Automatic extraction of the intracranial volume in fetal and neonatal MR scans using convolutional neural networks

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Automatic intracranial volume segmentation.

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Fetal and neonatal MRI.

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Deep learning.

MR images of infants and fetuses allow non-invasive analysis of the brain. Quantitative analysis of brain development requires automatic brain tissue segmentation that is typically preceded by segmentation of the intracranial volume (ICV). Fast changes in the size and morphology of the developing brain, motion artifacts, and large variation in the field of view make ICV segmentation a challenging task.

We propose an automatic method for segmentation of the ICV in fetal and neonatal MRI scans. The method was developed and tested with a diverse set of scans regarding image acquisition parameters (i.e. field strength, image acquisition plane, image resolution), infant age (23–45 weeks post menstrual age), and pathology (posthaemorrhagic ventricular dilatation, stroke, asphyxia, and Down syndrome). The results demonstrate that the method achieves accurate segmentation with a Dice coefficient (DC) ranging from 0.98 to 0.99 in neonatal and fetal scans regardless of image acquisition parameters or patient characteristics. Hence, the algorithm provides a generic tool for segmentation of the ICV that may be used as a preprocessing step for brain tissue segmentation in fetal and neonatal brain MR scans.

Volumetric assessment of extracranial carotid artery aneurysms

The extracranial carotid artery aneurysm (ECAA) is a rare pathology for which clinical treatment guidelines are lacking. In general, symptoms or growth of the aneurysm sac are thought to indicate intervention. ECAAs may present in a large variety of shapes and sizes, and conventional diameter measurements fail to indicate geometrical differences. Therefore, we propose a protocol to measure ECAA size by 3D volumetric assessment. The volumes of 40 ECAAs in computed tomography angiography (CTA) images were measured through manual segmentation, by two independent operators. Volumes of the entire internal carotid artery (ICA) and the ECAA were measured separately. Excellent inter- and intraoperator reliability was found for both ICA and ECAA volumes, with all intraclass correlation coefficients above 0.94. Bland-Altman analysis revealed normal differences for both inter- and intraoperator agreement. For all volumes, similarity of the segmentations was excellent. Outliers were explained by presence of intraluminal ECAA thrombus, which hampered identification of the aneurysm outer wall. These results implicate robustness of our protocol, which is designed as a step-up towards (semi)automatic volumetric measurements to monitor patients with ECAA. Future (semi)automatic volumetric assessments are recommended and such techniques can be developed and validated using the proposed protocol and manual reference segmentations.

MR-based source localization for MR-guided HDR brachytherapy

For the purpose of MR-guided high-dose-rate (HDR) brachytherapy, a method for real-time localization of an HDR brachytherapy source was developed, which requires high spatial and temporal resolutions. MR-based localization of an HDR source serves two main aims. First, it enables real-time treatment verification by determination of the HDR source positions during treatment. Second, when using a dummy source, MR-based source localization provides an automatic detection of the source dwell positions after catheter insertion, allowing elimination of the catheter reconstruction procedure. Localization of the HDR source was conducted by simulation of the MR artifacts, followed by a phase correlation localization algorithm applied to the MR images and the simulated images, to determine the position of the HDR source in the MR images. To increase the temporal resolution of the MR acquisition, the spatial resolution was decreased, and a subpixel localization operation was introduced. Furthermore, parallel imaging (sensitivity encoding) was applied to further decrease the MR scan time. The localization method was validated by a comparison with CT, and the accuracy and precision were investigated. The results demonstrated that the described method could be used to determine the HDR source position with a high accuracy (0.4-0.6 mm) and a high precision (⩽0.1 mm), at high temporal resolutions (0.15-1.2 s per slice). This would enable real-time treatment verification as well as an automatic detection of the source dwell positions.

Preoperative indication for systemic therapy extended to patients with early-stage breast cancer using multiparametric 7-tesla breast MRI

Purpose

To establish a preoperative decision model for accurate indication of systemic therapy in early-stage breast cancer using multiparametric MRI at 7-tesla field strength.

Materials and methods

Patients eligible for breast-conserving therapy were consecutively included. Patients underwent conventional diagnostic workup and one preoperative multiparametric 7-tesla breast MRI. The postoperative (gold standard) indication for systemic therapy was established from resected tumor and lymph-node tissue, based on 10-year risk-estimates of breast cancer mortality and relapse using Adjuvant! Online. Preoperative indication was estimated using similar guidelines, but from conventional diagnostic workup. Agreement was established between preoperative and postoperative indication, and MRI-characteristics used to improve agreement. MRI-characteristics included phospomonoester/phosphodiester (PME/PDE) ratio on 31-phosphorus spectroscopy (³¹P-MRS), apparent diffusion coefficients on diffusion-weighted imaging, and tumor size on dynamic contrast-enhanced (DCE)-MRI. A decision model was built to estimate the postoperative indication from preoperatively available data.

Results

We included 46 women (age: 43-74yrs) with 48 invasive carcinomas. Postoperatively, 20 patients (43%) had positive, and 26 patients (57%) negative indication for systemic therapy. Using conventional workup, positive preoperative indication agreed excellently with positive postoperative indication (N = 8/8; 100%). Negative preoperative indication was correct in only 26/38 (68%) patients. However, ³¹P-MRS score (p = 0.030) and tumor size (p = 0.002) were associated with the postoperative indication. The decision model shows that negative indication is correct in 21/22 (96%) patients when exempting tumors larger than 2.0cm on DCE-MRI or with PME>PDE ratios at ³¹P-MRS.

Conclusions

Preoperatively, positive indication for systemic therapy is highly accurate. Negative indication is highly accurate (96%) for tumors sized ≤2,0cm on DCE-MRI and with PME≤PDE ratios on ³¹P-MRS.

Improved estimation of MR relaxation parameters using complex-valued data

PURPOSE

In MR image analysis, T₁ , T₂ , and T2* maps are generally calculated using magnitude MR data. Without knowledge of the underlying noise variance, parameter estimates at low signal to noise ratio (SNR) are usually biased. This leads to confounds in studies that compare parameters across SNRs and or across scanners. This article compares several estimation techniques which use real or complex-valued MR data to achieve unbiased estimation of MR relaxation parameters without the need for additional preprocessing.

THEORY AND METHODS

Several existing and new techniques to estimate relaxation parameters using complex-valued data were compared with widely used magnitude-based techniques. Their bias, variance and processing times were studied using simulations covering various aspects of parameter variations. Validation on noise-degraded experimental measurements was also performed.

RESULTS

Simulations and experiments demonstrated the superior performance of techniques based on complex-valued data, even in comparison with magnitude-based techniques that account for Rician noise characteristics. This was achieved with minor modifications to data modeling and at computational costs either comparable to or higher ( ≈two fold) than magnitude-based estimators. Theoretical analysis shows that estimators based on complex-valued data are statistically efficient.

CONCLUSION

The estimation techniques that use complex-valued data provide minimum variance unbiased estimates of parametric maps and markedly outperform commonly used magnitude-based estimators under most conditions. They additionally provide phase maps and field maps, which are unavailable with magnitude-based methods. Magn Reson Med 77:385-397, 2017. © 2016 Wiley Periodicals, Inc.

Dual-head gamma camera system for intraoperative localization of radioactive seeds

Breast-conserving surgery is a standard option for the treatment of patients with early-stage breast cancer. This form of surgery may result in incomplete excision of the tumor. Iodine-125 labeled titanium seeds are currently used in clinical practice to reduce the number of incomplete excisions. It seems likely that the number of incomplete excisions can be reduced even further if intraoperative information about the location of the radioactive seed is combined with preoperative information about the extent of the tumor. This can be combined if the location of the radioactive seed is established in a world coordinate system that can be linked to the (preoperative) image coordinate system. With this in mind, we propose a radioactive seed localization system which is composed of two static ceiling-suspended gamma camera heads and two parallel-hole collimators. Physical experiments and computer simulations which mimic realistic clinical situations were performed to estimate the localization accuracy (defined as trueness and precision) of the proposed system with respect to collimator-source distance (ranging between 50 cm and 100 cm) and imaging time (ranging between 1 s and 10 s). The goal of the study was to determine whether or not a trueness of 5 mm can be achieved if a collimator-source distance of 50 cm and imaging time of 5 s are used (these specifications were defined by a group of dedicated breast cancer surgeons). The results from the experiments indicate that the location of the radioactive seed can be established with an accuracy of 1.6 mm  ±  0.6 mm if a collimator-source distance of 50 cm and imaging time of 5 s are used (these experiments were performed with a 4.5 cm thick block phantom). Furthermore, the results from the simulations indicate that a trueness of 3.2 mm or less can be achieved if a collimator-source distance of 50 cm and imaging time of 5 s are used (this trueness was achieved for all 14 breast phantoms which were used in this study). Based on these results we conclude that the proposed system can be a valuable tool for (real-time) intraoperative breast cancer localization.

Connectivity and phase coherence in neural network models of interconnected Z(4)-bi-stable units

A phenomenological neural network model with bi-stable oscillatory units is used to model up- and down-states. These states have been observed in vivo in biological neuronal systems and feature oscillatory, limit cycle type of behavior in the up-states. A network is formed by a set of interconnected units. Two different types of network layouts are considered in this work: networks with hierarchical connections and hubs and networks with random connections. The phase coherence between the different units is analyzed and compared to the connectivity distance between nodes. In addition the connectivity degree of a node is associated to the average phase coherence with all other units. The results show that we may be able to identify the set of hubs in a network based on the phase coherence estimates between the different nodes. If the network is very dense or randomly connected, the underlying network structure, however, can not be derived uniquely from the phase coherence.

CT brain perfusion protocol to eliminate the need for selecting a venous output function

BACKGROUND AND PURPOSE

In CTP, an arterial input function is used for cerebral blood volume measurement. AIFs are often influenced by partial volume effects resulting in overestimated CBV. A venous output function is manually selected to correct for partial volume. This can introduce variability. Our goal was to develop a CTP protocol that enables AIF selection unaffected by partial volume.

MATERIALS AND METHODS

First, the effects of partial volume on artery sizes/types including the MCA were estimated by using a CTP phantom with 9 protocols (section thicknesses of 1, 1.8, and 5 mm and image resolutions of 0.5, 1, and 1.5 mm). Next, these protocols were applied to clinical CTP studies from 6 patients. The influence of the partial volume effect was measured by comparison of the time-attenuation curves from different artery locations with reference veins.

RESULTS

AIFs from MCAs were unaffected by partial volume effects when using high image resolution (1 mm) and medium section thickness (1.8 mm). For the clinical data, a total of 104 arteries and 60 veins was selected. The data confirmed that high image resolution and thin section thickness enable selection of MCAs for AIFs free of partial volume influences. In addition, we found that large veins were not insusceptible to partial volume effects relative to large arteries, questioning the use of veins for partial volume correction.

CONCLUSIONS

A CTP protocol with 1.8-mm section thickness and 1-mm image resolution allows AIF selection unaffected by partial volume effects in MCAs.

Improved arterial visualization in cerebral CT perfusion-derived arteriograms compared with standard CT angiography: a visual assessment study

BACKGROUND AND PURPOSE

Invasive cerebral DSA has largely been replaced by CTA, which is noninvasive but has a compromised arterial view due to superimposed bone and veins. The purpose of this study was to evaluate whether arterial visualization in CTPa is superior to standard CTA, which would eliminate the need for an additional CTA scan to assess arterial diseases and therefore reduce radiation dose.

MATERIALS AND METHODS

In this study, we included 24 patients with subarachnoid hemorrhage for whom CTA and CTP were available. Arterial quality and presence of superimposed veins and bone in CTPa were compared with CTA and scored by 2 radiologists by using a VAS (0%-100%). Average VAS scores were determined and VAS scores per patient were converted to a 10-point NRS. Arterial visualization was considered to be improved when the highest rate (NRS 10, VAS > 90%) was scored for arterial quality, and the lowest rate (NRS 1, VAS < 10%), for the presence of superimposed veins and bone. A sign test with continuity correction was used to test whether the number of cases with these rates was significant.

RESULTS

Average VAS scores in the proximal area were 94% (arterial quality), 4% (presence of bone), and 7% (presence of veins). In this area, the sign test showed that a significant number of cases scored NRS 10 for arterial quality (P < .02) and NRS 1 for the presence of superimposed veins and bone (P < .01).

CONCLUSIONS

Cerebral CTPa shows improved arterial visualization in the proximal area compared with CTA, with similar arterial quality but no superimposed bone and veins.

Spatially regularized region-based perfusion estimation in peripherals using angiographic C-arm systems

The outcome assessment of endovascular revascularization procedures in the lower limbs is currently carried out by x-ray digital subtraction angiography (DSA). Due to the two-dimensional nature of this technique, only visual assessment of arterial blood flow is possible and no tissue blood flow information (i.e. perfusion) is available to assess the effective restoration of blood supply to the tissue. In this work, we propose a method for interventional perfusion estimation in peripherals using C-arms which is based on DSA and two additional 3D images reconstructed from rotational scans. The method assumes spatial homogeneity of contrast within multiple regions identified by segmentation of the reconstructed 3D images. A dedicated segmentation method which relies on local contrast homogeneity and connectivity of anatomical structures is introduced. Region-based perfusion is obtained by mapping the 2D blood flow information from DSA to the 3D segments by solving an inverse problem. Instability of the solution due to the spatial overlap of the regions is addressed by applying spatial and temporal regularizations. The method was evaluated on data simulated from CT perfusion scans of the lower limb. Blood flow values estimated with the optimal number of segmented regions exhibited errors of 1 ± 4  and 2 ± 11 ml/100 ml min(-1) for the two analyzed cases, respectively, which showed to be sufficient to differentiate hypoperfused and normally perfused areas. The use of spatial and temporal regularization proved to be an effective way to limit inaccuracies due to instability in the solution of the inverse problem. Results in general proved the feasibility of C-arm interventional perfusion imaging by a combination of temporal information derived from DSA and spatial information derived from 3D reconstructions.

Influence of variation in semiflexed knee positioning during image acquisition on separate quantitative radiographic parameters of osteoarthritis, measured by Knee Images Digital Analysis

OBJECTIVE

The clinical application of quantitative measurement of separate radiographic parameters of knee osteoarthritis (OA) might be hampered by a lack of reproducible semiflexed joint positioning during acquisition of radiographs. The influence of systematic variations in knee positioning on measurement of separate quantitative radiographic parameters was studied.

METHODS

Five components of knee position during radiographic acquisition (beam height, lower and upper leg extension, internal rotation, and lateral shift) were systematically varied within a clinically relevant range, using three cadaver legs. The influence of these variations on the measurement of the separate quantitative radiographic parameters by Knee Images Digital Analysis (KIDA) was evaluated. Significant changes were validated in vivo. Changes were compared with differences during 2-year follow-up in a radiographic progression cohort of early OA.

RESULTS

Systematic variation in upper and lower leg extension induced changes in the measurement of joint space width (JSW). Lower leg extension also influenced osteophyte area and eminence height measurement. Also bone density measurement was influenced by variation in all five position components. Variations were of clinical relevance compared with 2-year differences in knees with radiographic progression, and were confirmed in vivo.

CONCLUSIONS

Variations in semiflexed knee positioning, which are considered to occur easily during image acquisition in trials and clinical practice despite standardization, are of significant influence on the quantitative measurement of most separate radiographic parameters of OA using KIDA. The additional value of quantitative measurement might improve significantly by better standardization during radiographic acquisition; with radiography still being the gold standard for structure-modification in OA.

Evaluation of separate quantitative radiographic features adds to the prediction of incident radiographic osteoarthritis in individuals with recent onset of knee pain: 5-year follow-up in the CHECK cohort

OBJECTIVE

Detailed radiographic evaluation might enable the identification of osteoarthritis (OA) earlier in the disease. This study evaluated whether and which separate quantitative features on knee radiographs of individuals with recent onset knee pain are associated with incidence of radiographic OA and persistence and/or progression of clinical OA during 5-year follow-up.

METHOD

From the Cohort Hip & Cohort Knee study participants with knee pain at baseline were evaluated. Radiographic OA development was defined as Kellgren & Lawrence (K&L) grade ≥ II at 5-year follow-up. Clinical OA was defined as persistent knee pain and as progression of Westen Ontario & McMaster Universities Osteoarthritis index (WOMAC) pain and function score during follow-up. At baseline radiographic damage was determined by quantitative measurement of separate features using Knee Images Digital Analysis, and by K&L-grading.

RESULTS

Measuring osteophyte area [odds ratio (OR) =7.0] and minimum joint space width (OR=0.7), in addition to demographic and clinical characteristics, improved the prediction of radiographic OA 5 years later [area under curve receiver operating characteristic=0.74 vs 0.64 without radiographic features]. When the predictive score (based on multivariate regression coefficients) was larger than the cut-off for optimal specificity, the chance of incident radiographic OA was 54% instead of the prior probability of 19%. Evaluating separate quantitative features performed slightly better than K&L-grading (AUC=0.70). Radiographic characteristics hardly added to prediction of clinical OA.

CONCLUSION

In individuals with onset knee pain, radiographic characteristics added to the prediction of radiographic OA development 5 years later. Quantitative radiographic evaluation in individuals with suspected OA is worthwhile when determining treatment strategies and designing clinical trials.

Visual cerebral microbleed detection on 7T MR imaging: reliability and effects of image processing

SUMMARY

MR imaging at 7T has a high sensitivity for cerebral microbleed detection. We identified mIP processing conditions with an optimal balance between the number of visually detected microbleeds and the number of sections on 7T MR imaging. Even with optimal mIP processing, the limited size of some of the microbleeds and the susceptibility effects of other adjacent structures were a challenge for visual detection, which led to a modest inter-rater agreement, mainly due to missed microbleeds. Automated lesion-detection techniques may be required to optimally benefit from the increased spatial resolution offered by 7T MR imaging.

Feasibility of bone density evaluation using plain digital radiography

OBJECTIVE

For the radiographic evaluation of subchondral bone changes (sclerosis) in osteoarthritis (OA), bone density (BD) is commonly subjectively assessed. BD evaluation using plain digital radiography might be influenced by acquisition and post-processing (PP) settings. Objective of this study was to evaluate the effects of these settings on the measurement of BD using digital radiographs.

METHODS

A bone density standard (BDS) of hydroxyapatite (HA) mimicked a BD range of 1.0-5.75 g/cm(2). Digital radiographs were acquired with variation in acquisition settings, and with clinical and minimal PP. An aluminum step wedge served as an internal reference to express the gray values of the BDS in mm aluminum equivalents (mmAl). The relation (R(2)) between actual BD and BD normalized to the reference wedge was evaluated with linear regression analyses for radiographs with variations in PP and acquisition settings. Precision of BD measurement of the BDS was evaluated for application in clinical practice.

RESULTS

The correlation between actual BD and BD normalized to the reference was improved by changing PP from clinical (R(2)=0.96) to minimal (R(2)=0.98). Higher tube voltage [kilovolt (kV)] improved the correlation further. Even for clinical PP, average standard deviation (SD) was 0.97 mmAl, much smaller than the change of 2.51 mmAl clinically observed in early OA, which implies the feasibility of BD measurements on digital radiographs.

CONCLUSION

Changing PP and acquisition settings in clinical practice can have profound effect on outcome. If done with care, accurate BD measurement is feasible using plain digital radiography.

A systematic review of the association between radiographic and clinical osteoarthritis of hip and knee

OBJECTIVE

There is ongoing debate on whether an association between radiographic and clinical osteoarthritis (OA) exists. We hypothesized that the inconsistency in the detection of an association might be caused by different definitions of OA, by different radiographic protocols, and by scoring methods for radiographic damage and symptoms. The goal of this study was to evaluate which methodological criteria are important to detect an association between radiographic and clinical OA of hip and knee.

METHODS

A literature search was performed with the keywords 'OA', 'hip', 'knee', 'radiographic', and 'clinical' and results were screened for relevant studies. Quality criteria for study characteristics and methodology were developed. Studies were classified according to these criteria and the presence of an association between radiographic and clinical OA was scored. The importance of methodological quality and patient characteristics on the presence of an association was evaluated.

RESULTS

The literature search resulted in 39 studies describing an association between radiographic and clinical OA. The frequency of an association between radiographic and clinical OA outcome measures diminished when less quality criteria were fulfilled. Specifically the criterion for standardized outcome measures appeared important in the detection of an association. The association was not influenced by patient characteristics. Only four studies were identified that fulfilled all quality criteria and in these studies an association was found for the knee joint and an inconsistent association was found for the hip joint.

CONCLUSION

Methodological quality criteria are of importance to reveal an association between radiographic and clinical OA.

The first taste is always with the eyes: a meta-analysis on the neural correlates of processing visual food cues

Food selection is primarily guided by the visual system. Multiple functional neuro-imaging studies have examined the brain responses to visual food stimuli. However, the results of these studies are heterogeneous and there still is uncertainty about the core brain regions involved in the neural processing of viewing food pictures. The aims of the present study were to determine the concurrence in the brain regions activated in response to viewing pictures of food and to assess the modulating effects of hunger state and the food's energy content. We performed three Activation Likelihood Estimation (ALE) meta-analyses on data from healthy normal weight subjects in which we examined: 1) the contrast between viewing food and nonfood pictures (17 studies, 189 foci), 2) the modulation by hunger state (five studies, 48 foci) and 3) the modulation by energy content (seven studies, 86 foci). The most concurrent brain regions activated in response to viewing food pictures, both in terms of ALE values and the number of contributing experiments, were the bilateral posterior fusiform gyrus, the left lateral orbitofrontal cortex (OFC) and the left middle insula. Hunger modulated the response to food pictures in the right amygdala and left lateral OFC, and energy content modulated the response in the hypothalamus/ventral striatum. Overall, the concurrence between studies was moderate: at best 41% of the experiments contributed to the clusters for the contrast between food and nonfood. Therefore, future research should further elucidate the separate effects of methodological and physiological factors on between-study variations.

Robust initialization of 2D-3D image registration using the projection-slice theorem and phase correlation

PURPOSE

The image registration literature comprises many methods for 2D-3D registration for which accuracy has been established in a variety of applications. However, clinical application is limited by a small capture range. Initial offsets outside the capture range of a registration method will not converge to a successful registration. Previously reported capture ranges, defined as the 95% success range, are in the order of 4-11 mm mean target registration error. In this article, a relatively computationally inexpensive and robust estimation method is proposed with the objective to enlarge the capture range.

METHODS

The method uses the projection-slice theorem in combination with phase correlation in order to estimate the transform parameters, which provides an initialization of the subsequent registration procedure.

RESULTS

The feasibility of the method was evaluated by experiments using digitally reconstructed radiographs generated from in vivo 3D-RX data. With these experiments it was shown that the projection-slice theorem provides successful estimates of the rotational transform parameters for perspective projections and in case of translational offsets. The method was further tested on ex vivo ovine x-ray data. In 95% of the cases, the method yielded successful estimates for initial mean target registration errors up to 19.5 mm. Finally, the method was evaluated as an initialization method for an intensity-based 2D-3D registration method. The uninitialized and initialized registration experiments had success rates of 28.8% and 68.6%, respectively.

CONCLUSIONS

The authors have shown that the initialization method based on the projection-slice theorem and phase correlation yields adequate initializations for existing registration methods, thereby substantially enlarging the capture range of these methods.

CHECK (Cohort Hip and Cohort Knee): similarities and differences with the Osteoarthritis Initiative

OBJECTIVE

To describe the osteoarthritis study population of CHECK (Cohort Hip and Cohort Knee) in comparison with relevant selections of the study population of the Osteoarthritis Initiative (OAI) based on clinical status and radiographic parameters.

METHODS

In The Netherlands a prospective 10-year follow-up study was initiated by the Dutch Arthritis Association on participants with early osteoarthritis-related complaints of hip and/or knee: CHECK. In parallel in the USA an observational 4-year follow-up study, the OAI, was started by the National Institutes of Health, on patients with or at risk of symptomatic knee osteoarthritis. For comparison with CHECK, the entire cohort and a subgroup of individuals excluding those with exclusively hip pain were compared with relevant subpopulations of the OAI.

RESULTS

At baseline, CHECK included 1002 participants with in general similar characteristics as described for the OAI. However, significantly fewer individuals in CHECK had radiographic knee osteoarthritis at baseline when compared with the OAI (p<0.001). In contrast, at baseline, the CHECK cohort reported higher scores on pain, stiffness and functional disability (Western Ontario and McMaster osteoarthritis index) when compared with the OAI (all p<0.001). These differences were supported by physical health status in contrast to mental health (Short Form 36/12) was at baseline significantly worse for the CHECK participants (p<0.001).

CONCLUSION

Although both cohorts focus on the early phase of osteoarthritis, they differ significantly with respect to structural (radiographic) and clinical (health status) characteristics, CHECK expectedly representing participants in an even earlier phase of disease.

Knee Images Digital Analysis (KIDA): a novel method to quantify individual radiographic features of knee osteoarthritis in detail

OBJECTIVE

Radiography is still the golden standard for imaging features of osteoarthritis (OA), such as joint space narrowing, subchondral sclerosis, and osteophyte formation. Objective assessment, however, remains difficult. The goal of the present study was to evaluate a novel digital method to analyse standard knee radiographs.

METHODS

Standardized radiographs of 20 healthy and 55 OA knees were taken in general practise according to the semi-flexed method by Buckland-Wright. Joint Space Width (JSW), osteophyte area, subchondral bone density, joint angle, and tibial eminence height were measured as continuous variables using newly developed Knee Images Digital Analysis (KIDA) software on a standard PC. Two observers evaluated the radiographs twice, each on two different occasions. The observers were blinded to the source of the radiographs and to their previous measurements. Statistical analysis to compare measurements within and between observers was performed according to Bland and Altman. Correlations between KIDA data and Kellgren & Lawrence (K&L) grade were calculated and data of healthy knees were compared to those of OA knees.

RESULTS

Intra- and inter-observer variations for measurement of JSW, subchondral bone density, osteophytes, tibial eminence, and joint angle were small. Significant correlations were found between KIDA parameters and K&L grade. Furthermore, significant differences were found between healthy and OA knees.

CONCLUSION

In addition to JSW measurement, objective evaluation of osteophyte formation and subchondral bone density is possible on standard radiographs. The measured differences between OA and healthy individuals suggest that KIDA allows detection of changes in time, although sensitivity to change has to be demonstrated in long-term follow-up studies.

Automated measurement of brain and white matter lesion volume in type 2 diabetes mellitus

AIMS/HYPOTHESIS

Type 2 diabetes mellitus has been associated with brain atrophy and cognitive decline, but the association with ischaemic white matter lesions is unclear. Previous neuroimaging studies have mainly used semiquantitative rating scales to measure atrophy and white matter lesions (WMLs). In this study we used an automated segmentation technique to investigate the association of type 2 diabetes, several diabetes-related risk factors and cognition with cerebral tissue and WML volumes.

SUBJECTS AND METHODS

Magnetic resonance images of 99 patients with type 2 diabetes and 46 control participants from a population-based sample were segmented using a k-nearest neighbour classifier trained on ten manually segmented data sets. White matter, grey matter, lateral ventricles, cerebrospinal fluid not including lateral ventricles, and WML volumes were assessed. Analyses were adjusted for age, sex, level of education and intracranial volume.

RESULTS

Type 2 diabetes was associated with a smaller volume of grey matter (-21.8 ml; 95% CI -34.2, -9.4) and with larger lateral ventricle volume (7.1 ml; 95% CI 2.3, 12.0) and with larger white matter lesion volume (56.5%; 95% CI 4.0, 135.8), whereas white matter volume was not affected. In separate analyses for men and women, the effects of diabetes were only significant in women.

CONCLUSIONS/INTERPRETATION

The combination of atrophy with larger WML volume indicates that type 2 diabetes is associated with mixed pathology in the brain. The observed sex differences were unexpected and need to be addressed in further studies.

Neuronavigation and surgery of intracerebral tumours

Approximately four decades after the successful clinical introduction of framebased stereotactic neurosurgery by Spiegel and Wycis, frameless stereotaxy emerged to enable more elaborate image guidance in open neurosurgical procedures. Frameless stereotaxy, or neuronavigation, relies on one of several different localizing techniques to determine the position of an operative instrument relative to the surgical field, without the need for a coordinate frame rigidly fixed to the patients' skull. Currently, most systems are based on the optical triangulation of infrared light sources fixed to the surgical instrument. In its essence, a navigation system is a three-dimensional digitiser that correlates its measurements to a reference data set, i.e. a preoperatively acquired CT or MRI image stack. This correlation is achieved through a patient-to-image registration procedure resulting in a mathematical transformation matrix mapping each position in 'world space' onto 'image space'. Thus, throughout the remainder of the surgical procedure, the position of the surgical instrument can be demonstrated on a computer screen, relative to the CT or MRI images. Though neuronavigation has become a routinely used addition to the neurosurgical armamentarium, its impact on surgical results has not yet been examined sufficiently. Therefore, the surgeon is left to decide on a case-by-case basis whether to perform surgery with or without neuronavigation. Future challenges lie in improvement of the interface between the surgeon and the neuronavigator and in reducing the brainshift error, i.e. inaccuracy introduced by changes in tissue positions after image acquisition.

Computed tomography versus magnetic resonance imaging of endoleaks after EVAR

AIM

The aim of study was to compare the sensitivity of MRI and CTA for endoleak detection and classification after EVAR.

PATIENTS & METHODS

Twenty-eight patients, between 2 days and 65 months after EVAR, were evaluated with both CT and MRI. Twenty-five patients had an Ancure graft and the other three had an Excluder. The MRI protocol for endoleak evaluation included: a T1-weighted spin echo, a high-resolution 3D CE-MRA, and a post-contrast T1-weighted spin echo. In total 40 ml Gadolinium was administered. The CT protocol consisted of a blank survey followed by a spiral CT angiography (CTA) using 140 ml of Ultravist. An experienced, blinded observer evaluated all CTs and MRIs.

RESULTS

Using MRI and MRA techniques significantly more endoleaks (23/35) were detected than with CTA (11/35) (p=0.01, Chi-Square). CT could not determine the type of endoleak in 3 of the 11 endoleaks detected and was uncertain in one. MRI was uncertain about the type in 14 of the 23 endoleaks detected. All endoleaks visible on CT were visible by MRI as well.

CONCLUSIONS

MRI techniques are more sensitive for the detection of endoleak after endovascular AAA repair than CT.

Normalized mutual information based registration using k-means clustering and shading correction

In this paper the influence of intensity clustering and shading correction on mutual information based image registration is studied. Instead of the generally used equidistant re-binning, we use k-means clustering in order to achieve a more natural binning of the intensity distribution. Secondly, image inhomogeneities occurring notably in MR images can have adverse effects on the registration. We use a shading correction method in order to reduce these effects. The method is validated on clinical MR, CT and PET images, as well as synthetic MR images. It is shown that by employing clustering with inhomogeneity correction the number of misregistrations is reduced without loss of accuracy thus increasing robustness as compared to the standard non-inhomogeneity corrected and equidistant binning based registration.

The impact of auditory feedback on neuronavigation

OBJECT

We aimed to develop an auditory feedback system to be used in addition to regular neuronavigation, in an attempt to improve the usefulness of the information offered by neuronavigation systems.

INSTRUMENTATION

Using a serial connection, instrument co-ordinates determined by a commercially available neuronavigation system were transferred to a laptop computer. Based on preoperative segmentation of the images, the software on the laptop computer produced an audible signal whenever the instrument moved into an area the surgeon wanted to avoid.

METHODS

To evaluate the impact of our setup on volumetric resections, phantom experiments were conducted. CT scans were acquired from eight blocks of floral foam. In each of these scans, a target-volume was segmented. This target-volume was subsequently resected using either regular neuronavigation or neuronavigation extended with auditory feedback. A 'postoperative' CT scan was used to compare the resection cavity to the preoperatively planned target-volume.

FINDINGS

The resemblance between the resection cavity and the target-volume was greater each time auditory feedback had been used. This corresponded with more complete removal of the target-volume. However, it also corresponded with the removal of more non-target 'tissue' in two out of four cases.

CONCLUSIONS

The usefulness of auditory feedback was made plausible and the use of a new type of navigation phantom was illustrated. Based on these results, we recommend incorporation of auditory feedback in commercially available neuronavigation systems, especially since this is relatively inexpensive.

A quantitative analysis of 3-D coronary modeling from two or more projection images

A method is introduced to examine the geometrical accuracy of the three-dimensional (3-D) representation of coronary arteries from multiple (two and more) calibrated two-dimensional (2-D) angiographic projections. When involving more then two projections, (multiprojection modeling) a novel procedure is presented that consists of fully automated centerline and width determination in all available projections based on the information provided by the semi-automated centerline detection in two initial calibrated projections. The accuracy of the 3-D coronary modeling approach is determined by a quantitative examination of the 3-D centerline point position and the 3-D cross sectional area of the reconstructed objects. The measurements are based on the analysis of calibrated phantom and calibrated coronary 2-D projection data. From this analysis a confidence region (alpha degrees approximately equal to [35 degrees - 145 degrees]) for the angular distance of two initial projection images is determined for which the modeling procedure is sufficiently accurate for the applied system. Within this angular border range the centerline position error is less then 0.8 mm, in terms of the Euclidean distance to a predefined ground truth. When involving more projections using our new procedure, experiments show that when the initial pair of projection images has an angular distance in the range alpha degrees approximately equal to [35 degrees - 145 degrees], the centerlines in all other projections (gamma = 0 degrees - 180 degrees) were indicated very precisely without any additional centering procedure. When involving additional projection images in the modeling procedure a more realistic shape of the structure can be provided. In case of the concave segment, however, the involvement of multiple projections does not necessarily provide a more realistic shape of the reconstructed structure.

Registration-based interpolation

A method is presented to interpolate between neighboring slices in a grey-scale tomographic data set. Spatial correspondence between adjacent slices is established using a nonrigid registration algorithm based on B-splines which optimizes the normalized mutual information similarity measure. Linear interpolation of the image intensities is then carried out along the directions calculated by the registration algorithm. The registration-based method is compared to both standard linear interpolation and shape-based interpolation in 20 tomographic data sets. Results show that the proposed method statistically significantly outperforms both linear and shape-based interpolation.

Clinical evaluation of stereotactic brain biopsies with an MKM-mounted instrument holder

OBJECT

The aim of this study was to assess the clinical usefulness and accuracy of robot-assisted frameless stereotactic brain biopsies with a recently introduced MKM-mounted instrument holder.

METHODS

Twenty-three patients with intracranial lesions participated in this study. Depending on the size of the intracranial lesion, fiducials for image-to-patient co-ordinate transformation consisted either of bone screws or adhesive markers. Shortly after surgery, postoperative MRI-imaging was performed to demonstrate the location of the biopsy site. These images were compared with the preoperative images to assess the biopsy localisation error.

RESULTS

Postoperative biopsy sites could be demonstrated in six patients with bone screws and in 14 with adhesive markers. These two subgroups yielded average biopsy localisation errors of 3.3 mm (SD 1.7 mm) and 4.5 mm (SD 2.0 mm) respectively. This difference was not statistically significant. One biopsy was located in a liquefied haematoma. All others yielded pathological tissue. There were two postoperative haemorrhages, of which only one was temporarily symptomatic. There was no mortality in the first 30 days after surgery.

CONCLUSIONS

Robot-assisted frameless point-stereotactic techniques represent an alternative to frame-based techniques for the performance of stereotactic biopsies.

Simultaneous (99m)Tc/(201)Tl dual-isotope SPET with Monte Carlo-based down-scatter correction

In simultaneous technetium-99m/thallium-201 dual-isotope (DI) single-photon emission tomography (SPET), down-scatter of (99m)Tc photons contaminates the (201)Tl image, which leads to a decrease in lesion contrast and loss of quantitative accuracy. Correction for down-scatter can be achieved by first reconstructing the (99m)Tc activity distribution. Subsequently, the (99m)Tc down-scatter in the (201)Tl photopeak window is simulated and used for correction during iterative reconstruction of the (201)Tl image. In this work, the down-scatter projections are calculated using a dedicated Monte Carlo simulator which is able to efficiently model the detection of lead X-rays from the collimator. An anthropomorphic torso phantom with a cardiac insert with and without cold lesions was used for evaluation of the proposed method. Excellent agreement in lesion contrast and quantitative accuracy was found between the down-scatter corrected DI-SPET (201)Tl image and the virgin (i.e. separately acquired) (201)Tl image, in particular when the effects of lead X-rays were included. Compensation for the noise added by down-scatter to the (201)Tl image can be achieved by using a 15% lower dose of (99m)Tc, a 15% increase in scan time and a 12% increase in (201)Tl dose. In conclusion, the Monte Carlo-based down-scatter correction recovers lesion contrast and quantitative accuracy in DI-SPET (201)Tl images almost perfectly. In addition, degradations due to the added noise of down-scatter in simultaneous DI-SPET can be prevented by slight adaptations to the data acquisition protocol.

Retrospective correction of MR intensity inhomogeneity by information minimization

In this paper, the problem of retrospective correction of intensity inhomogeneity in magnetic resonance (MR) images is addressed. A novel model-based correction method is proposed, based on the assumption that an image corrupted by intensity inhomogeneity contains more information than the corresponding uncorrupted image. The image degradation process is described by a linear model, consisting of a multiplicative and an additive component which are modeled by a combination of smoothly varying basis functions. The degraded image is corrected by the inverse of the image degradation model. The parameters of this model are optimized such that the information of the corrected image is minimized while the global intensity statistic is preserved. The method was quantitatively evaluated and compared to other methods on a number of simulated and real MR images and proved to be effective, reliable, and computationally attractive. The method can be widely applied to different types of MR images because it solely uses the information that is naturally present in an image, without making assumptions on its spatial and intensity distribution. Besides, the method requires no preprocessing, parameter setting, nor user interaction. Consequently, the proposed method may be a valuable tool in MR image analysis.

Automatic scoliosis detection based on local centroids evaluation on moiré topographic images of human backs

This paper presents a technique for automating human scoliosis detection by computer based on moiré topographic images of human backs. Scoliosis is a serious disease often suffered by teenagers. For prevention, screening is performed at schools in Japan employing a moiré method in which doctors inspect moiré images of subjects' backs visually. The inspection of a large number of moiré images collected by the school screening causes exhaustion of doctors and leads to misjudgment. Computer-aided diagnosis of scoliosis has, therefore, been requested eagerly by orthopedists. To automate the inspection process, unlike existent three-dimensional techniques, displacement of local centroids is evaluated two-dimensionally between the left-hand side and the right-hand side of the moiré images in the present technique. The technique was applied to real moiré images to draw a distinction between normal and abnormal cases. According to the leave-out method, the entire 120 image data (60 normal and 60 abnormal) were separated into three data sets. The linear discriminant function based on Mahalanobis distance was defined on the two-dimensional feature space employing one of the data sets containing 40 moiré images and classified 80 images in the remaining two sets. The technique finally achieved the average classification rate of 88.3%.

An MKM-mounted instrument holder for frameless point-stereotactic procedures: a phantom-based accuracy evaluation

To enable the use of the Mehrkoordinaten Manipulator (MKM) robotic navigation system for frameless point stereotactic procedures, a new instrument holder is presented. A phantom-based accuracy study was performed in which this new method was compared with frame-based procedures performed using the Brown-Roberts-Wells (BRW) stereotactic frame. The authors acquired computerized tomography scans of a test phantom, consisting of 19 acrylic plastic target rods on a circular base. These images were used in frame-based (BRW) and frameless (MKM) localization experiments. In both cases the authors calculated the distances between the actual target positions and the positions reached stereotactically. The mean application accuracy (target registration error) was 0.68 mm when the BRW frame was used and 0.96 mm when the MKM system was used after manual repositioning of the microscope (p < 0.001). Positioning accomplished using robotics only demonstrated a slightly larger inaccuracy: 1.47 mm (p < 0.005). Because the surgeon is concerned with the largest error in an individual case rather than the mean error in a large number of cases, the mean + three standard deviations was also compared. This value differed very little between the manually positioned MKM system and the BRW frame (2.04 mm and 1.84 mm, respectively). Although repeatability per target appeared to be slightly better when the BRW frame was used, accuracy was more homogeneous over the phantom volume when the MKM system was used (both differences were not significant). In conclusion, the accuracy of point stereotactic procedures performed using an instrument holder attached to the system is comparable with the accuracy of procedures involving a stereotactic frame. Moreover, the frameless techniques and robotic features of the MKM enable a more surgeon- and patient-friendly stereotactic procedure.

Computer-aided diagnosis in chest radiography: a survey

The traditional chest radiograph is still ubiquitous in clinical practice, and will likely remain so for quite some time. Yet, its interpretation is notoriously difficult. This explains the continued interest in computer-aided diagnosis for chest radiography. The purpose of this survey is to categorize and briefly review the literature on computer analysis of chest images, which comprises over 150 papers published in the last 30 years. Remaining challenges are indicated and some directions for future research are given.

Localization of intravascular devices with paramagnetic markers in MR images

Magnetic resonance imaging (MRI) offers potential advantages over conventional X-ray techniques for guiding and evaluating intravascular interventions. The development of methods to safely and robustly localize and track devices under MRI guidance is mandatory to enable automatic scan plane adaptation so as to exploit the three-dimensional imaging capabilities of the MRI scanner. With regard to the issue of radiofrequency-induced heating, passive approaches to catheter tracking are inherently safe. These techniques visualize intravascular devices by exploiting the susceptibility artifacts associated with the devices. To promote conspicuity, the devices are equipped with paramagnetic markers. This paper introduces a method to enable automatic localization of devices by its ability to recognize markers in two-dimensional MR images. The method requires a coarse segmentation of the vasculature of interest, and consists of two steps. First, it performs a series of postprocessing operations including calculation of the winding number image and of the Laplacian image to detect marker candidates in the image. Second, the device is localized by matching the detected pattern of candidates to the known distance template of the device markers. Results of an animal experiment and of a clinical application are demonstrated. Validation in phantom experiments shows that the method is able to localize the device in 95% of the cases.

Automatic segmentation of the ventricular system from MR images of the human brain

An algorithm was developed that automatically segments the lateral and third ventricles from T1-weighted 3-D-FFE MR images of the human brain. The algorithm is based upon region-growing and mathematical morphology operators and starts from a coarse binary total brain segmentation, which is obtained from the 3-D-FFE image. Anatomical knowledge of the ventricular system has been incorporated into the method in order to find all constituting parts of the system, even if they are disconnected, and to avoid inclusion of nonventricle cerebrospinal fluid (CSF) regions. A test of the method on a synthetic MR brain image produced a segmentation overlap of 0.98 between the simulated ventricles ("model") and those defined by the algorithm. Further tests were performed on a large data set of 227 1.5 T MR brain images. The algorithm yielded useful results for 98% of the images. The automatic segmentations had intra-class correlation coefficients of 0.996 for the lateral ventricles and 0.86 for the third ventricle, with manually edited segmentations. Comparison of ventricular volumes of schizophrenia patients compared with those of healthy control subjects showed results in agreement with the literature.

Background suppression using magnetization preparation for contrast-enhanced MR projection angiography

In contrast-enhanced MR projection angiography, vessel conspicuity is determined by the T(1)-weighted signal difference between blood and surrounding tissues. For slice-selective excitation pulses, the excitation angle varies across the slice, leading to poor saturation of the background signal at the slice edge and reducing the blood-background signal difference. This work reports on the use of magnetization preparation to enhance the T(1)-weighted contrast between blood and background tissue. Applying the prepulse nonselectively reduces the influence of the slice profile imperfections of the excitation pulse by keeping the background tissue at the slice edge saturated. Analytical calculations and in vitro experiments show that a prepulse angle of 110 degrees -130 degrees and a delay time of 20-25 ms enhance the contrast between contrast-enhanced blood (T(1) < 50 ms) and background tissues (T(1) > 200 ms), and improve the slice weighting profile. Magnetization preparation is shown to effectively suppress signal from background tissue, resulting in a threefold increase of the vessel-to-background signal ratio. Magnetization preparation eliminates the need for subtraction at the cost of a slight increase in scan time. Possible applications, such as projection MRA, detection of contrast arrival, and test-bolus tracking are demonstrated in a pig model. Magn Reson Med 46:78-87, 2001.

Correcting partial volume artifacts of the arterial input function in quantitative cerebral perfusion MRI

To quantify cerebral perfusion with dynamic susceptibility contrast MRI (DSC-MRI), one needs to measure the arterial input function (AIF). Conventionally, one derives the contrast concentration from the DSC sequence by monitoring changes in either the amplitude or the phase signal on the assumption that the signal arises completely from blood. In practice, partial volume artifacts are inevitable because a compromise has to be reached between the temporal and spatial resolution of the DSC acquisition. As the concentration of the contrast agent increases, the vector of the complex blood signal follows a spiral-like trajectory. In the case of a partial-volume voxel, the spiral is located around the static contribution of the surrounding tissue. If the static contribution of the background tissue is disregarded, estimations of the contrast concentration will be incorrect. By optimizing the correspondence between phase information and amplitude information one can estimate the origin of the spiral, and thereupon correct for partial volume artifacts. This correction is shown to be accurate at low spatial resolutions for phantom data and to improve the AIF determination in a clinical example. Magn Reson Med 45:477-485, 2001.

Quantitative evaluation of convolution-based methods for medical image interpolation

Interpolation is required in a variety of medical image processing applications. Although many interpolation techniques are known from the literature, evaluations of these techniques for the specific task of applying geometrical transformations to medical images are still lacking. In this paper we present such an evaluation. We consider convolution-based interpolation methods and rigid transformations (rotations and translations). A large number of sinc-approximating kernels are evaluated, including piecewise polynomial kernels and a large number of windowed sinc kernels, with spatial supports ranging from two to ten grid intervals. In the evaluation we use images from a wide variety of medical image modalities. The results show that spline interpolation is to be preferred over all other methods, both for its accuracy and its relatively low computational cost.

MRA of hemodialysis access grafts and fistulae using selective contrast injection and flow interruption

MR is a potentially attractive modality for evaluating hemodialysis access anatomy and function. However, the wide range of flow rates in the hemodialysis access complicates interpretation of phase contrast, time-of-flight, and even contrast-enhanced MR angiograms. At high flow rates, signal voids may easily arise at mild narrowings or sharp-angled anastomoses. A method is proposed which visualizes hemodialysis accesses without flow artifacts. Diluted Gd-DTPA is hand-injected directly into the access, while a cuff is used to reduce and subsequently interrupt access flow. Filling of the access is monitored using a fast projection technique with complex subtraction. When filling is satisfactory, a 3D acquisition is started. The feasibility of this selective contrast-enhanced MR angiography technique is demonstrated in four Cimino-fistulae and four PTFE grafts. Magn Reson Med 45:557-561, 2001.