Peter A. Rinck

Feature extraction and classification of dynamic contrast-enhanced T2*-weighted breast image data

The relatively low specificity of dynamic contrast-enhanced T1-weighted magnetic resonance imaging (MR) imaging of breast cancer has lead several groups to investigate different approaches to data acquisition, one of them being the use of rapid T2*-weighted imaging. Analyses of such data are difficult due to susceptibility artifacts and breathing motion. One-hundred-twenty-seven patients with breast tumors underwent MR examination with rapid, single-slice T2*-weighted imaging of the tumor. Different methods for classifying the image data set using leave-one-out cross validation were tested. Furthermore, a semi-automatic region of interest (ROI) definition tool was presented and compared with manual ROI definitions from a previous study. Finally, pixel-by-pixel analysis was done and compared with ROI analysis. The analyses were done with and without noise reduction. The minimum enhancement parameter was the most robust and accurate of the parameters tested. The semi-automatic ROI definition method was fast and produced similar results as the manually defined ROIs. Noise reduction improved both sensitivity and specificity, but the improvement was not statistically significant. The pixel-based analysis methods used in the present study did not improve classification results. In conclusion, analysis of T2*-weighted breast images can be done in a rapid and robust manner by using semi-automatic ROI definition tools in combination with noise reduction. Minimum enhancement gives an indication of malignancy in T2*-weighted imaging.

Use of the mean transit time of an intravascular contrast agent as an exchange-insensitive index of myocardial perfusion

A simple two‐compartment model was used to study the effects of water exchange on the signal produced by an inversion recovery prepared rapid gradient‐echo sequence during the first passage of a low dose of an intravascular contrast agent. Water exchange at intermediate rates of exchange (1–10 Hz) between the vascular and extravascular spaces caused the form of the signal changes during the first pass to be dependent on both the fractional sizes of the vascular and extravascular compartments and on the exchange rate. Unless the effects of exchange are minimized by using a very short inversion time, parameters such as the peak height and area under the curve will be affected by regional and/or pathological variations in the exchange rate and the size of the vascular fraction. The mean transit time (MTT) is, however, less affected by water exchange. Experimental first‐pass data produced by intravascular low‐dose injections of iron oxide particles were studied in five pigs at 0.5 T. The MTT as derived from the first‐pass curves, without deconvolution with the arterial input function, was well correlated with the myocardial blood flow (MBF) as measured using radioactive microspheres (r = 0.70, n = 52, P < 0.01). Other first‐pass parameters such as the peak height or area under the curve exhibited either a poorer, or no, correlation with the MBF. The data suggest that the MTT of the first pass of an intravascular contrast agent may be a robust, quantitative method for assessing myocardial blood flow in patients. J. Magn. Reson. Imaging 1999;9:402–408.

Analysis of dynamic magnetic resonance images

Dynamic magnetic resonance (MR) imaging with contrast agents is a very promising technique for studying tissue perfusion in vivo. A temporal series of magnetic resonance images of the same slice are acquired following the injection of a contrast agent into the blood stream. The image intensity depends on the local concentration of the contrast agent, so that tissue perfusion can be studied by the image series. A new method of analyzing such series is described here. Nonparametric linear regression is used for modeling the image intensity along the series on a pixel by pixel basis. After modeling, some relevant quantities describing the time series are obtained and displayed as images. Due to its flexibility, this approach is preferred to parametric modeling when pathology is present since this can induce a wide spread of patterns for the pixel image intensity along time. Results of the application of the method to series of dynamic magnetic resonance images from ischaemic rat brains after the injection of the susceptibility agent Sprodiamide Inj. (Dy-DTPA-BMA) are shown and compared to results from a related known method.

Approach to equilibrium in snapshot imaging

The snapshot FLASH sequence uses a subsecond scan time and a small flip angle in conjunction with nonsteady-state acquisition to produce high-contrast images with minimum motion artifacts. The magnetisation evolves towards an equilibrium state in the course of a scan and it is the form of this approach to equilibrium which determines the contrast and signal-to-noise ratio (SNR) obtained. The contrast obtained is strongly dependent on the phase encoding scheme used. If the flip angle is increased, and the resulting transverse coherences refocused, then the SNR is improved while the contrast is little changed.

Relationship between function and perfusion early after acute myocardial infarction

To assess the relationship between baseline left ventricle function, functional reserve and resting myocardial perfusion in patients with acute myocardial infarction (AMI). After AMI the presence of dysfunctioning but viable myocardium plays a determinant role in clinical outcome. Regional ventricular function was evaluated by echocardiography both in resting conditions and during dobutamine infusion (10 μg/kg/min). Perfusion was assessed by magnetic resonance imaging in a single slice approach where the first pass of an intravenously injected bolus of gadolinium-based contrast agent was followed through six regions of interest within the myocardium. In each patient a region with normal function was used as reference and the cross-correlation coefficient (CCC), which described the myocardial perfusion relatively to the reference region (CCC = 1 means equivalent perfusion), was obtained for the other five myocardial regions. Twenty-two patients were enrolled into the study. Sixty-one segments had normal function and normal perfusion (CCC = 0.92 ± 0.23). The perfusion deficit was more marked in the 29 regions with resting akinesia–dyskinesia than in the 20 hypokinetic regions (CCC = 0.71 ± 0.45 vs. 0.84 ± 0.23; p < 0.05). Out of the 29 regions with resting akinesia–dyskinesia the 13 segments which showed functional improvement following dobutamine had a higher resting perfusion than the 16 segments which were unresponsive to dobutamine (CCC = 0.83 ± 0.32 vs. 0.61 ± 0.52, p < 0.05). Similarly, out of the 20 regions with resting hypokinesia the 11 segments having functional reserve showed an higher resting perfusion than the segments which did not (0.96 ± 0.21 vs. 0.69 ± 0.19; p < 0.05). Early after AMI, the perfusion deficit reflects the severity of the mechanical dysfunction. In regions with baseline dyssynergy resting perfusion is, in general, higher when contractile reserve can be elicited by stress-echo.

Correction to Analysis of Dynamic Magnetic Resonance Images

The value 79.2 would then be added to the output array at location x = 10, y = 20, with the other values added to the corresponding surrounding pixels. 4) If multiple slices are being rotated to the same angle, the relative contributions of each origin pixel to the destination image will be independent of the count distribution in the origin image. This allows some of the computational cost to be spread over multiple slices.