Cardiac ejection fraction: phantom study comparing cine MR imaging, radionuclide blood pool imaging, and ventriculography.

The accuracy and reproducibility of cardiac ejection fraction (EF) measurements based on cine magnetic resonance (MR) imaging, radionuclide multigated acquisition (MUGA) blood pool imaging, and angiographic ventriculography were evaluated by comparing them with a volumetrically determined standard. A biventricular, compliant, fluid-filled heart phantom was developed to mimic normal cardiac anatomy and physiology. Ventricular EFs were measured with cine MR imaging by summation of nine contiguous 10-mm-thick sections in short and long axis, with single-plane ventriculography, and with MUGA. Three measurements were performed with each modality for each of three EFs. Ventriculography was least accurate, with average relative errors ranging from 7.9% for the largest EF to 60.1% for the smallest. Cine MR was most accurate, with average relative errors ranging from 4.4% to 8.5%. MUGA EF measurements showed good correlation, with average relative errors ranging from 7.1% to 22.4%. Comparison of the error variances for the three modalities with the F test revealed that MR and MUGA EF measurements were significantly more accurate than those based on ventriculography (P less than .01). No significant difference was demonstrated between the accuracy of short- and long-axis cine MR acquisitions.     

Evaluation of the European spine phantom in a multi-centre clinical trial

The European Spine Phantom (ESP) has recently been developed as a universal standard for instruments measuring bone density. The ESP is composed of three semi-anthropomorphic hydroxyapatite vertebrae of varying densities surrounded by soft tissue equivalent plastic designed to resemble human bone and soft tissue when scanned on bone densitometers. In multi-centre studies it is particularly important to verify that each participating bone densitometer is performing in a stable and linear fashion. The aim of this study was to evaluate the ESP within the context of a multi-centre clinical trial. Eighteen centres in the UK and Canada with dual-energy X-ray absorptiometry (DXA) instruments (Lunar DPX, DPX-alpha and DPX-L) participated in the study. The ESP was scanned 10 times on each instrument without repositioning using standardized protocols. The precision of the bone mineral density (BMD) measurements (L1–3) expressed as a coefficient of variation ranged from 0.4% to 1.1% (mean 0.7%). The mean BMD of each instrument was expressed as a percentage difference from the overall mean and ranged from −1.33% to 1.33%. Linear regression analysis showed that all instruments behaved in a linear fashion across the range of densities with correlation coefficients all ≥0.999 and standard errors of the estimate <1.5% of the mean BMD ESP value. The data from this study demonstrate that the ESP is a useful phantom for assessing the linearity, stability and differences between DXA instruments from one manufacturer.     

Modified iterative model based on data extrapolation method to reduce Gibbs ringing.

An iterative algorithm that involves image filtering and data replacement (as suggested by Constable and Henkelman) is investigated for reducing the Gibbs artifact in magnetic resonance imaging. The image is processed with an edge-preserving filter to estimate the height and location of a set of model elements (delta functions or boxes) for generating the missing high-frequency information. Filtering was performed in the complex image domain to account for discontinuities in phase as well as magnitude. The process is repeated after each data replacement to handle varying degrees of contrast. The convergence and signal-to-noise characteristics of the algorithm are investigated by means of simulated and clinical examples. The results indicate that the algorithm performs reasonably well in reducing ringing artifacts due to nearby edge contrast seen in most of the homogeneous, isointense regions. Nevertheless, it may generate some spurious thickening of structures that do not match the assumed step-edge models.     

Fast,interactive algorithm for segmentation of a series of related images: Application to volumetric analysis of MR images of the heart

Magnetic resonance (MR) cine images of the beating heart have excellent spatial and temporal resolution. Extracting the boundaries of the heart from MR images for volumetric measurements is of considerable interest; however, since the number of images involved is large, tracing the boundaries by hand is tedious and prohibitively time consuming. The authors have developed an interactive method of boundary detection that uses the correlation between the cardiac boundaries on temporally or spatially adjacent images to increase the speed of the process and reproducibility of the measurement. A simulated cine MR study of a phantom (total of 155 images) and cardiac cine studies of two patients (192 images each) were analyzed by two independent observers. Analysis of the phantom data was completed in 5.6 minutes (2.16 seconds per image) by observer 1 and 6.3 minutes (2.4 seconds per image) by observer 2. The percent measurement errors for 31 phantom volumes (30-120 mL) were 0.96% and 0.83% for observers 1 and 2, respectively. The observers analyzed the patient studies in 14-23 minutes (4.4-7.2 seconds per image), with interobserver variabilities of 5.8% and 3.7% for the two patients, respectively. The authors conclude that their flexible, semiautomatic, interactive algorithm allows rapid and reproducible detection of structural boundaries.     

Pulsatile flow artifacts in two-dimensional time-of-flight MR angiography: Initial studies in elastic models of human carotid arteries

Initial experimental and numerical analysis of artifacts due to pulsatile flow in two-dimensional time-of-flight (2D-TOF) magnetic resonance (MR) angiography are presented. The experimental studies used elastic models of the carotid artery bifurcation cast from fresh cadavers and accurately reproducing the twisting and tapering of the human blood vessels, allowing direct comparison of images with and without flow. Prominent image artifacts, including periodic ghosts and signal loss, were produced by pulsatile flow even though flow-compensated gradient waveforms were used. The dependence of artifacts due to partial saturation on pulse sequence parameters (TR and flip angle) was investigated theoretically for a simple pulsatile velocity profile and compared with experimental results from a model of a normal carotid artery. Signal reduction was observed proximal and distal to the stenosis in a model with a 70% internal carotid artery (ICA) stenosis and a model with 90% stenoses in both the ICA and the external carotid artery. Although this study deals exclusively with 2D-TOF imaging, the methods can also be applied to evaluate other MR angiography techniques.