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Determination of Peak Velocity in Stenotic Areas: Echocardiography versus k-t SENSE Accelerated MR Fourier Velocity Encoding¹

¹ From the Institute for Biomedical Engineering, University and Swiss Federal Institute of Technology Zurich, Wolfgang-Pauli-Str 10, CH-8093 Zurich, Switzerland (C.B., S.K., P.B.); Division of Imaging Sciences, Guy's Hospital, King's College, London, England (M.S.H., R.R.); Novartis Institutes for BioMedical Research, Cambridge, Mass (J.T.); and MR-Centre, Aarhus University Hospital, Skejby, Aarhus, Denmark (E.M.P.). Received August 7, 2006; revision requested October 13; revision received December 22; accepted January 23, 2007; final version accepted April 2. Supported by the Strategic Excellence Project Life Sciences (grant TH7/02-2) of the Swiss Federal Institute of Technology Zurich, the Danish Heart Foundation (grant #02-2-3-43-22021), and Philips Medical Systems, Best, the Netherlands. Address correspondence to C.B. (e-mail: baltes{at}biomed.ee.ethz.ch).

View larger version (33K): [in this window] [in a new window] [Download PPT slide]   Figure 1: Images illustrate information content of four-dimensional MR FVE data set (two spatial dimensions and one temporal and one velocity dimension). Left: Cross-sectional view of ascending aorta at peak systole shows lowest velocity range from –18.75 to +18.75 cm/sec encoded by using FVE (16 FVE steps; encoding velocity, 150 cm/sec), corresponding to static and slow-moving tissues. Top and middle right: Systolic and diastolic v-y spaces generated by calculating the maximum intensity projection (MIP) in the x direction show the velocity spectrum in the aorta (moving blood) in comparison to the spectrum of the surrounding static tissues. Bottom right: The v-t space of area in left image encircled by black dashed line after calculation of the maximum intensity projection in x and y directions. The v-t space illustrates the temporal evolution of the velocity spectrum, similar to the data provided by Doppler US.

View larger version (24K): [in this window] [in a new window] [Download PPT slide]   Figure 2: Flowchart summarizes k-t SENSE reconstruction applied to accelerate FVE data collection. The acquisition consists of two stages: a high-spatial-resolution undersampling stage (left) and a low-spatial-resolution fully sampled training stage (right). Top: Optimal sampling pattern (18) at eightfold acceleration is shown for an example kv-ky-t space (kv = 16, ky = 56, t = 32; 62 training profiles per cardiac phase), where an elliptic shutter is applied to further reduce data acquisition. The ky lines are evenly skipped in the undersampling stage but not in the training stage. Middle: Corresponding v-y spaces. The undersampled data set (left) exhibits aliasing, while the training data set (right) does not. Bottom: In the k-t SENSE reconstruction, the low-resolution training data are used in conjunction with the encoding capabilities of the coil array to resolve aliasing of the undersampled data, yielding an aliasing-free high-resolution reconstruction (v-y spaces).

View larger version (26K): [in this window] [in a new window] [Download PPT slide]   Figure 3: Phantom setup used to measure peak velocities in a tube (tube diameter [dTube ], 20 mm) with stenoses of different diameters (dStenosis); 1 – (dStenosis/dTube): 75% and 90%. At the inlet, pulsatile flow was generated, resulting in peak velocities of 150–590 cm/sec. Peak velocities were detected by using conventional MR phase-contrast (PC) velocity mapping and nonaccelerated, eightfold-accelerated, and 16-fold–accelerated k-t SENSE MR FVE data and were compared with values at Doppler US.

View larger version (11K): [in this window] [in a new window] [Download PPT slide]   Figure 4a: Graphs show results of Bland and Altman comparison of relative differences (Rel. Diff.) among in vitro results. (a) Peak velocities detected by using nonaccelerated MR FVE (FVE ref) underestimated the Doppler US values only slightly. (b–d) Accelerated MR FVE and conventional phase-contrast (PC) MR imaging led to a marginal increase in this underestimation at a spatial resolution of 0.9 x 0.9 mm2. (e–g) At a lower spatial resolution of 2.3 x 2.3 mm2, accelerated MR FVE still yielded only moderate underestimation, whereas phase-contrast velocity mapping resulted in substantial underestimation. 8xFVE = eightfold-accelerated FVE, 16xFVE = 16-fold–accelerated FVE, SD = standard deviation.

View larger version (11K): [in this window] [in a new window] [Download PPT slide]   Figure 4b: Graphs show results of Bland and Altman comparison of relative differences (Rel. Diff.) among in vitro results. (a) Peak velocities detected by using nonaccelerated MR FVE (FVE ref) underestimated the Doppler US values only slightly. (b–d) Accelerated MR FVE and conventional phase-contrast (PC) MR imaging led to a marginal increase in this underestimation at a spatial resolution of 0.9 x 0.9 mm2. (e–g) At a lower spatial resolution of 2.3 x 2.3 mm2, accelerated MR FVE still yielded only moderate underestimation, whereas phase-contrast velocity mapping resulted in substantial underestimation. 8xFVE = eightfold-accelerated FVE, 16xFVE = 16-fold–accelerated FVE, SD = standard deviation.

View larger version (11K): [in this window] [in a new window] [Download PPT slide]   Figure 4c: Graphs show results of Bland and Altman comparison of relative differences (Rel. Diff.) among in vitro results. (a) Peak velocities detected by using nonaccelerated MR FVE (FVE ref) underestimated the Doppler US values only slightly. (b–d) Accelerated MR FVE and conventional phase-contrast (PC) MR imaging led to a marginal increase in this underestimation at a spatial resolution of 0.9 x 0.9 mm2. (e–g) At a lower spatial resolution of 2.3 x 2.3 mm2, accelerated MR FVE still yielded only moderate underestimation, whereas phase-contrast velocity mapping resulted in substantial underestimation. 8xFVE = eightfold-accelerated FVE, 16xFVE = 16-fold–accelerated FVE, SD = standard deviation.

View larger version (11K): [in this window] [in a new window] [Download PPT slide]   Figure 4d: Graphs show results of Bland and Altman comparison of relative differences (Rel. Diff.) among in vitro results. (a) Peak velocities detected by using nonaccelerated MR FVE (FVE ref) underestimated the Doppler US values only slightly. (b–d) Accelerated MR FVE and conventional phase-contrast (PC) MR imaging led to a marginal increase in this underestimation at a spatial resolution of 0.9 x 0.9 mm2. (e–g) At a lower spatial resolution of 2.3 x 2.3 mm2, accelerated MR FVE still yielded only moderate underestimation, whereas phase-contrast velocity mapping resulted in substantial underestimation. 8xFVE = eightfold-accelerated FVE, 16xFVE = 16-fold–accelerated FVE, SD = standard deviation.

View larger version (11K): [in this window] [in a new window] [Download PPT slide]   Figure 4e: Graphs show results of Bland and Altman comparison of relative differences (Rel. Diff.) among in vitro results. (a) Peak velocities detected by using nonaccelerated MR FVE (FVE ref) underestimated the Doppler US values only slightly. (b–d) Accelerated MR FVE and conventional phase-contrast (PC) MR imaging led to a marginal increase in this underestimation at a spatial resolution of 0.9 x 0.9 mm2. (e–g) At a lower spatial resolution of 2.3 x 2.3 mm2, accelerated MR FVE still yielded only moderate underestimation, whereas phase-contrast velocity mapping resulted in substantial underestimation. 8xFVE = eightfold-accelerated FVE, 16xFVE = 16-fold–accelerated FVE, SD = standard deviation.

View larger version (11K): [in this window] [in a new window] [Download PPT slide]   Figure 4f: Graphs show results of Bland and Altman comparison of relative differences (Rel. Diff.) among in vitro results. (a) Peak velocities detected by using nonaccelerated MR FVE (FVE ref) underestimated the Doppler US values only slightly. (b–d) Accelerated MR FVE and conventional phase-contrast (PC) MR imaging led to a marginal increase in this underestimation at a spatial resolution of 0.9 x 0.9 mm2. (e–g) At a lower spatial resolution of 2.3 x 2.3 mm2, accelerated MR FVE still yielded only moderate underestimation, whereas phase-contrast velocity mapping resulted in substantial underestimation. 8xFVE = eightfold-accelerated FVE, 16xFVE = 16-fold–accelerated FVE, SD = standard deviation.

View larger version (11K): [in this window] [in a new window] [Download PPT slide]   Figure 4g: Graphs show results of Bland and Altman comparison of relative differences (Rel. Diff.) among in vitro results. (a) Peak velocities detected by using nonaccelerated MR FVE (FVE ref) underestimated the Doppler US values only slightly. (b–d) Accelerated MR FVE and conventional phase-contrast (PC) MR imaging led to a marginal increase in this underestimation at a spatial resolution of 0.9 x 0.9 mm2. (e–g) At a lower spatial resolution of 2.3 x 2.3 mm2, accelerated MR FVE still yielded only moderate underestimation, whereas phase-contrast velocity mapping resulted in substantial underestimation. 8xFVE = eightfold-accelerated FVE, 16xFVE = 16-fold–accelerated FVE, SD = standard deviation.

View larger version (52K): [in this window] [in a new window] [Download PPT slide]   Figure 5: Representative FVE data (spatial resolution, 2.7 x 2.7 x 5 mm3; 6.1/3.1; flip angle, 15°; bandwidth, 65 kHz; number of FVE steps, 16; encoding velocity, 180 cm/sec; number of cardiac phases, 32) acquired in healthy volunteer by using eightfold acceleration (8x) (breath-hold duration, 18 seconds for high-spatial-resolution data acquisition and 9 seconds for training data acquisition) and 16-fold acceleration (16 x) (breath-hold duration, 20 seconds). Top: Transverse views during (left) systole (130 and 140 msec after R wave) and (right) diastole (619 and 674 msec after R wave) at lowest velocity range, from vFVE = –22.5 to +22.5 cm/sec. Aorta (arrows) appears dark during systole (high velocities) and bright during diastole (low velocities), allowing the easy segmentation required for peak velocity determination. The quality of the FVE data can be inspected from the corresponding v-y (second row) and v-t (third row) spaces, which show the velocity spectrum over space and the change of the velocity spectrum over time, respectively. Bottom: Graph shows results of comparison of peak velocities over time at Doppler US and accelerated MR FVE imaging.

View larger version (80K): [in this window] [in a new window] [Download PPT slide]   Figure 6a: Images in patient with pulmonary stenosis. (a) Systolic frames of cine balanced steady-state free precession scout image acquired in coronal (top) and sagittal (bottom) orientations of pulmonary artery. Imaging plane (dashed line) for subsequent velocity measurements with phase-contrast MR imaging and accelerated MR FVE was perpendicular to flow jet (arrows). (b) Sixteen-fold–accelerated FVE data (spatial resolution, 2.0 x 2.0 x 5 mm3; 4.6/2.3; flip angle: 15°; bandwidth, 65 kHz; number of FVE steps, 16; encoding velocity, 450 cm/sec; number of cardiac phases, 32; breath-hold duration, 21 seconds for high-spatial-resolution data acquisition and 9 seconds for training data acquisition). Images on right are systolic (160 msec) and diastolic (762) frames in transverse orientation (magnified from image on left), with  vFVE  less than 56.25 cm/sec. Corresponding v-y (middle) and v-t (bottom) spaces are shown. Arrows indicate spatial extension of flow jet during systole. The resulting high-velocity spectrum is clearly visible in both the systolic v-y space and the v-t space. (c) Doppler US image shows peak velocities over time; lines and data points show peak velocities over time at accelerated MR FVE imaging.

View larger version (41K): [in this window] [in a new window] [Download PPT slide]   Figure 6b: Images in patient with pulmonary stenosis. (a) Systolic frames of cine balanced steady-state free precession scout image acquired in coronal (top) and sagittal (bottom) orientations of pulmonary artery. Imaging plane (dashed line) for subsequent velocity measurements with phase-contrast MR imaging and accelerated MR FVE was perpendicular to flow jet (arrows). (b) Sixteen-fold–accelerated FVE data (spatial resolution, 2.0 x 2.0 x 5 mm3; 4.6/2.3; flip angle: 15°; bandwidth, 65 kHz; number of FVE steps, 16; encoding velocity, 450 cm/sec; number of cardiac phases, 32; breath-hold duration, 21 seconds for high-spatial-resolution data acquisition and 9 seconds for training data acquisition). Images on right are systolic (160 msec) and diastolic (762) frames in transverse orientation (magnified from image on left), with  vFVE  less than 56.25 cm/sec. Corresponding v-y (middle) and v-t (bottom) spaces are shown. Arrows indicate spatial extension of flow jet during systole. The resulting high-velocity spectrum is clearly visible in both the systolic v-y space and the v-t space. (c) Doppler US image shows peak velocities over time; lines and data points show peak velocities over time at accelerated MR FVE imaging.

View larger version (52K): [in this window] [in a new window] [Download PPT slide]   Figure 6c: Images in patient with pulmonary stenosis. (a) Systolic frames of cine balanced steady-state free precession scout image acquired in coronal (top) and sagittal (bottom) orientations of pulmonary artery. Imaging plane (dashed line) for subsequent velocity measurements with phase-contrast MR imaging and accelerated MR FVE was perpendicular to flow jet (arrows). (b) Sixteen-fold–accelerated FVE data (spatial resolution, 2.0 x 2.0 x 5 mm3; 4.6/2.3; flip angle: 15°; bandwidth, 65 kHz; number of FVE steps, 16; encoding velocity, 450 cm/sec; number of cardiac phases, 32; breath-hold duration, 21 seconds for high-spatial-resolution data acquisition and 9 seconds for training data acquisition). Images on right are systolic (160 msec) and diastolic (762) frames in transverse orientation (magnified from image on left), with  vFVE  less than 56.25 cm/sec. Corresponding v-y (middle) and v-t (bottom) spaces are shown. Arrows indicate spatial extension of flow jet during systole. The resulting high-velocity spectrum is clearly visible in both the systolic v-y space and the v-t space. (c) Doppler US image shows peak velocities over time; lines and data points show peak velocities over time at accelerated MR FVE imaging.

View larger version (12K): [in this window] [in a new window] [Download PPT slide]   Figure 7a: In both volunteers and patients, peak velocities were detected by using eightfold-accelerated (8x) and 16-fold–accelerated (16x) MR FVE (breath-hold acquisitions) and conventional phase-contrast (PC) MR imaging (free breathing). Results were compared with those at US. (a) Graph shows that peak velocities for both volunteers and patients spanned a wide range. In one patient (arrow), eightfold-accelerated and 16-fold–accelerated MR FVE and phase-contrast MR imaging showed similar peak velocities, while these results deviated substantially from the US value. Consequently, data in this patient were excluded from the comparison in e and f. These discrepancies were most likely due to an erroneous US measurement, because this patient's medical records revealed a series of suboptimal US examinations caused by poor acoustic window conditions. Peak velocities are compared between pairs of image acquisitions separately for (b–d) healthy volunteers and (e–g) patients. Rel. Diff. = relative difference, SD = standard deviation.

View larger version (10K): [in this window] [in a new window] [Download PPT slide]   Figure 7b: In both volunteers and patients, peak velocities were detected by using eightfold-accelerated (8x) and 16-fold–accelerated (16x) MR FVE (breath-hold acquisitions) and conventional phase-contrast (PC) MR imaging (free breathing). Results were compared with those at US. (a) Graph shows that peak velocities for both volunteers and patients spanned a wide range. In one patient (arrow), eightfold-accelerated and 16-fold–accelerated MR FVE and phase-contrast MR imaging showed similar peak velocities, while these results deviated substantially from the US value. Consequently, data in this patient were excluded from the comparison in e and f. These discrepancies were most likely due to an erroneous US measurement, because this patient's medical records revealed a series of suboptimal US examinations caused by poor acoustic window conditions. Peak velocities are compared between pairs of image acquisitions separately for (b–d) healthy volunteers and (e–g) patients. Rel. Diff. = relative difference, SD = standard deviation.

View larger version (10K): [in this window] [in a new window] [Download PPT slide]   Figure 7c: In both volunteers and patients, peak velocities were detected by using eightfold-accelerated (8x) and 16-fold–accelerated (16x) MR FVE (breath-hold acquisitions) and conventional phase-contrast (PC) MR imaging (free breathing). Results were compared with those at US. (a) Graph shows that peak velocities for both volunteers and patients spanned a wide range. In one patient (arrow), eightfold-accelerated and 16-fold–accelerated MR FVE and phase-contrast MR imaging showed similar peak velocities, while these results deviated substantially from the US value. Consequently, data in this patient were excluded from the comparison in e and f. These discrepancies were most likely due to an erroneous US measurement, because this patient's medical records revealed a series of suboptimal US examinations caused by poor acoustic window conditions. Peak velocities are compared between pairs of image acquisitions separately for (b–d) healthy volunteers and (e–g) patients. Rel. Diff. = relative difference, SD = standard deviation.

View larger version (10K): [in this window] [in a new window] [Download PPT slide]   Figure 7d: In both volunteers and patients, peak velocities were detected by using eightfold-accelerated (8x) and 16-fold–accelerated (16x) MR FVE (breath-hold acquisitions) and conventional phase-contrast (PC) MR imaging (free breathing). Results were compared with those at US. (a) Graph shows that peak velocities for both volunteers and patients spanned a wide range. In one patient (arrow), eightfold-accelerated and 16-fold–accelerated MR FVE and phase-contrast MR imaging showed similar peak velocities, while these results deviated substantially from the US value. Consequently, data in this patient were excluded from the comparison in e and f. These discrepancies were most likely due to an erroneous US measurement, because this patient's medical records revealed a series of suboptimal US examinations caused by poor acoustic window conditions. Peak velocities are compared between pairs of image acquisitions separately for (b–d) healthy volunteers and (e–g) patients. Rel. Diff. = relative difference, SD = standard deviation.

View larger version (10K): [in this window] [in a new window] [Download PPT slide]   Figure 7e: In both volunteers and patients, peak velocities were detected by using eightfold-accelerated (8x) and 16-fold–accelerated (16x) MR FVE (breath-hold acquisitions) and conventional phase-contrast (PC) MR imaging (free breathing). Results were compared with those at US. (a) Graph shows that peak velocities for both volunteers and patients spanned a wide range. In one patient (arrow), eightfold-accelerated and 16-fold–accelerated MR FVE and phase-contrast MR imaging showed similar peak velocities, while these results deviated substantially from the US value. Consequently, data in this patient were excluded from the comparison in e and f. These discrepancies were most likely due to an erroneous US measurement, because this patient's medical records revealed a series of suboptimal US examinations caused by poor acoustic window conditions. Peak velocities are compared between pairs of image acquisitions separately for (b–d) healthy volunteers and (e–g) patients. Rel. Diff. = relative difference, SD = standard deviation.

View larger version (10K): [in this window] [in a new window] [Download PPT slide]   Figure 7f: In both volunteers and patients, peak velocities were detected by using eightfold-accelerated (8x) and 16-fold–accelerated (16x) MR FVE (breath-hold acquisitions) and conventional phase-contrast (PC) MR imaging (free breathing). Results were compared with those at US. (a) Graph shows that peak velocities for both volunteers and patients spanned a wide range. In one patient (arrow), eightfold-accelerated and 16-fold–accelerated MR FVE and phase-contrast MR imaging showed similar peak velocities, while these results deviated substantially from the US value. Consequently, data in this patient were excluded from the comparison in e and f. These discrepancies were most likely due to an erroneous US measurement, because this patient's medical records revealed a series of suboptimal US examinations caused by poor acoustic window conditions. Peak velocities are compared between pairs of image acquisitions separately for (b–d) healthy volunteers and (e–g) patients. Rel. Diff. = relative difference, SD = standard deviation.

View larger version (10K): [in this window] [in a new window] [Download PPT slide]   Figure 7g: In both volunteers and patients, peak velocities were detected by using eightfold-accelerated (8x) and 16-fold–accelerated (16x) MR FVE (breath-hold acquisitions) and conventional phase-contrast (PC) MR imaging (free breathing). Results were compared with those at US. (a) Graph shows that peak velocities for both volunteers and patients spanned a wide range. In one patient (arrow), eightfold-accelerated and 16-fold–accelerated MR FVE and phase-contrast MR imaging showed similar peak velocities, while these results deviated substantially from the US value. Consequently, data in this patient were excluded from the comparison in e and f. These discrepancies were most likely due to an erroneous US measurement, because this patient's medical records revealed a series of suboptimal US examinations caused by poor acoustic window conditions. Peak velocities are compared between pairs of image acquisitions separately for (b–d) healthy volunteers and (e–g) patients. Rel. Diff. = relative difference, SD = standard deviation.