[An experimental organ model for magnetic resonance imaging]. / Ein experimentelles Organmodell für die MR-Bildgebung.

The investigation of reperfusion of organs after an ischaemic phase is of great interest in transplantation medicine. This work presents an experimental organ model for the examination of isolated canine livers by means of magnetic resonance imaging in reperfusion experiments. The perfusion of the organs inside a perfusate container in the scanner's head coil was performed with approximately physiological conditions for 1.5 hours. The pumps for the perfusate circulation were installed outside the scanner's room, where oxygenation and heating of the perfusate was also performed. In the MR examinations, T1- and T2-weighted sequences could be used to display the morphology of the organs and the spatial perfusion distribution after administration of contrast media. The images revealed oedema development during reperfusion and an inhomogenous perfusion distribution. Thus, the model allows for the non-invasive investigation of morphology and perfusion distribution in isolated reperfused organs.

[STEAM-sequence with multi-echo-readout for static magnetic resonance elastography]. / STEAM-sequenz mit Multi-Echo-Auslese für die statische Magnetresonanz-Elastographie.

In static magnetic resonance elastography, the elasticity of an object is determined by measuring the internal displacement between two compression states. To reduce signal loss during the long time delay between application of external deformation and the static compression state, a STEAM sequence with a long mixing time is used This results in long scan times. The aim of this work was the development of a STEAM sequence with a multi-echo-readout, which allows the reduction of scan time and number of necessary external deformations. This new sequence was compared to the standard STEAM sequence on an agarose gel phantom with a hard inclusion. In addition, the elasticity of thermal tissue lesions was investigated, which were induced using high-intensity focused ultrasound (HIFU). During a given measurement time, more acquisitions per image can be taken using the multi-echo-readout. As a result the signal-to-noise ratio is increased and errors in the data become clearly smaller. Drawbacks of. the new sequence are its higher signal loss due to T2-decay and its greater sensitivity against ghosting artefacts caused by k-space segmentation. During the investigation of the thermally-induced lesions, a clear contrast in elasticity between normal tissue and the treated region was observed. Taking advantage of the greater accuracy of the new STEAM sequence, it was shown, that this contrast is significantly larger than the one in conventional MR parameters.

Quantification of aortic elasticity: development and experimental validation of a method using computed tomography.

Aortic distensibility depending on aortic cross-sectional area changes is an important parameter for the grading of vascular diseases. This study measured aortic area changes by multidetector computed tomography. An image reconstruction algorithm was developed to assess aorta diameter and area as a function of the cardiac cycle with sufficient time resolution along the entire length of the aorta by four-detector row computed tomography. The algorithm was tested on porcine aortic specimens and compared with an optical reference method. The error of the relative vessel area change comparing the two methods was found to be about 3%. Initial tests on patient datasets indicate that clinical application is feasible. The proposed method has the advantage that it can easily be integrated into a modified routine CT angiography study and allows the measurement of aortic cross-sectional area changes.

Multi-detector row computed tomography of the heart: does a multi-segment reconstruction algorithm improve left ventricular volume measurements?

A multi-segment cardiac image reconstruction algorithm in multi-detector row computed tomography (MDCT) was evaluated regarding temporal resolution and determination of left ventricular (LV) volumes and global LV function. MDCT and cine magnetic resonance (CMR) imaging were performed in 12 patients with known or suspected coronary artery disease. Patients gave informed written consent for the MDCT and the CMR exam. MDCT data were reconstructed using the standard adaptive cardiac volume (ACV) algorithm as well as a multi-segment algorithm utilizing data from three, five and seven rotations. LV end-diastolic (LV-EDV) and end-systolic volumes and ejection fraction (LV-EF) were determined from short-axis image reformations and compared to CMR data. Mean temporal resolution achieved was 192+/-24 ms using the ACV algorithm and improved significantly utilizing the three, five and seven data segments to 139+/-12, 113+/-13 and 96+/-11 ms (P<0.001 for each). Mean LV-EDV was without significant differences using the ACV algorithm, the multi-segment approach and CMR imaging. Despite improved temporal resolution with multi-segment image reconstruction, end-systolic volumes were less accurately measured (mean differences 3.9+/-11.8 ml to 8.1+/-13.9 ml), resulting in a consistent underestimation of LV-EF by 2.3-5.4% in comparison to CMR imaging (Bland-Altman analysis). Multi-segment image reconstruction improves temporal resolution compared to the standard ACV algorithm, but this does not result in a benefit for determination of LV volume and function.

Comparison of spatial resolution in high-resolution multislice computed tomography and digital subtraction angiography using renal specimens.

OBJECTIVE: To compare maximum spatial resolution in multislice computed tomography (MS-CT) and digital subtraction angiography (DSA) using an arterial tree model for canine kidney specimens. METHODS: Twenty-three isolated fox terrier kidneys were catheterized with a 4F catheter and underwent contrast-enhanced MS-CT with a maximum spatial resolution of 0.23-mm isotropic voxel size in an early arterial phase. In addition, a digital subtraction angiogram was performed on all kidneys. The kidneys were segmented semiautomatically, and each parenchymal vessel that was identified in the kidney was marked. The maximum intensity projections of arterial vessels in the CT datasets were evaluated in a comparison with the DSA datasets. RESULTS: No significant difference in vessel delineation and count was found at any level up to the fourth level of intrarenal branching. CONCLUSION: MS-CT has the potential of replacing DSA in the diagnosis of intrarenal arteries.

Determination of aortic compliance from magnetic resonance images using an automatic active contour model.

The possibility of monitoring changes in aortic elasticity in humans has important applications for clinical trials because it estimates the efficacy of plaque-reducing therapies. The elasticity is usually quantified by compliance measurements. Therefore, the relative temporal change in the vessel cross-sectional area throughout the cardiac cycle has to be determined. In this work we determined and compared the compliance between three magnetic resonance (MR) methods (FLASH, TrueFISP and pulse-wave). Since manual outlining of the aortic wall area is a very time-consuming process and depends on an operator's variability, an algorithm for the automatic segmentation of the artery wall from MR images through the entire heart cycle is presented. The reliable detection of the artery cross-sectional area over the whole heart cycle was possible with a relative error of about 1%. Optimizing the temporal resolution to 60 ms we obtained a relative error in compliance of about 7% from TrueFISP (1.0 x 1.0 x 10 mm3, signal-to-noise ratio (SNR) > 12) and FLASH (0.7 x 0.7 x 10 mm3, SNR > 12) measurements in volunteers. Pulse-wave measurements yielded an error of more than 9%. In a study of ten volunteers, a compliance between C = 3 x 10(-5) Pa(-1) and C = 8 x 10(-5) Pa(-1) was determined, depending on age. The results of the TrueFISP and the pulse-wave measurements agreed very well with one another (confidence interval of 1 x 10(-5) Pa(-1)) while the results of the FLASH method more clearly deviated from the TrueFISP and pulse-wave (confidence interval of more than 2 x 10(-5) Pa(-1)).

In vivo validation of cardiac spiral computed tomography using retrospective gating.

BACKGROUND: Cardiac functional assessment represents the basis for diagnostics and cardiac operation planning. Spiral computed tomography (CT) combines the advantages of three-dimensional imaging and high temporal resolution when using gating techniques. However, in vivo validation data of this novel imaging technology are lacking. The purpose of this study was to validate in vivo the new imaging method using retrospective gating and to evaluate the clinical usefulness of the achieved temporal resolution. METHODS: In domestic pigs (n = 10, weight 35 to 40 kg) a flowmeter was placed surgically on the ascending aorta. Flow velocity integrated over systole served as the gold standard for left ventricular (LV) stroke volume (LVSV-FM). CT signal, projection data, pacemaker signal, and flow velocity were recorded simultaneously at constant heart rate (pacemaker, 90 beats per minute). End-systolic and end-diastolic frames were calculated by retrospective gating. LV volumes were traced, the difference representing CT stroke volume (LVSV-CT). Image data were three-dimensionally reconstructed using ray-tracing. RESULTS: Temporal resolution was 170 ms. Correlation of stroke volumes was high (r = 0.94, mean difference 1.75 mL). Intraobserver (0.49 mL for LVEDV, 0.31 for LVESV) and interobserver variability (p = 0.21 and p = 0.06, respectively) were low. Postprocessing resulted in four-dimensional beating-heart models useful for operation planning. CONCLUSIONS: Spiral CT using retrospective gating was validated in vivo. Clinically acceptable temporal resolution and accuracy in determining cardiac stroke volumes were found. As a true volumetric imaging modality the method may now play an important role in computer-assisted diagnostics and surgery.