Bob S Hu

Palo Alto Research Center, Palo Alto, California, United States

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Publications (74)342.85 Total impact

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    ABSTRACT: To develop a rapid single-breath-hold 3D late gadolinium enhancement (LGE) magnetic resonance imaging (MRI) method, and demonstrate its feasibility in cardiac patients. An inversion recovery dual-density 3D stack-of-spirals imaging sequence was developed. The spiral acquisition was 2-fold accelerated by self-consistent parallel imaging reconstruction (SPIRiT), which resulted in a total scan time of 12 heartbeats. Field map-based linear off-resonance correction was incorporated to the SPIRiT reconstruction. The 3D spiral LGE scans were performed in 15 patients who were referred for clinically ordered cardiac MR examinations that included the standard 2D multislice LGE imaging. Image sharpness and overall quality were qualitatively assessed based on 5-point scales. Scar-induced hyper-LGE was identified in 4 out of the 15 patients by both 3D spiral and 2D multislice LGE tests. On average over all datasets (n = 15), the image sharpness scores were 3.9 (3D spiral) and 4.0 (2D multislice), and the image quality scores were 4.1 (3D spiral) and 4.0 (2D multislice) with no significant difference in both metrics (paired t-test; P > 0.1). The average scar contrast enhancement ratios were 0.72 and 0.75 in 3D and 2D images, respectively (n = 4). The average difference of fractional scar volumes measured in 3D and 2D images was 4.3% (n = 3). Stack-of-spiral acquisition combined with non-Cartesian SPIRiT parallel imaging enables rapid 3D LGE MRI in a 12 heartbeat-long breath-hold.J. Magn. Reson. Imaging 2013. © 2013 Wiley Periodicals, Inc.
    Journal of Magnetic Resonance Imaging 12/2014; 40(6). DOI:10.1002/jmri.24494 · 2.79 Impact Factor
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    ABSTRACT: PurposeTo develop a magnetization preparation sequence for simultaneous outer volume suppression (OVS) and T2 weighting in whole-heart coronary magnetic resonance angiography.MethodsA combined OVS and T2 preparation sequence (OVS-T2 Prep) was designed with a nonselective adiabatic 90° tipdown pulse, two adiabatic 180° refocusing pulses, and a 2D spiral −90° tipup pulse. The OVS-T2 Prep preserves the magnetization inside an elliptic cylinder with T2 weighting, while saturating the magnetization outside the cylinder. Its performance was tested on phantoms and on 13 normal subjects with coronary magnetic resonance angiography using 3D cones trajectories.ResultsPhantom studies showed expected T2-dependent signal amplitude in the spatial passband and suppressed signal in the spatial stopband. In vivo studies with full-field-of-view cones yielded a passband-to-stopband signal ratio of 3.18 ± 0.77 and blood-myocardium contrast-to-noise ratio enhancement by a factor of 1.43 ± 0.20 (P < 0.001). In vivo studies with reduced-field-of-view cones showed that OVS-T2 Prep well suppressed the aliasing artifacts, as supported by significantly reduced signal in the regions with no tissues compared to the images acquired without preparation (P < 0.0001).ConclusionOVS-T2 Prep is a compact sequence that can accelerate coronary magnetic resonance angiography by suppressing signals from tissues surrounding the heart while simultaneously enhancing the blood-myocardium contrast. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 12/2014; DOI:10.1002/mrm.25575 · 3.40 Impact Factor
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    ABSTRACT: PurposeTo develop a new sequence for non-contrast-enhanced peripheral angiography using a sliding interleaved cylinder (SLINCYL) acquisition.MethodsA venous saturation pulse was incorporated into a three-dimensional magnetization-prepared balanced steady-state free precession sequence for non-contrast-enhanced peripheral angiography to improve artery-vein contrast. The SLINCYL acquisition, which consists of a series of overlapped thin slabs for volumetric coverage similar to the original sliding interleaved ky (SLINKY) acquisition, was used to evenly distribute the venous-suppression effects over the field of view. In addition, the thin-slab-scan nature of SLINCYL and the centric-ordered sampling geometry of its readout trajectory were exploited to implement efficient fluid-suppression and parallel imaging schemes. The sequence was tested in healthy subjects and a patient.ResultsCompared to a multiple overlapped thin slab acquisition, both SLINKY and SLINCYL suppressed the venetian blind artifacts and provided similar artery-vein contrast. However, SLINCYL achieved this with shorter scan times and less noticeable artifacts from k-space amplitude modulation than SLINKY. The fluid-suppression and parallel imaging schemes were also validated. A patient study using the SLINCYL-based sequence well identified stenoses at the superficial femoral arteries, which were also confirmed with digital subtraction angiography.Conclusion Non-contrast-enhanced angiography using SLINCYL can provide angiograms with improved artery-vein contrast in the lower extremities. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 09/2014; DOI:10.1002/mrm.25452 · 3.40 Impact Factor
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    ABSTRACT: To implement a nonrigid autofocus motion correction technique to improve respiratory motion correction of free-breathing whole-heart coronary magnetic resonance angiography acquisitions using an image-navigated 3D cones sequence. 2D image navigators acquired every heartbeat are used to measure superior-inferior, anterior-posterior, and right-left translation of the heart during a free-breathing coronary magnetic resonance angiography scan using a 3D cones readout trajectory. Various tidal respiratory motion patterns are modeled by independently scaling the three measured displacement trajectories. These scaled motion trajectories are used for 3D translational compensation of the acquired data, and a bank of motion-compensated images is reconstructed. From this bank, a gradient entropy focusing metric is used to generate a nonrigid motion-corrected image on a pixel-by-pixel basis. The performance of the autofocus motion correction technique is compared with rigid-body translational correction and no correction in phantom, volunteer, and patient studies. Nonrigid autofocus motion correction yields improved image quality compared to rigid-body-corrected images and uncorrected images. Quantitative vessel sharpness measurements indicate superiority of the proposed technique in 14 out of 15 coronary segments from three patient and two volunteer studies. The proposed technique corrects nonrigid motion artifacts in free-breathing 3D cones acquisitions, improving image quality compared to rigid-body motion correction. Magn Reson Med, 2013. © 2013 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 08/2014; 72(2). DOI:10.1002/mrm.24924 · 3.40 Impact Factor
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    ABSTRACT: PurposeTo develop a self-gated alternating repetition time balanced steady-state free precession (ATR-SSFP) pulse sequence for fat-suppressed cardiac cine imaging.Methods Cardiac gating is computed retrospectively using acquired magnetic resonance self-gating data, enabling cine imaging without the need for electrocardiogram (ECG) gating. Modification of the slice-select rephasing gradients of an ATR-SSFP sequence enables the acquisition of a one-dimensional self-gating readout during the unused short repetition time (TR). Self-gating readouts are acquired during every TR of segmented, breath-held cardiac scans. A template-matching algorithm is designed to compute cardiac trigger points from the self-gating signals, and these trigger points are used for retrospective cine reconstruction. The proposed approach is compared with ECG-gated ATR-SSFP and balanced steady-state free precession in 10 volunteers and five patients.ResultsThe difference of ECG and self-gating trigger times has a variability of 13 ± 11 ms (mean ± SD). Qualitative reviewer scoring and ranking indicate no statistically significant differences (P > 0.05) between self-gated and ECG-gated ATR-SSFP images. Quantitative blood-myocardial border sharpness is not significantly different among self-gated ATR-SSFP ( ), ECG-gated ATR-SSFP ( ), or conventional ECG-gated balanced steady-state free precession cine MRI ( ).Conclusion The proposed self-gated ATR-SSFP sequence enables fat-suppressed cardiac cine imaging at 1.5 T without the need for ECG gating and without decreasing the imaging efficiency of ATR-SSFP. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 05/2014; 73(5). DOI:10.1002/mrm.25291 · 3.40 Impact Factor
  • Journal of Cardiovascular Magnetic Resonance 01/2014; 16(Suppl 1):W10. DOI:10.1186/1532-429X-16-S1-W10 · 5.11 Impact Factor
  • Journal of Cardiovascular Magnetic Resonance 01/2014; 16(Suppl 1):P380. DOI:10.1186/1532-429X-16-S1-P380 · 5.11 Impact Factor
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    ABSTRACT: PURPOSE: To develop a new velocity-selective (VS) excitation pulse sequence which is robust to field inhomogeneity, and demonstrate its application to non-contrast-enhanced peripheral MR angiography (MRA). METHODS: The off-resonance-robust VS saturation pulse is designed by incorporating 180° refocusing pulses into the k-space-based reference design and tailoring sequence parameters in a velocity region of interest. The VS saturation pulse is used as magnetization preparation for non-contrast-enhanced peripheral MRA to suppress background tissues but not arterial blood based on their velocities. Non-contrast-enhanced peripheral MRA using the proposed VS preparation was tested in healthy volunteers and a patient with arterial stenosis. RESULTS: Calf angiograms obtained using the new VS preparation show more uniform background suppression than the reference VS preparation, as demonstrated by larger mean values and smaller standard deviations of artery-to-vein and artery-to-muscle contrast-to-noise ratios (71.0 ± 11.4 and 75.3 ± 12.1 versus 61.7 ± 22.7 and 58.5 ± 27.8). Two-station peripheral MRA using the new VS preparation identifies stenosis of the femoral and popliteal arteries in the patient, as validated by digital subtraction angiography. CONCLUSION: Non-contrast-enhanced MRA using the new VS magnetization preparation can reliably provide high angiographic contrast in the lower extremities with significantly improved immunity to field inhomogeneity. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 11/2013; 70(5):spcone. DOI:10.1002/mrm.25042 · 3.40 Impact Factor
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    ABSTRACT: Non-contrast-enhanced MR angiography is a promising alternative to the established contrast-enhanced approach as it reduces patient discomfort and examination costs and avoids the risk of nephrogenic systemic fibrosis. Inflow-sensitive slab-selective inversion recovery imaging has been used with great promise, particularly for abdominal applications, but has limited craniocaudal coverage due to inflow time constraints. In this work, a new non-contrast-enhanced MR angiography method using velocity-selective inversion preparation is developed and applied to renal and abdominal angiography. Based on the excitation k-space formalism and Shinnar-Le-Roux transform, a velocity-selective excitation pulse is designed that inverts stationary tissues and venous blood while preserving inferiorly flowing arterial blood. As the magnetization of the arterial blood in the abdominal aorta and iliac arteries is well preserved during the magnetization preparation, artery visualization over a large abdominal field of view is achievable with an inversion delay time that is chosen for optimal background suppression. Healthy volunteer tests demonstrate that the proposed method significantly increases the extent of visible arteries compared with the slab-selective approach, covering renal arteries through iliac arteries over a craniocaudal field of view of 340 mm. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 05/2013; 69(5). DOI:10.1002/mrm.24356 · 3.40 Impact Factor
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    ABSTRACT: Noninvasive visualization of the coronary arteries in vivo is one of the most important goals in cardiovascular imaging. Compared to other paradigms for coronary MR angiography, a free-breathing three-dimensional whole-heart iso-resolution approach simplifies prescription effort, requires less patient cooperation, reduces overall exam time, and supports retrospective reformats at arbitrary planes. However, this approach requires a long continuous acquisition and must account for respiratory and cardiac motion throughout the scan. In this work, a new free-breathing coronary MR angiography technique that reduces scan time and improves robustness to motion is developed. Data acquisition is accomplished using a three-dimensional cones non-Cartesian trajectory, which can reduce the number of readouts 3-fold or more compared to conventional three-dimensional Cartesian encoding and provides greater robustness to motion/flow effects. To further enhance robustness to motion, two-dimensional navigator images are acquired to directly track respiration-induced displacement of the heart and enable retrospective compensation of all acquired data (none discarded) for image reconstruction. In addition, multiple cardiac phases are imaged to support retrospective selection of the best phase(s) for visualizing each coronary segment. Experimental results demonstrate that whole-heart coronary angiograms can be obtained rapidly and robustly with this proposed technique. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 04/2013; 69(4). DOI:10.1002/mrm.24346 · 3.40 Impact Factor
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    ABSTRACT: Three-dimensional cardiac magnetic resonance perfusion imaging is promising for the precise sizing of defects and for providing high perfusion contrast, but remains an experimental approach primarily due to the need for large-dimensional encoding, which, for traditional 3DFT imaging, requires either impractical acceleration factors or sacrifices in spatial resolution. We demonstrated the feasibility of rapid three-dimensional cardiac magnetic resonance perfusion imaging using a stack-of-spirals acquisition accelerated by non-Cartesian k-t SENSE, which enables entire myocardial coverage with an in-plane resolution of 2.4 mm. The optimal undersampling pattern was used to achieve the largest separation between true and aliased signals, which is a prerequisite for k-t SENSE reconstruction. Flip angle and saturation recovery time were chosen to ensure negligible magnetization variation during the transient data acquisition. We compared the proposed three-dimensional perfusion method with the standard 2DFT approach by consecutively acquiring both data during each R-R interval in cardiac patients. The mean and standard deviation of the correlation coefficients between time intensity curves of three-dimensional versus 2DFT were 0.94 and 0.06 across seven subjects. The linear correlation between the two sets of upslope values was significant (r = 0.78, P < 0.05). Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 03/2013; 69(3). DOI:10.1002/mrm.24303 · 3.40 Impact Factor
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    Journal of Cardiovascular Magnetic Resonance 01/2013; 15(1). DOI:10.1186/1532-429X-15-S1-E78 · 5.11 Impact Factor
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    ABSTRACT: When evaluating the severity of valvular stenosis, the peak velocity of the blood flow is routinely used to estimate the transvalvular pressure gradient. One-dimensional Fourier velocity encoding effectively detects the peak velocity with an ungated time series of spatially resolved velocity spectra in real time. However, measurement accuracy can be degraded by the pulsatile and turbulent nature of stenotic flow and the existence of spatially varying off-resonance. In this work, we investigate the feasibility of improving the peak velocity detection capability of one-dimensional Fourier velocity encoding for stenotic flow using a novel echo-shifted interleaved readout combined with a variable-density circular k-space trajectory. The shorter echo and readout times of the echo-shifted interleaved acquisitions are designed to reduce sensitivity to off-resonance. Preliminary results from limited phantom and in vivo results also indicate that some artifacts from pulsatile flow appear to be suppressed when using this trajectory compared to conventional single-shot readouts, suggesting that peak velocity detection may be improved. The efficiency of the new trajectory improves the temporal and spatial resolutions. To realize the proposed readout, a novel multipoint-traversing algorithm is introduced for flexible and automated gradient-waveform design. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.
    Magnetic Resonance in Medicine 12/2012; 68(6). DOI:10.1002/mrm.24212 · 3.40 Impact Factor
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    Journal of Cardiovascular Magnetic Resonance 02/2012; 14 Suppl 1(Suppl 1):P237. DOI:10.1186/1532-429X-14-S1-P237 · 5.11 Impact Factor
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    Journal of Cardiovascular Magnetic Resonance 02/2012; 14 Suppl 1(Suppl 1):P250. DOI:10.1186/1532-429X-14-S1-P250 · 5.11 Impact Factor
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    ABSTRACT: Accurate depiction of the vessels of the lower leg, foot or hand benefits from suppression of bright MR signal from lipid (such as bone marrow) and long-T1 fluid (such as synovial fluid and edema). Signal independence of blood flow velocities, good arterial/muscle contrast and arterial/venous separation are also desirable. The high SNR, short scan times and flow properties of balanced steady-state free precession (SSFP) make it an excellent candidate for flow-independent angiography. In this work, a new magnetization-prepared 3D SSFP sequence for flow-independent peripheral angiography is presented. The technique combines a number of component techniques (phase-sensitive fat detection, inversion recovery, T2-preparation and square-spiral phase-encode ordering) to achieve high-contrast peripheral angiograms at only a modest scan time penalty over simple 3D SSFP. The technique is described in detail, a parameter optimization performed and preliminary results presented achieving high contrast and 1-mm isotropic resolution in a normal foot.
    Magnetic Resonance Imaging 06/2011; 29(8):1119-24. DOI:10.1016/j.mri.2011.04.007 · 2.02 Impact Factor
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    ABSTRACT: To propose a new noncontrast-enhanced flow-independent angiography sequence based on balanced steady-state free precession (bSSFP) that produces reliable vessel contrast despite the reduced blood flow in the extremities. The proposed technique addresses a variety of factors that can compromise the exam success including insufficient background suppression, field inhomogeneity, and large volumetric coverage requirements. A bSSFP sequence yields reduced signal from venous blood when long repetition times are used. Complex-sum bSSFP acquisitions decrease the sensitivity to field inhomogeneity but retain phase information, so that data can be processed with the Iterative Decomposition of Water and Fat with Echo Asymmetry and Least-Squares Estimation (IDEAL) method for robust fat suppression. Meanwhile, frequent magnetization preparation coupled with parallel imaging reduces the muscle and long-T(1) fluid signals without compromising scan efficiency. In vivo flow-independent peripheral angiograms with reliable background suppression and high spatial resolution are produced. Comparisons with phase-sensitive bSSFP angiograms (that yield out-of-phase fat and water signals, and exploit this phase difference to suppress fat) demonstrate enhanced vessel depiction with the proposed technique due to reduced partial-volume effects and improved venous suppression. Magnetization-prepared complex-sum bSSFP with IDEAL fat/water separation can create reliable flow-independent angiographic contrast in the lower extremities.
    Journal of Magnetic Resonance Imaging 04/2011; 33(4):931-9. DOI:10.1002/jmri.22479 · 2.79 Impact Factor
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    Journal of Cardiovascular Magnetic Resonance 01/2011; DOI:10.1186/1532-429X-13-S1-P238 · 5.11 Impact Factor
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    ABSTRACT: As a noninvasive modality, MR is attractive for in vivo skin imaging. Its unique soft tissue contrast makes it an ideal imaging modality to study the skin water content and to resolve the different skin layers. In this work, the challenges of in vivo high-resolution skin imaging are addressed. Three 3D Cartesian sequences are customized to achieve high-resolution imaging and their respective performance is evaluated. The balanced steady-state free precession (bSSFP) and gradient echo (GRE) sequences are fast but can be sensitive to off-resonance artifacts. The fast large-angle spin echo (FLASE) sequence provides a sharp depiction of the hypodermis structures but results in more specific absorption rate (SAR). The effect of increasing the field strength is assessed. As compared to 1.5 T, signal-to-noise ratio at 3 T slightly increases in the hypodermis and almost doubles in the dermis. The need for fat/water separation is acknowledged and a solution using an interleaved three-point Dixon method and an iterative reconstruction is shown to be effective. The effects of motion are analyzed and two techniques to prevent motion and correct for it are evaluated. Images with 117 x 117 x 500 microm(3) resolution are obtained in imaging times under 6 min.
    Magnetic Resonance in Medicine 03/2010; 63(3):790-6. DOI:10.1002/mrm.22271 · 3.40 Impact Factor
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    ABSTRACT: Parallel imaging using sophisticated receiver coils has improved the clinical feasibility of magnetic resonance coronary angiography (MRCA). These techniques, however, are not readily available outside advanced imaging centers. Our custom-made 2-element phased array coil is readily and inexpensively assembled to address this limitation and enable the widespread application of MRCA. The 2-element phased array coil, comprised of two 4-inch, overlapping circular coils, is specifically designed for MRCA. We compare our prototype coil to two commercially available coils commonly used for MRCA. MRCA has been performed in 14 normal volunteers. Anatomic coverage, image quality, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) are calculated for each coil. The prototype coil has imaged 92.6% (125/135) of the segments compared to 83.7% (113/135) and 76.3% (103/135) (p = 0.002) using the surface coil and cardiac phased array coil, respectively. Excellent or good (grade 1-2) image quality has been attained in 85.9% (116/135) of all the coronary segments using the prototype coil compared to 77.0% (104/135) and 71 % (96/135) using the surface and cardiac phased array coils, respectively (p = 0.025). Overall, higher SNR and CNR have been achieved by the prototype coil compared to the surface and the cardiac phased array coils (SNR: 13.5 ± 5.3 vs 12.2 ± 3.7 vs 9.0 ± 3.1 and CNR: 7.5 ± 5.5 vs 6.2 ± 3.3 vs 3.7 ± 2.7, respectively). Compared to two commercially available coils, the 2-element phased array coil is associated with overall improved SNR and CNR and provides higher image quality with wider anatomic coverage.
    The Open Cardiovascular Imaging Journal 09/2009; 1(1). DOI:10.2174/1876538600901010030

Publication Stats

2k Citations
342.85 Total Impact Points

Institutions

  • 2014
    • Palo Alto Research Center
      Palo Alto, California, United States
  • 1992–2014
    • Stanford University
      • • Department of Electrical Engineering
      • • Magnetic Resonance Systems Research Laboratory
      • • Division of Cardiovascular Medicine
      • • Department of Medicine
      • • Information Systems Laboratory
      • • Department of Radiology
      Palo Alto, California, United States
  • 2003–2013
    • Palo Alto Medical Foundation
      Palo Alto, California, United States
  • 2005
    • University of Southern California
      • Department of Electrical Engineering
      Los Angeles, CA, United States
  • 1991–2002
    • Stanford Medicine
      • Division of Cardiovascular Medicine
      Stanford, California, United States
  • 1994
    • University of Pennsylvania
      Filadelfia, Pennsylvania, United States