute hydrocephalus on cerebral perfusion in patients with SAH seems more pronounced in the vicinity of the ventricles than in remote sites.

Interpretation of radiological images: towards a framework of knowledge and skills

The knowledge and skills that are required for radiological image interpretation are not well documented, even though medical imaging is gaining importance. This study aims to develop a comprehensive framework of knowledge and skills, required for two-dimensional and multiplanar image interpretation in radiology. A mixed-method study approach was applied. First, a literature search was performed to identify knowledge and skills that are important for image interpretation. Three databases, PubMed, PsycINFO and Embase, were searched for studies using synonyms of image interpretation skills or visual expertise combined with synonyms of radiology. Empirical or review studies concerning knowledge and skills for medical image interpretation were included and relevant knowledge and skill items were extracted. Second, a preliminary framework was built and discussed with nine selective experts in individual semi-structured interviews. The expert team consisted of four radiologists, one radiology resident, two education scientists, one cognitive psychologist and one neuropsychologist. The framework was optimised based on the experts comments. Finally, the framework was applied to empirical data, derived from verbal protocols of ten clerks interpreting two-dimensional and multiplanar radiological images. In consensus meetings adjustments were made to resolve discrepancies of the framework with the verbal protocol data. We designed a framework with three main components of image interpretation: perception, analysis and synthesis. The literature study provided four knowledge and twelve skill items. As a result of the expert interviews, one skill item was added and formulations of existing items were adjusted. The think-aloud experiment showed that all knowledge items and three of the skill items were applied within all three main components of the image interpretation process. The remaining framework items were apparent only within one of the main components. After combining two knowledge items, we finally identified three knowledge items and thirteen skills, essential for image interpretation by trainees. The framework can serve as a guideline for education and assessment of two- and three-dimensional image interpretation. Further validation of the framework in larger study groups with different levels of expertise is needed.

How Visual Search Relates to Visual Diagnostic Performance: A Narrative Systematic Review of Eye-Tracking Research in Radiology.

Eye tracking research has been conducted for decades to gain understanding of visual diagnosis such as in radiology. For educational purposes, it is important to identify visual search patterns that are related to high perceptual performance and to identify effective teaching strategies. This review of eye-tracking literature in the radiology domain aims to identify visual search patterns associated with high perceptual performance. Databases PubMed, EMBASE, ERIC, PsycINFO, Scopus and Web of Science were searched using ‘visual perception’ OR ‘eye tracking’ AND ‘radiology’ and synonyms. Two authors independently screened search results and included eye tracking studies concerning visual skills in radiology published between January 1, 1994 and July 31, 2015. Two authors independently assessed study quality with the Medical Education Research Study Quality Instrument, and extracted study data with respect to design, participant and task characteristics, and variables. A thematic analysis was conducted to extract and arrange study results, and a textual narrative synthesis was applied for data integration and interpretation. The search resulted in 22 relevant full-text articles. Thematic analysis resulted in six themes that informed the relation between visual search and level of expertise: (1) time on task, (2) eye movement characteristics of experts, (3) differences in visual attention, (4) visual search patterns, (5) search patterns in cross sectional stack imaging, and (6) teaching visual search strategies. Expert search was found to be characterized by a global-focal search pattern, which represents an initial global impression, followed by a detailed, focal search-to-find mode. Specific task-related search patterns, like drilling through CT scans and systematic search in chest X-rays, were found to be related to high expert levels. One study investigated teaching of visual search strategies, and did not find a significant effect on perceptual performance. Eye tracking literature in radiology indicates several search patterns are related to high levels of expertise, but teaching novices to search as an expert may not be effective. Experimental research is needed to find out which search strategies can improve image perception in learners.