In vivo evaluation of myocardial infarction by computed tomography: an experiment with electrocardiographic gating.

Retrospectively gated and ungated images of normal and 24 h post-infarction mini-pig hearts were obtained. The 11 imaged infarcts were transcatheter embolisation of the branches of the left anterior descending coronary artery. Using a contrast infusion technique and scanning during infusion, four infarcts were clearly detected as low-attenuation areas in the myocardium, one of these showing adjacent contrast enhancement. Two other infarcts showed as enhancing regions. Two small infarcts (0.8-1.0 cm3) were not detected and three others were in doubt. Streaking and other artefacts presented difficulties in image interpretation, which were sometimes resolved by gating. A comparison is made of these findings with those obtained from experiments with dogs, of comparable methodology. Differences are considered to result from anatomical differences between the two species, more particularly in the collateral blood supply to the myocardium.

A method for increasing the resolution of scanned projection radiography and other digital X-ray systems.

A method for increasing the sampling frequency of digital X-ray systems is described. The method employs discrete, stepwise magnetic deflection of the focal spot and the recombination of the resulting displaced images. The technique was applied to a GE CT/T 8800 scanner operating in "Scout View" mode. The hardware and software modifications were minor. The transverse resolution was shown to improve from 0.63 to 0.9 line pairs per mm, using a test phantom. The effect of the improvement in resolution is also illustrated in the human. The possibility of further improving the resolution of the system is discussed.

Simultaneous mapping of blood volume and endothelial permeability surface area product in gliomas using iterative analysis of first-pass dynamic contrast enhanced MRI data.

We describe a novel method for the calculation of endothelial permeability surface area product from dynamic contrast enhanced MRI. The technique uses iterative estimation to automatically decompose tissue residue function into intravascular and extravascular components, which are subsequently used to generate tumour blood volume, which is equal to relative cerebral blood volume calculated from T(1) weighted images and corrected for contamination by contrast agent leakage (rCBV(T1)(corrected), and endothelial permeability (k(fp)) maps. The technique was assessed in patients with cerebral glioma (n=5) by examining the reproducibility of endothelial permeability and rCBV(T1)(corrected) between two separate examinations conducted with a 2-day interval. The technique produces maps of endothelial permeability that appear to be free of any contribution from intravascular contrast agent. Maps of rCBV(T1)(corrected) show close correlation with maps of blood volume calculated from independently acquired dynamic susceptibility weighted MRI examinations, with no evidence of residual permeability effects. The results were highly reproducible with strong intra-class correlation between the two examinations for mean values and for 97.5 percentiles of endothelial permeability and rCBV(T1)(corrected). The excellent reproducibility of this technique and the ability to calculate endothelial permeability and rCBV(T1)(corrected) values from rapidly acquired data sets offer considerable advantages over conventional approaches and support the use of this methodology for therapeutic monitoring or trials of novel therapeutic agents.

Reproducibility of quantitative dynamic contrast-enhanced MRI in newly presenting glioma.

We have investigated the reproducibility of dynamic contrast enhanced imaging techniques in nine patients with cerebral glioma. Patients were imaged twice with a 2 day interval between scans. Maps were produced of the time taken to achieve 90% enhancement (T90), the maximal intensity change per time interval ratio (MITR), the volume transfer coefficient between plasma and the extravascular extracellular space (K(trans)) and the extravascular extracellular contrast distribution volume, v(e). Measurements of K(trans) greater than 1.2 min(-1) were used to exclude pixels where first pass perfusion effects dominated the measurement. Measures of the test-retest coefficient of variation (CoV) and intraclass correlation coefficients were used to assess reproducibility for measurements from a volume of interest containing enhancing tissue from the whole tumour. MITR showed poor reproducibility (mean CoV 17.9%, 95% confidence limits for group comparisons 20.2%). T90 showed good reproducibility (mean CoV 7.1%, 95% confidence limits for group comparisons 5.2%). Calculated values of K(trans) and v(e) also showed good reproducibility (mean CoV 7.7% and 6.2% respectively, 95% confidence limits for group comparisons 6.2% and 4.8%, respectively). We conclude that the measurements of K(trans) and v(e) derived from pharmacokinetic analysis are sufficiently reproducible to support their use as a biological markers in therapeutic trials.

Active coil isolation in NMR imaging and spectroscopy using PIN diodes and tuned transmission line: a practical approach.

To improve signal-to-noise (S/N) ratios in biological NMR experiments we have regularly employed close-fitting receiver coils. The poor RF (radio-frequency) homogeneity often exhibited by these coils can be partly overcome by using them with large transmitter coils, provided that good between-coil isolation during the RF transmission and receive periods is achieved. With this in mind, we have used combined PIN diodes and tuned line to isolate transmitter and receiver and to remove transmitter noise. A series of experiments reported here demonstrate (a) distortion-free receiver detuning during free-induction decay, (b) the reduced effect of the receiver coil on the transmitter pulse, (c) an increase in S/N from 71:1 to 158:1, and (d) the effectiveness of transmitter noise isolation. Improvements in S/N, isolation, and image homogeneity illustrate the value of utilizing these devices. Hardware to allow PIN diode switching under computer control is described, utilizing mostly nonmagnetic materials and batteries.

Accuracy of area measurements made from MR images compared with computed tomography.

The accuracy of area measurements made on magnetic resonance (MR) and CT images with clinical general purpose scanners was compared using automatic boundary detection. Both backprojection and two-dimensional Fourier transform reconstruction were used with several pulse sequences for the MR images. Several phantoms were scanned with different receiver coils and image matrices. Errors caused by grey level variation in MR images can be reduced using local edge detection. The estimated values varied with the sequence, reconstruction algorithm, and the area under investigation. Magnetic resonance area measurements should be interpreted with caution.

An investigation into the problems of attenuation and area measurements made from CT images of pulmonary nodules.

Pulmonary nodules were modelled with Perspex rods of various diameters. Computed tomographic (CT) images of these rods were obtained in the centre of the scan field, both in air and in a water bath. The numerical values at the centres of the rods were compared with values obtained from scan simulations that isolated the effects of the filtered backprojection. Both real scan and simulated data were reconstructed with different algorithms. Air was found to give markedly depressed attenuation values in real scans, but not in the simulations. This effect was considered to be the result of smoothing or inappropriate beam hardening corrections. In the simulated data, air resulted in elevated attenuation values due to overshoot of the point spread function. Such overshoots varied with object size and had a more extensive influence than could be appreciated from the real scan data. Different algorithms produced different central attenuation values as a result of differing overshoot effects. These and other artifact influences make attenuation measurements taken from CT images unreliable for the assessment of pulmonary nodules. Area measurements made over a period of time may more reliably discriminate between benign and malignant nodules.

Quantification of endothelial permeability, leakage space, and blood volume in brain tumors using combined T1 and T2* contrast-enhanced dynamic MR imaging.

This study describes a method for imaging brain tumors that combines T1-weighted (T1W) and T2*-weighted (T2*W) dynamic contrast-enhanced acquisitions. Several technical improvements have been made to produce high-quality three-dimensional mapping of endothelial permeability surface area product (k) and leakage space (vl), based on T1W data. Tumor blood volume maps are obtained from T2*W images with a complete removal of residual relaxivity effects. The method was employed in 15 patients with brain tumors (5 gliomas, 5 meningioma, and 5 acoustic schwannoma). Mean values of vl were significantly greater in acoustic schwannomas (53% +/- 9%) than in meningiomas (34% +/- 7%) or gliomas (22% +/- 4%). Mean values of vl in meningioma were significantly greater than those of gliomas. Mean values of rCBV correlated closely with k. There was also a positive correlation between k and vl for pixels with low k values. This relationship was weaker in areas of high k. The highest mean ratios of k to vl (k(ep)) were seen in two patients with glioblastoma, one patient with transitional cell meningioma, and one patient with angioblastic meningioma. Pixel-by-pixel comparison showed a strong correlation between rCBV and k in 11 of 15 patients. However, decoupling between pixel-wise rCBV and k was found in four patients who had lesions with moderate k and vl elevation but no increase of rCBV. Results from this study suggest that in assessing the angiogenic activities in brain tumors it is advisable to monitor simultaneously changes in tumor blood volume, vessel permeability, and leakage space of tumor neovasculature.