Jon-Fredrik Nielsen

University of Southern California, Los Angeles, CA, United States

Are you Jon-Fredrik Nielsen?

Claim your profile

Publications (6)18.96 Total impact

  • Source
    Joao L A Carvalho, Jon-Fredrik Nielsen, Krishna S Nayak
    [show abstract] [hide abstract]
    ABSTRACT: Arterial wall shear stress is widely believed to influence the formation and growth of atherosclerotic plaque; however, there is currently no gold standard for its in vivo measurement. The use of phase contrast MRI has proved to be challenging due to partial-volume effects and inadequate signal-to-noise ratio at the high spatial resolutions that are required. This work evaluates the use of spiral Fourier velocity encoded MRI as a rapid method for assessing wall shear rate in the carotid arteries. Wall shear rate is calculated from velocity histograms in voxels spanning the blood/vessel wall interface, using a method developed by Frayne and Rutt (Magn Reson Med 1995;34:378-387). This study (i) demonstrates the accuracy of the velocity histograms measured by spiral Fourier velocity encoding in a pulsatile carotid flow phantom compared with high-resolution two-dimensional Fourier transform phase contrast, (ii) demonstrates the accuracy of Fourier velocity encoding-based shear rate measurements in a numerical phantom designed using a computational fluid dynamics simulation of carotid flow, and (iii) demonstrates in vivo measurement of regional wall shear rate and oscillatory shear index in the carotid arteries of healthy volunteers at 3 T.
    Magnetic Resonance in Medicine 06/2010; 63(6):1537-47. · 3.27 Impact Factor
  • Source
    Jon-Fredrik Nielsen, Krishna S Nayak
    [show abstract] [hide abstract]
    ABSTRACT: To analyze steady-state signal distortions in interleaved balanced steady-state free precession (bSSFP) caused by slightly unbalanced eddy-current fields and develop a general strategy for mitigating these artifacts. We considered bSSFP sequences in which two gradient waveforms are interleaved in a "groupwise" fashion, ie, each waveform is executed consecutively two or more times before switching to the other waveform (we let "N" count the number of times each waveform is executed consecutively). The steady-state signal profile over the bSSFP passband was calculated using numerical Bloch simulations and measured experimentally in a uniform phantom. The proposed "grouped" interleaved bSSFP strategy was applied to cardiac velocity mapping using interleaved phase-contrast imaging with N=2 and N=6 in one healthy volunteer. Simulation and phantom measurements show that signal distortions are systematically reduced with increasing grouping number N. For most tissues, significant suppression was achieved with N=4, and increasing N beyond this value produced only marginal gains. However, signal distortions for blood remain relatively high even for N>4. In vivo cardiac velocity mapping using interleaved phase-contrast imaging with N=6 demonstrated reduced image artifact levels compared to the N=2 acquisition. Gradient waveform "grouping" offers a simple and general strategy for mitigating steady-state eddy-current distortions in bSSFP sequences that interleave two different gradients. Blood exhibits significant distortion even with "grouping," which is a major obstacle for cardiovascular bSSFP approaches that interleave multiple gradient waveforms. The grouping concept may also benefit applications that acquire images during the transient approach to steady state.
    Journal of Magnetic Resonance Imaging 03/2009; 29(3):745-50. · 2.57 Impact Factor
  • Source
    Jon-Fredrik Nielsen, Krishna S Nayak
    [show abstract] [hide abstract]
    ABSTRACT: Phase contrast MRI (PC-MRI) is an established technique for measuring blood flow velocities in vivo. Although spoiled gradient recalled echo (GRE) PC-MRI is the most widely used pulse sequence today, balanced steady state free precession (SSFP) PC-MRI has been shown to produce accurate velocity estimates with superior SNR efficiency. We propose a referenceless approach to flow imaging that exploits the intrinsic refocusing property of balanced SSFP, and achieves up to a 50% reduction in total scan time. With the echo time set to exactly one half of the sequence repetition time (TE = TR/2), we show that non-flow-related image phase tends to vary smoothly across the field-of-view, and can be estimated from static tissue regions to produce a phase reference for nearby voxels containing flowing blood. This approach produces accurate in vivo one-dimensional velocity estimates in half the scan time compared with conventional balanced SSFP phase-contrast methods. We also demonstrate the feasibility of referenceless time-resolved 3D flow imaging (called "7D" flow) in the carotid bifurcation from just three acquisitions.
    Magnetic Resonance in Medicine 03/2009; 61(5):1096-102. · 3.27 Impact Factor
  • Source
    Yoon-Chul Kim, Jon-Fredrik Nielsen, Krishna S Nayak
    [show abstract] [hide abstract]
    ABSTRACT: To develop a method that automatically corrects ghosting artifacts due to echo-misalignment in interleaved gradient-echo echo-planar imaging (EPI) in arbitrary oblique or double-oblique scan planes. An automatic ghosting correction technique was developed based on an alternating EPI acquisition and the phased-array ghost elimination (PAGE) reconstruction method. The direction of k-space traversal is alternated at every temporal frame, enabling lower temporal-resolution ghost-free coil sensitivity maps to be dynamically estimated. The proposed method was compared with conventional one-dimensional (1D) phase correction in axial, oblique, and double-oblique scan planes in phantom and cardiac in vivo studies. The proposed method was also used in conjunction with two-fold acceleration. The proposed method with nonaccelerated acquisition provided excellent suppression of ghosting artifacts in all scan planes, and was substantially more effective than conventional 1D phase correction in oblique and double-oblique scan planes. The feasibility of real-time reconstruction using the proposed technique was demonstrated in a scan protocol with 3.1-mm spatial and 60-msec temporal resolution. The proposed technique with nonaccelerated acquisition provides excellent ghost suppression in arbitrary scan orientations without a calibration scan, and can be useful for real-time interactive imaging, in which scan planes are frequently changed with arbitrary oblique orientations.
    Journal of Magnetic Resonance Imaging 02/2008; 27(1):239-45. · 2.57 Impact Factor
  • Source
    Jon-Fredrik Nielsen, Krishna S Nayak
    [show abstract] [hide abstract]
    ABSTRACT: A technique for rapid in-plane phase-contrast imaging with high signal-to-noise ratio (SNR) is described. Velocity-encoding is achieved by oscillating the readout gradient, such that each 2DFT phase-encode is acquired three times following a single RF slice-selective excitation. Three images are reconstructed, from which both flow velocity and local resonance offset are calculated. This technique is compatible with both gradient-recalled echo (GRE) and balanced steady-state free precession (SSFP) imaging using a single steady-state. The proposed technique enables 1D velocity mapping with 40% higher temporal resolution and 80% higher SNR, compared to conventional PC-MRI using bipolar velocity-encoding gradient pulses.
    Magnetic Resonance in Medicine 01/2008; 58(6):1288-93. · 3.27 Impact Factor
  • Source
    Taehoon Shin, Jon-Fredrik Nielsen, Krishna S Nayak
    [show abstract] [hide abstract]
    ABSTRACT: The temporal resolution of dynamic magnetic resonance imaging (MRI) can be increased by sampling a fraction of k-space in an interleaved fashion, which introduces spatial and temporal aliasing. We describe algebraically and graphically the aliasing process caused by dynamic undersampled spiral imaging within 3-D xyf space (the Fourier transform of k(x)k(y)t space) and formulate the unaliasing problem as a set of independent linear inversions. Since each linear system is numerically underdetermined, the use of prior knowledge in the form of bounded support regions is proposed. To overcome the excessive memory requirements for handling large matrices, a fast implementation of the conjugate gradient (CG) method is used. Numerical simulation and in vivo experiments using spiral twofold undersampling demonstrate reduced motion artifacts and the improved depiction of fine cardiac structures. The achieved reduction of motion artifacts and motion blur is comparable to simple filtering, which is computationally more efficient, while the proposed algebraic framework offers greater flexibility to incorporate additional algebraic acceleration techniques and to handle arbitrary sampling schemes.
    IEEE Transactions on Medical Imaging 08/2007; 26(7):917-24. · 4.03 Impact Factor