A methodology to study the morphologic changes in lesions during in vitro angioplasty using MRI and image processing.

The assessment of morphologic changes in atherosclerotic lesions during interventional procedures such as transluminal balloon angioplasty is an issue of highest clinical importance. We propose a methodology that allows realistic 3D morphomechanical modeling of the vessel, the plaque and the lumen at different stages of in vitro angioplasty. We elaborate on a novel device designed to guide angioplasty under controlled experimental conditions. The device allows to reproduce in vivo conditions as good as possible, i.e. axial in situ pre-stretch, 100mmHg intraluminal pressure, 37 degrees C Tyrode solution, balloon inflation without external constraints using a high-pressure syringe and contrast medium. With a standard 1.5T MR-system we accomplish multi-spectral images at different stages of the angioplasty experiment. After MR image acquisition the specimen is used for histopathological analysis and biomechanical tests. A segmentation process is used to generate NURBS-based 3D geometric models of the individual vessel and plaque components at different balloon pressures. Tissue components are segmented automatically using generalized gradient vector flow active contours. We investigated 10 human femoral arteries. The effects of balloon compression on the individual artery components is particularly described for two obstructed arteries with an intact collagenous cap, a pronounced lipid pool and with calcification. In both arteries we observe a significant increase in lumen area after angioplasty. Dissection between intima and media and reduction of the lipid pool are primary mechanisms of dilatation. This methodology provides a basis for studying plaque biomechanics under supra-physiological loading conditions. It has the potential to improve and validate finite element models of atherosclerotic plaques which may allow a better prediction of angioplasty procedures.

Minimizing macrovessel signal in cerebral perfusion imaging using independent component analysis.

The pronounced susceptibility effect of macrovessels in MR bolus-tracking studies induces spots of artificially high blood flow and volume in perfusion parameter images. These high-intensity regions impede the detection of perfusion changes and lead to elevated perfusion parameters in adjacent tissues. The purpose of this work was to explore postprocessing methods to reduce the influence of macrovessel signal in dynamic MRI. After data reduction was performed with the use of a principal component analysis (PCA), an independent component analysis (ICA) was applied to separate signal components of different compartments. Based on this decomposition, the dynamic time series were reconstructed with minimized contributions of macrovessel signal and noise. The influence of the temporal resolution and signal-to-noise ratio (SNR) of the source data were investigated by means of a simulation study. A region-of-interest (ROI)-based analysis of corrected and uncorrected in vivo data demonstrated that the influence of arteries and veins was reduced at least by 50%, while gray matter (GM) and white matter (WM) tissues were nearly unaffected by the correction process. Hemodynamic parameter images of the cerebral blood volume (CBV), cerebral blood flow (CBF), and mean transit time (MTT) were calculated from corrected and uncorrected scans. The corrected parameter images showed a clearly reduced macrovessel signal and an improved perceptibility of microvascular perfusion changes compared to the uncorrected ones.

T1 maps from shifted spin echoes and stimulated echoes.

A new pulse sequence for fast multislice T1 mapping is presented. This method is based on calculating T1 from spin echo (SE) and stimulated echo (STE) images obtained with different degrees of T1 weighting, and uses the interleaved acquisition scheme of the fast phase acquisition of composite echoes (FastPACE) technique. In contrast to the FastPACE technique, the two echoes are sampled separately. Experimental comparisons confirm that the new sequence layout overcomes most of the FastPACE restrictions, such as its motion sensitivity and the need for a fully complex data set. Moreover, this method offers a higher precision for long T1 values and a further reduction of acquisition time.

Improved diffusion-weighted single-shot echo-planar imaging (EPI) in stroke using sensitivity encoding (SENSE).

Diffusion-weighted single-shot EPI (sshEPI) is one of the most important tools for the diagnostic assessment of stroke patients, but it suffers from well known artifacts. Therefore, sshEPI was combined with SENSitivity Encoding (SENSE) to further increase EPI's potential for stroke imaging. Eight healthy volunteers and a consecutive series of patients (N = 8) with suspected stroke were examined with diffusion-weighted SENSE-sshEPI using different reduction factors (1.0 < or = R < or = 3.0). Additionally, a high-resolution diffusion-weighted SENSE-sshEPI scan was included. All examinations were diagnostic and of better quality than conventional sshEPI. No ghostings or aliasing artifacts were discernible, and EPI-related image distortions were markedly diminished. Chemical shift artifacts and eddy current-induced image warping were still present, although to a markedly smaller extent. Measured direction-dependent diffusion-coefficients and isotropic diffusion values were comparable to previous findings but showed less fluctuation. We have demonstrated the technical feasibility and clinical applicability of diffusion-weighted SENSE-sshEPI in patients with subacute stroke. Because of the faster k-space traversal, this novel technique is able to reduce typical EPI artifacts and increase spatial resolution while simultaneously remaining insensitive to bulk motion.

Assessing abdominal fatness with local bioimpedance analysis: basics and experimental findings.

OBJECTIVE: Abdominal fat is of major importance in terms of body fat distribution but is poorly reflected in conventional body impedance measurements. We developed a new technique for assessing the abdominal subcutaneous fat layer thickness (SFL) with single-frequency determination of the electrical impedance across the waist (SAI). SUBJECTS AND MEASUREMENTS: The method uses a tetrapolar arrangement of surface electrodes which are placed symmetrically to the umbilicus in a plane perpendicular to the body axis. Twenty-four test subjects (12 male, 12 female) underwent SAI and abdominal magnetic resonance imaging (MRI). The SFL below the sensing electrodes was determined from MRI and correlated with the SAI data at four different frequencies (5, 20, 50 and 204 kHz). RESULTS: A highly significant linear correlation (r2=0.99) between SFL and SAI over a wide range of the abdominal SFL was found. Separate regression models for female and male subjects did not differ significantly, except at 50 kHz. CONCLUSION: SAI represents a good predictor of the SFL and provides an excellent tool for the assessment of central obesity.

Source localization of induced cortical oscillations during tactile finger stimulation.

We investigated the EEG beta event-related synchronization (ERS) after tactile finger stimulation in three subjects. Prior studies from our group using electrical stimulation and self-paced movement showed a beta rebound within one second after stimulation respectively movement offset. As the tactile-stimulation-data showed a similar ERS behaviour, we extracted the cortical sources for this beta rebound by the linear estimation method in order to see whether the representation areas of different fingers were distinguishable (as is possible with MEG data). Although realistic head models of two subjects were used for the calculations the fingers could not be spatially distinguished. However, regarding the whole spatio-temporal pattern of the ERS for different fingers clear differences can be observed.

A comparison of magnetization transfer ratio, magnetization transfer rate, and the native relaxation time of water protons related to relapsing-remitting multiple sclerosis.

BACKGROUND AND PURPOSE: Magnetization transfer (MT) imaging and measurements of the magnetization transfer ratio (MTR) have extended our capability to depict and characterize pathologic changes associated with multiple sclerosis (MS). We wanted to investigate whether the analysis of other MT parameters, such as magnetization transfer rate (k(for)) and relative measure of water content (T1(free)), adds insight into MS-related tissue changes. METHODS: Quantitative MT imaging by use of phase acquisition of composite echoes was performed in nine patients with clinically definite relapsing-remitting MS and eight healthy control subjects on a 1.5-T MR system. We analyzed a total of 360 regions of interest and compared control white matter with various types of lesions and normal-appearing white matter in MS. RESULTS: We found a strong correlation between the MTR and k(for), but this relation was non-linear. A slight but significant reduction of the MTR in normal-appearing white matter of patients with MS was attributable to a reduced transfer rate only, whereas a lower MTR was associated with both a reduction of k(for) and an increase of T1(free) in regions of dirty white matter. Moreover, areas such as edema and T1-isointense lesions had a similar MTR but could be differentiated on the basis of Tl(free). CONCLUSION: Estimates of k(for) and T1(free) appear to complement MTR measurements for the understanding of MT changes that occur with different types of MS abnormalities in the brain.

Magnetic resonance diffusion tensor imaging for characterizing diffuse and focal white matter abnormalities in multiple sclerosis.

High-resolution diffusion tensor imaging (DTI) was performed in 14 patients with clinically definite multiple sclerosis (MS) and the trace of the diffusion tensor () and the fractional anisotropy (FA) were determined in normal appearing white matter (NAWM) and in different types of focal MS lesions. A small but significant increase of the in NAWM compared to control white matter ((840 +/- 85) x 10(-6) mm(2)/sec vs. (812 +/- 59) x 10(-6) mm(2)/sec; P < 0.01) was found. In addition, there was a significant decrease in the FA of normal-appearing regions containing well-defined white matter tracts, such as the genu of the internal capsule. In non-acute lesions, the of T(1)-hypointense areas was significantly higher than that of T(1)-isointense lesions ((1198 +/- 248) x 10(-6) mm(2)/sec vs. (1006 +/- 142) x 10(-6) mm(2)/sec; P < 0. 001), and there was a corresponding inverse relation of FA. Diffusion characteristics of active lesions with different enhancement patterns were also significantly different. DTI with a phase navigated interleaved echo planar imaging technique may be used to detect abnormalities of isotropic and anisotropic diffusion in the NAWM and selected fiber tracts of patients with MS throughout the entire brain, and it demonstrates substantial differences between various types of focal lesions.

Estimation of magnetization transfer rates from PACE experiments with pulsed RF saturation.

A new imaging method has been developed for estimating the magnetization transfer rate (MTR) in a biologic two-pool system such as the brain tissue. The transfer rate is calculated from the ratio of the MTR to T(1sat), where T(1sat) is the apparent longitudinal relaxation time under complete saturation of the macromolecular pool. MTR and T(1sat) maps were obtained with a phase acquisition of composite echo (PACE) technique combined with pulsed radiofrequency (RF) saturation. The influences of RF saturation power and frequency offset on quantitative results were investigated with phantom and in vivo measurements. In white matter of seven healthy volunteers we found a mean transfer rate of 1.5 sec(-1), where the highest transfer rate was found in the genu of the corpus callosum (k(f) = 1. 9 sec(-1)). It could be shown that conditions near to complete saturation can also be reached under common restrictions by the specific absorption rate.

Diffusion-weighted imaging of patients with subacute cerebral ischemia: comparison with conventional and contrast-enhanced MR imaging.

BACKGROUND AND PURPOSE: The importance of diffusion-weighted imaging (DWI) for delineating acute ischemic lesions has been investigated extensively; however, few studies have investigated the role of DWI in the subacute stage of stroke. Because these lesions tend to appear bright throughout the first days of ischemia, owing to restricted diffusion, we speculated that DWI could also improve the detection of subacute infarcts as compared with conventional and contrast-enhanced MR imaging. METHODS: Interleaved echo-planar DWI with phase navigation was performed on a 1.5-T MR unit in a consecutive series of 53 patients (mean age, 66 +/- 14 years) with suspected recent cerebral ischemia. The interval between onset of clinical symptoms and MR imaging ranged from 1 to 14 days (mean, 6 +/- 4 days). Contrast material was given to 28 patients in a dose of 0.1 mmol/kg. RESULTS: DWI clearly delineated recent ischemic damage in 39 patients (74%) as compared with 33 (62%) in whom lesions were identified or suspected on conventional T2-weighted images. DWI provided information not accessible with T2-weighted imaging in 17 patients when evidence of lesion multiplicity or detection of clinically unrelated recent lesions was included for comparison. Subacute ischemic lesions were also seen more frequently on DWI sequences than on contrast-enhanced images (20 versus 13 patients). DWI was more likely to make a diagnostic contribution in the first week of stroke and in patients with small lesions or preexisting ischemic cerebral damage than was conventional MR imaging. CONCLUSION: Recent ischemic damage is better shown on DWI sequences than on conventional and contrast-enhanced MR images throughout the first days after stroke and may provide further information about the origin of clinical symptoms. Adding DWI to imaging protocols for patients with subacute cerebral ischemia is recommended.

Novel imaging technologies in the assessment of cerebral ageing and vascular dementia.

Introduction of magnetic resonance imaging (MRI) has opened new possibilities for detecting age-related brain tissue changes. The majority of these abnormalities consists of hyperintense foci in the deep and subcortical white matter probably related to microvascular disturbances and of signal hyperintensities around the lateral ventricles. It has also been suggested that these abnormalities may contribute to the development of cognitive impairment. The correlation between age-related signal abnormalities on conventional MRI and neuropsychologic dysfunction is limited, however, and a threshold beyond which such a relation may come into existence has not yet been defined. Poor tissue characterisation by conventional MRI may be one explanation. Therefore, new pulse sequences are expected not only to provide a higher lesion contrast such as the fluid attenuated inversion recovery (FLAIR) technique but also to offer new insights concerning the composition of incidental brain lesions. In this context both magnetisation transfer imaging (MTI) and diffusion weighted imaging (DWI) may serve to gain information about the integrity of cell membranes and organelles and the preservation of axons and fibre tracts. We will review the technical background of these recently developed MR sequences and their first applications to age-associated brain abnormalities.

Diffusion-weighted MR imaging of the spinal cord.

Diffusion-weighted MR imaging may increase the sensitivity and specificity of MR imaging for certain pathologic conditions of the spinal cord but is rarely performed because of several technical issues. We therefore tested a novel phase-navigated spin-echo diffusion-weighted interleaved echo-planar imaging sequence in seven healthy volunteers and six patients with intramedullary lesions. We performed diffusion-weighted MR imaging of the spinal cord with high spatial resolution. Different patterns of diffusion abnormalities observed in patient studies support the possible diagnostic impact of diffusion-weighted MR imaging for diseases of the spinal cord.

Fast multislice T(1) and t(1sat) imaging using a phase acquisition of composite echoes (PACE) technique.

An optimized multislice data acquisition scheme for phase acquisition of composite echoes (PACE) imaging is presented. This new scheme uses a repetition time equal to the mixing time and an appropriate phase cycling scheme, which allows for more efficient exploitation of all composite echoes. These modifications provide true multislice capability in parallel to a reduction of the acquisition time by a factor >2 compared with the original PACE method. Moreover, T(1) values can be obtained directly from phase images without the use of a lookup table. Because of the symmetrical application of the radio frequency pulses, this method is also well suited for T(1sat) imaging. Phantom studies showed a significantly better accuracy of the multislice fast PACE technique compared with a conventional two-point method in multislice acquisition mode, although precision was limited at high T(1) values. T(1) and T(1sat) measurements in brain tissue of eight healthy volunteers confirmed the stability of the fast PACE technique in a clinical setting. Magn Reson Med 42:1089-1097, 1999.

Diffusion-weighted imaging with navigated interleaved echo-planar imaging and a conventional gradient system.

PURPOSE: To demonstrate the technical feasibility and precision of a navigated diffusion-weighted (DW) MR imaging method with interleaved echo-planar imaging and test its diagnostic sensitivity for detection of ischemic stroke. MATERIALS AND METHODS: Apparent diffusion coefficient (ADC) measurements were performed in phantoms, and six healthy adult volunteers were examined to determine intrasubject (precision) and intersubject (reference range) variations in absolute ADC and relative ADC (rADC) measurements. DW imaging maps and lesion rADC values were also obtained in 34 consecutive stroke patients to evaluate the sensitivity and reliability of DW-interleaved echo-planar imaging for detection of ischemic brain damage. RESULTS: Phantom and volunteer ADC values were in excellent agreement with published data. The intrasubject variation of rADC was 6.2%. The ADC precision ranged from 6.5% in the subcortical white matter in the frontal lobe to 12.9% in the head of the caudate nucleus. Interleaved echo-planar imaging enabled rapid acquisition of high-quality images of the entire brain without substantial artifacts. Within the 1st week, the sensitivity of DW-interleaved echo-planar imaging for detection of acute infarction was 90% (18 of 20 true-positive studies) and independent of lesion location. CONCLUSION: DW-interleaved echo-planar imaging with phase navigation and cardiac triggering is robust, reliable, and fast. With high sensitivity for detection of early ischemic infarction, it is useful for examining stroke patients by using MR systems with conventional gradient hardware.

T1 imaging using phase acquisition of composite echoes.

A new magnetic resonance method for T1 imaging is presented. It is based on the simultaneous acquisition of a spin echo and a phase-shifted, stimulated echo. The phase of this composite echo image is modulated heavily by the spin-lattice relaxation time T1, whereas spin density and T2 do not influence the phase information. This phase information can be assigned easily to corresponding T1 values. Although this method is sensitive to radiofrequency inhomogenities, phantom studies showed that high precision and reasonable accuracy of T1 determination can be achieved. Moreover, this method was found to be very robust, because no fitting is required. T1 imaging of the human brain was performed to confirm the in vivo utility of this method.

Forearm composition contributes to differences in reactive hyperaemia between healthy men and women.

BACKGROUND: Post-ischaemic reactive hyperaemia in the forearm has been suggested as a marker of resistance vessel function. The contribution of forearm composition to the kinetics of reactive hyperaemia is largely unknown. The body composition of men and women differs in that women have a higher body fat content and less lean body mass. METHODS: In the present study, we investigated whether the kinetics of reactive hyperaemia in the forearm in 14 healthy subjects (seven men and seven women) show gender-specific differences and whether forearm composition contributes to such differences. RESULTS: Peak reactive hyperaemic flow as well as 1-min-flow debt repayment (measured by venous occlusion plethysmography) were significantly higher in male than in female study participants. This difference was explained to > 60% by gender-specific differences in forearm relative muscle mass (as determined by magnetic resonance imaging). The half-life of the reactive hyperaemic response, on the other hand, was not different between men and women and did not show an association with forearm muscle. CONCLUSION: Our results demonstrate that forearm composition must be considered if peak reactive hyperaemic or flow debt repayment is used as a target, and that dynamic measurements of the reactive hyperaemic process are more suitable to describe the function of resistance arteries than single-point observations.

Temperature monitoring of interstitial thermal tissue coagulation using MR phase images.

The temperature-dependent water proton frequency shift was investigated for temperature monitoring of interstitial thermal coagulation. A procedure for on-line temperature calculation was developed, and errors due to temperature-dependent susceptibility were investigated by finite element analysis and reference measurements. The temperature coefficient of magnetic susceptibility and proton chemical shift were determined for brain tissue and other substances. With the proposed procedure, the location of isotherms could be well visualized during laser-induced interstitial coagulation in vitro and in vivo. Systematic errors caused by magnetic susceptibility changes with temperature depend strongly on the characteristics of the heat source and can exceed susceptibility effects caused by physiologic tissue changes. For the laser applicators discussed here, however, a first order compensation for this effect was found to be satisfactory, because it reduces the absolute error to the range of +/- 1 degrees C. The proposed method represents a very promising approach for monitoring of the interstitial thermal coagulation.