Paul Malcolm

Optimization of metal artefact reduction (MAR) sequences for MRI of total hip prostheses

AIM To describe the relative contribution of matrix size and bandwidth to artefact reduction in order to define optimal sequence parameters for metal artefact reduction (MAR) sequences for MRI of total hip prostheses. METHODS AND MATERIALS A phantom was created using a Charnley total hip replacement. Mid-coronal T1-weighted (echo time 12ms, repetition time 400ms) images through the prosthesis were acquired with increasing bandwidths (150, 300, 454, 592, and 781Hz/pixel) and increasing matrixes of 128, 256, 384, 512, 640, and 768 pixels square. Signal loss from the prosthesis and susceptibility artefact was segmented using an automated tool. RESULTS Over 90% of the achievable reduction in artefacts was obtained with matrixes of 256x256 or greater and a receiver bandwidth of approximately 400Hz/pixel or greater. Thereafter increasing the receiver bandwidth or matrix had little impact on reducing susceptibility artefacts. Increasing the bandwidth produced a relative fall in the signal-to-noise ratio (SNR) of between 49 and 56% for a given matrix, but, in practice, the image quality was still satisfactory even with the highest bandwidth and largest matrix sizes. The acquisition time increased linearly with increasing matrix parameters. CONCLUSION Over 90% of the achievable metal artefact reduction can be realized with mid-range matrices and receiver bandwidths on a clinical 1.5T system. The loss of SNR from increasing receiver bandwidth, is preferable to long acquisition times, and therefore, should be the main tool for reducing metal artefact.

Specific food structures supress appetite through reduced gastric emptying rate

The aim of this study was to determine the extent to which gastric layering and retention of a meal could be used to reduce appetite using the same caloric load. Liquid (control) and semi-solid (active) meals were produced with the same protein, fat, carbohydrate, and mass. These were fed to 10 volunteers on separate days in a crossover study, and subjective appetite ratings, gastric contents, and plasma cholecystokinin (CCK) were assessed over a period of 3 h. The active meal showed food boluses in the stomach persisting for ~45 min, slower emptying rates, and lower plasma CCK levels over the first hour. After the first hour, both gastric emptying rates and plasma CCK levels were similar for both systems and slightly increased compared with the unfed situation. Despite the lower plasma CCK levels for the active meal over the first hour, this meal reduced appetite more than the control meal over the 3 h of the study. For a moderately increased plasma CCK level in the fed state, appetite was correlated with the volume of gastric contents rather than gastric emptying rates or plasma CCK. This suggests that enhanced gastric retention was the key factor in decreasing appetite and was probably mediated by a combination of intestinal nutrient sensing and increased viscosity in the stomach.

Advantages of blood pool contrast agents in MR angiography: a pictorial review.

Gadofosveset trisodium (Ablavar®, formerly Vasovist®) is the first intravascular contrast agent approved for clinical use in peripheral vascular disease. The purpose of this review is to illustrate the clinical uses of gadofosveset‐enhanced magnetic resonance angiography in patients referred for assessment of arteriovenous disease.

High fat food increases gastric residence and thus thresholds for objective symptoms in allergic patients

SCOPE We have tested the hypothesis that high fat foods such as chocolate induce reduced rates of gastric emptying in comparison to lower fat foods and that this can impact uptake of allergens and subsequent reactions in allergic patients. METHODS AND RESULTS In four volunteers, magnetic resonance imaging was used to measure gastric emptying of a series of nine doses of either dark chocolate bars containing 35% fat or a chocolate dessert containing 8% fat. Analysis showed a mean rate of decrease in gastric volume with an 8% fat dessert was 0.33 ± 0.09 mL/min compared to an average rate of increase in gastric volume of 0.09 ± 0.10 mL/min for the chocolate bars. In parallel, eight allergic patients were challenged for either peanut or hazelnut in the same two matrices and doses using a standardized protocol. A statistical analysis of the objective symptoms in the allergic patients showed that the chocolate bars gave a significantly higher threshold for objective symptoms than the dessert. CONCLUSIONS Chocolate bars induced lower gastric emptying rates and in food challenges with allergic patients gave a higher threshold of elicitation for objective reactions than a dessert.

Early failure of a Birmingham resurfacing hip replacement with lymphoreticular spread of metal debris: pre-operative diagnosis with MR

Metal-on-metal hip replacements are the subject of much current debate. There is some evidence that there may be a hypersensitivity reaction, specific to metal-on-metal total hip replacements (THRs), which is associated with early failure of these prostheses. It has to date only been described in total replacements and not in metal-on-metal hip resurfacing. We present the case of a 68-year-old man who underwent bilateral metal-on-metal hip resurfacing for osteoarthrosis. The patient presented 6 months after surgery with pain and lateral thigh swelling. Pre-operative ultrasound and MRI demonstrated findings similar to those described in early failing metal-on-metal THRs, as well as evidence of lymphoreticular spread of metal debris. The operative findings included extensive aseptic soft-tissue necrosis. Histology revealed necrosis and a dense perivascular lymphocytic infiltrate along with metal debris within sinus histiocytes. The surgical, radiological and histological findings are similar to soft-tissue reactions described in metal-on-metal THRs.

Aortic arch morphology and late systemic hypertension following correction of coarctation of aorta.

OBJECTIVE To reproduce in an adult population a pediatric study that found an association between aortic arch geometry and late systemic hypertension following successful repair of aortic coarctation. DESIGN AND RESULTS Fifty-one patients with successful repair of coarctation of the aorta had blood pressure measurement at rest and during exercise. After cross-sectional imaging of the aortic arch, patients were assigned to 1 of 3 previously defined morphological categories: normal, gothic, or crenel. The degree of residual stenosis and the ratio of the height/transverse diameter of the arch (A/T ratio) were calculated. No relationship was found between arch geometry and either resting- or exercise-induced hypertension. CONCLUSIONS We found the classification into 3 morphological types difficult and did not find an association between gothic arch or a high A/T ratio and hypertension.

Computer-aided detection of prostate cancer in T2-weighted MRI within the peripheral zone

In this paper we propose a prostate cancer computer-aided diagnosis (CAD) system and suggest a set of discriminant texture descriptors extracted from T2-weighted MRI data which can be used as a good basis for a multimodality system. For this purpose, 215 texture descriptors were extracted and eleven different classifiers were employed to achieve the best possible results. The proposed method was tested based on 418 T2-weighted MR images taken from 45 patients and evaluated using 9-fold cross validation with five patients in each fold. The results demonstrated comparable results to existing CAD systems using multimodality MRI. We achieved an area under the receiver operating curve (A z ) values equal to [Formula: see text], [Formula: see text], [Formula: see text] and [Formula: see text] for Bayesian networks, ADTree, random forest and multilayer perceptron classifiers, respectively, while a meta-voting classifier using average probability as a combination rule achieved [Formula: see text].

Developing a new measure of small bowel peristalsis with dynamic MR: a proof of concept study

Background Small bowel peristalsis is a complex of many individual motion elements. Although each element of peristalsis can be measured there is no current global measure of peristalsis. Purpose To examine the feasibility of automated computerized assessment of global small bowel motility using simple computational methods. Material and Methods Coronal dynamic MR images were obtained from five healthy volunteers who had fasted for 9 h and drunk 1.5 L of water. Images were taken using single breath-hold and ECG triggering. Acquisitions were repeated at 10 and 20 min after an intramuscular injection of hyoscine butylbromide. Parametric maps were generated representing the mean change in signal amplitude (MSA) per voxel for each dynamic acquisition. Two observers independently assessed thresholding for optimal segmentation of small bowel from other sources of signal. Total voxel activity (TVA) for each study was calculated as a sum of MSA per slice and whole examination and TVA profiles were generated. Results Independent observations suggest that the automated segmentation method described usefully segments small bowel activity from other signal. Small bowel movement represented as TVA varied three-fold in the five volunteers and was inhibited by anti-muscarinic injection. Conclusion It is possible to develop a new measure, based on automated segmentation of mean signal amplitude changes, of small bowel peristalsis using dynamic MR.

Minimization of errors in biexponential T2 measurements of the prostate

To determine the echo times that provide the greatest precision in measurements of prostate T2s. T2 relaxation time measurements in the prostate are complicated by the structure of prostate tissue, which consists of fluid‐filled glands surrounded by epithelial and stromal cells. Since the glands are large relative to diffusion distances, there is little water exchange between the two compartments and T2s are biexponential. Because the relative size and characteristics of the two compartments change in prostate tumors, accurate measurement of the characteristics of each may provide useful information on tumor grade.

Test–retest reliability of rapid whole body and compartmental fat volume quantification on a widebore 3T MR system in normal-weight, overweight, and obese subjects

To measure the test–retest reliability of rapid (<15 min) whole body and visceral fat volume quantification in normal and obese subjects on a widebore 3T MR system and compare it with conventional manual segmentation.