Kerstin M. Lagerstrand

Method to correct for the effects of limited spatial resolution in phase-contrast flow MRI measurements

Phase‐contrast (PC) magnetic resonance imaging (MRI) flow measurements suffer from the effect of the point spread function (PSF) due to the limited sampling of k‐space. The PSF, which in this case is a sinc function, deforms the flow profile and forms a ringing pattern around the vessel. In this work, an empirical method is presented that corrects for errors due to the deformation of the flow profile. The ringing pattern is used to obtain a well‐defined vessel segmentation, which after correction provides more accurate vessel radius and volume flow rate (VFR). The correction method was developed from phantom measurements at constant flow and applied on phantom measurements at moderately pulsatile flow. After correction, the error of the estimated tube radius and the VFR was less than 10% and 5%, respectively. Corresponding errors without correction overestimated the radius by 60% and the VFR by 35%. Preliminary results indicate that the method is also valid in vivo. The variation in the estimated radius and VFR for different spatial resolution decreased when the method was applied. The presented method gives a more accurate estimation of the radius and VFR in vessels of the size of a few pixels without prior knowledge about the true vessel radius. Magn Reson Med 48:883–889, 2002.

Axial loading during MRI influences T2-mapping values of lumbar discs: a feasibility study on patients with low back pain.

PurposeTo investigate whether axial loading of the spine during MRI (alMRI) instantaneously induces changes in biochemical disc features as reflected by altered quantitative T2 values in patients with chronic low back pain (LBP).MethodsT2 mapping was performed on 11 LBP patients (54 lumbar discs) during the conventional unloaded MRI and subsequent alMRI. Each disc was divided into five volumetric regions of interests (ROIs), anterior annulus fibrosus (AF) (ROI 1), the interface anterior AF-nucleus pulposus (NP) (ROI 2), NP (ROI 3), the interface NP-posterior AF (ROI 4), and the posterior AF (ROI 5). The mean T2 values for each ROI were compared between MRI and alMRI and correlated with degeneration grade (Pfirrmann), disc angle, and disc level.ResultsWith alMRI, T2 values increased significantly in the whole disc as well as in various parts of the disc with an increase in ROI 1–3 and a decrease in ROI 5. The changes in T2 values correlated to degeneration grade, changes in disc angle, and lumbar level.ConclusionalMRI instantaneously induces T2-value changes in lumbar discs and is, thus, a feasible method to reveal dynamic, biochemical disc features in patients with chronic LBP.

COPD Patients Have Short Lung Magnetic Resonance T1 Relaxation Time.

Abstract Magnetic resonance imaging (MRI) may provide attractive biomarkers for assessment of pulmonary disease in clinical trials as it is free from ionizing radiation, minimally invasive and allows regional information. The aim of this study was to characterize lung MRI T1 relaxation time as a biomarker of chronic obstructive pulmonary disease (COPD); and specifically its relationship to smoking history, computed tomography (CT), and pulmonary function test (PFT) measurements in comparison to healthy age-matched controls. Lung T1 and inter-quartile range (IQR) of T1 maps from 24 COPD subjects and 12 healthy age-matched non-smokers were retrospectively analyzed from an institutional review board approved study. The subjects underwent PFTs and two separate MR imaging sessions at 1.5 tesla to test T1 repeatability. CT scans were performed on the COPD subjects. T1 repeatability (intraclass correlation coefficient) was 0.72 for repeated scans acquired on two visits. The lung T1 was significantly shorter (p < 0.0001) and T1 IQR was significantly larger (p = 0.0002) for the COPD subjects compared to healthy controls. Lung T1 significantly (p = 0.001) correlated with lung density assessed with CT. Strong significant correlations (p < 0.0001) between lung T1 and all PFT measurements were observed. Cigarette exposure did not correlate with lung T1 in COPD subjects. In conclusion, lung MRI T1 mapping shows potential as a repeatable, radiation free, non-invasive imaging technique in the evaluation of COPD.

Quantitative phase-contrast flow MRI measurements in the presence of a second vessel closely positioned to the examined vessel

To examine the influence of the truncated sampling of k‐space data on the accuracy of phase‐contrast (PC) flow quantifications in the presence of nearby vessels.

T1 Relaxation Time in Lungs of Asymptomatic Smokers

Purpose Interest in using T1 as a potential MRI biomarker of chronic obstructive pulmonary disease (COPD) has recently increased. Since tobacco smoking is the major risk factor for development of COPD, the aim for this study was to examine whether tobacco smoking, pack-years (PY), influenced T1 of the lung parenchyma in asymptomatic current smokers. Materials and Methods Lung T1 measurements from 35 subjects, 23 never smokers and 12 current smokers were retrospectively analyzed from an institutional review board approved study. All 35 subjects underwent pulmonary function test (PFT) measurements and lung T1, with similar T1 measurement protocols. A backward linear model of T1 as a function of FEV1, FVC, weight, height, age and PY was tested. Results A significant correlation between lung T1 and PY was found with a negative slope of -3.2 ms/year (95% confidence interval [CI] [-5.8, -0.6], p = 0.02), when adjusted for age and height. Lung T1 shortens with ageing among all subjects, -4.0 ms/year (95%CI [-6.3, -1.7], p = 0.001), and among the never smokers, -3.7 ms/year (95%CI [-6.0, -1.3], p = 0.003). Conclusions A correlation between lung T1 and PY when adjusted for both age and height was found, and T1 of the lung shortens with ageing. Accordingly, PY and age can be significant confounding factors when T1 is used as a biomarker in lung MRI studies that must be taken into account to detect underlying patterns of disease.

Contrast agent influences MRI phase-contrast flow measurements in small vessels

Contrast‐enhanced MR angiography is often combined with phase contrast (PC) flow measurement to answer a particular clinical question. The contrast agent that is administered during contrast‐enhanced MR angiography may still be present in the blood during the consecutive PC flow measurement. The aim of this work was to evaluate the influence of contrast agent on PC flow measurements in small vessels. For that purpose, both in vivo measurements and computer simulations were performed. The dependence of the PC flow quantification on the signal amplitude difference between blood and stationary background tissue for various vessel sizes was characterized. Results show that the partial‐volume effect strongly affects the accuracy of the PC flow quantification when the imaged vessel is small compared to the spatial resolution. A higher blood‐to‐background‐contrast level during imaging significantly increases the partial‐volume effect and thereby reduces the accuracy of the flow quantification. On the other hand, a higher blood‐to‐background‐contrast level facilitated the segmentation of the vessel for flow rate determination. PC flow measurements should therefore be performed after contrast agent administration in large vessels, but before contrast agent administration in small vessels. Magn Reson Med, 2010.

Flow-induced disturbances in balanced steady-state free precession images: Means to reduce or exploit them

In this work computer simulations and phantom measurements are presented that show the effect of flow on in‐plane balanced steady‐state free precession images. The images were studied for various flow velocities, excitation regions, relaxation times, RF‐pulse angles, and off‐resonance frequencies. The work shows that flow‐induced disturbances are present in the images, but can be reduced by the application of inhomogeneous excitation regions. Also, a velocity quantification method that utilizes the disturbances was developed and proved to quantify flow velocities accurately. The work concluded that the flow‐induced disturbances can be reduced to improve image quality, but can also be exploited to quantify the flow velocity. Magn Reson Med, 2009.

Pulsed-Wave Doppler Recordings in the Proximal Descending Aorta in Patients with Chronic Aortic Regurgitation: Insights from Cardiovascular Magnetic Resonance

Background: The pulsed‐wave Doppler recording in the descending aorta (PWDDAO) is one of the parameters used in grading aortic regurgitation (AR) severity. The aim of the present study was to investigate the assessment of chronic AR by PWDDAO with insights from cardiovascular magnetic resonance (CMR). Methods: This prospective study comprised 40 patients investigated with echocardiography and CMR within 4 hours either prior to valve surgery (n = 23) or as part of their follow‐up (n = 17) due to moderate or severe AR. End‐diastolic flow velocity (EDFV) and the diastolic velocity time integral (dVTI) were measured. The appearance of diastolic forward flow (DFF) was noted. Phase‐contrast flow rate curves were obtained in the DAO. Results: Twenty‐five patients had severe and eight had moderate AR by echocardiography (seven were indeterminate). The EDFV was below the recommended threshold (>20 cm/sec) in 13 patients (52%) with severe AR. Lowering the EDFV threshold (>13 cm/sec) and with a dVTI threshold >13 cm showed negative likelihood ratios of 0.27 and 0.09, respectively. Detection of DFF with PWDDAO identified a nonuniform velocity profile by CMR with positive and negative likelihood ratios of 7.0 and 0.19, respectively. The relation between EDFV and DAO regurgitant volume (DAO‐RVolCMR) was strong in patients without (R = 0.88) and weak in patients with DFF (R = 0.49). The DAO‐RVolCMR as a percent of the total RVolCMR decreased with increasing ascending aorta (AAO) size and increased with increasing AR severity. Conclusions: Our findings suggest that PWDDAO provides semiquantitative parameters useful to assess chronic AR severity. The limitations are related to nonuniform velocity contour and variable degree of lower body contribution, which depends on AR severity but also on the AAO size.

Left ventricular volumes by echocardiography in chronic aortic and mitral regurgitations

ABSTRACT Objectives Cut-off values for left ventricular (LV) dimensions indicating severe valve regurgitation have not been defined. The aim of this study was to establish echocardiographic cut-off values for LV dimensions indicating severe chronic aortic (AR) or mitral (MR) regurgitation. Design The hemodynamic significance was confirmed by documented reduction of end-diastolic volume (EDV) and symptom relief after surgery. Eighty-three patients with moderate or severe regurgitation (AR, n = 41; MR, n = 42) without other cardiac conditions underwent prospectively two-dimensional (2DE), real-time three-dimensional (RT3DE) echocardiography and cardiovascular magnetic resonance (CMR) exams within 4 h. Results The relationship between EDVCMR and EDV2DE and EDVRT3DE were strong (R 0.95 and 0.91). EDV index cut-offs for 2DE/RT3DE >87/104 ml/m2 identified AR patients with severe regurgitation with a positive likelihood ratio (PLR) of 5.0/5.0. The corresponding in patients with MR EDV index cut-offs were >69/87 ml/m2 with a PLR of 14.9/5.5. LV linear dimensions could not identify patients with severe regurgitation. Conclusions LV volumes by echocardiography can support the diagnosis of severe chronic regurgitation. Importantly, other causes for LV enlargement have to be considered.

Adjustment of the velocity encoding parameter to the blood flow velocity is not necessary for accurate and precise quantification of aortic regurgitation severity with phase contrast magnetic resonance imaging

Background Accurate and precise quantification of aortic regurgitation (AVR) severity by cardiovascular magnetic resonance is essential for the clinical decission-making and timing of surgery. The regurgitant flow volume (RV) can be measured directly by 2D phase contrast (PC) velocity measurements. The velocity encoding parameter, venc, has been identified by others as an important factor for accurate and precise determination of RV. For large vessel and high signal to noise measurements, though, integration of the measured blood flow velocities over the vessel lumen and over the cardiac phases for calculation of RV should average out variations in the measured velocities and enable high precision estimates of RV independently of venc. Furthermore, application of a correction method that effectively reduces the background velocity offset in the PC image to a sufficiently low value should enable accurate estimation of RV and remove the venc dependency. The aim of the study was to demonstrate venc insensitivity in the estimated RV with effectively background offset corrected PC velocity measurements.