About

Krishna S. Nayak, PhD, is the Dean's Professor of Electrical and Computer Engineering at the Viterbi School of Engineering, University of Southern California. He serves as the Director of the Magnetic Resonance Engineering Laboratory and the Dynamic Imaging Science Center. His research interests focus on Magnetic Resonance Imaging (MRI), including MRI pulse sequence design, MRI reconstruction, artifact correction, and real-time imaging. He applies MRI techniques to assess cardiovascular disease and obesity, integrating signal and image processing methodologies to advance the field. Dr. Nayak has been recognized with numerous honors, including being named a Fellow of the IEEE, SCMR, and AIMBE, and has received awards such as the USC Viterbi School of Engineering Use-Inspired Research Award and the GE Healthcare Magnetic Resonance Thought Leader Award. His academic career includes a sabbatical at the NIH/NHLBI Laboratory for Cardiac Energetics and multiple faculty mentoring and research awards at USC. Dr. Nayak's work contributes significantly to the development and clinical implementation of advanced MRI technologies, particularly in cardiovascular imaging and real-time applications.

Research topics

• Computer Science
• Artificial Intelligence
• Medicine
• Radiology
• Machine Learning
• Physics
• Medical physics
• Speech recognition
• Risk analysis (engineering)
• Computer vision
• Mathematics
• Chemistry

Selected publications

• Improved dynamic MRI of the wrist and heart at 0.55 T enabled by rapid 3D printed flexible coils

  Nature Communications · 2026-04-20

  articleOpen access

  Dynamic MRI at low field is limited by reduced signal-to-noise ratio (SNR) and additional losses imposed by accelerated acquisitions required for high temporal resolution. Flexible, anatomically conformal coils can recover SNR by maintaining proximity during motion, but lower conductor conductivity introduces resistive losses, requiring careful material optimization. We demonstrate high-quality dynamic imaging at 0.55 T using coils fabricated via two rapid, low-cost digital methods: direct-ink-write silver printing and screen printing of copper-doped EGaIn. Four-element wrist arrays were fabricated in 8 minutes per element at approximately $30 in consumable materials. In three subjects, both arrays achieved up to 4 × higher contrast and 5 × greater sharpness than a commercial coil during dynamic wrist imaging. To demonstrate generalizability, a seven-element silver-ink cardiac array produced cine images comparable to a commercial coil. These results establish digitally fabricated flexible coils as a scalable and accessible solution for dynamic musculoskeletal and cardiac MRI at low field.

  PublisherDOI

• Online spatiotemporally constrained reconstruction for real-time interactive MRI

  Proceedings on CD-ROM - International Society for Magnetic Resonance in Medicine. Scientific Meeting and Exhibition/Proceedings of the International Society for Magnetic Resonance in Medicine, Scientific Meeting and Exhibition · 2025-09-16

  articleSenior author

  Motivation: Real-time interactive MRI is extremely useful during localization of moving structures and guidance of interventional procedures. View-sharing is currently used but presents an unfavorable tradeoff between aliasing artifacts and temporal resolution. Goal(s): To develop a low latency reconstruction that provides the spatial and temporal fidelity comparable to offline compressed sensing reconstruction. Approach: We implement online spatiotemporally constrained reconstruction (STCR) using a computationally efficient Augmented Lagrangian solution that terminates before the acquisition of the next frame. Results: Online STCR removes aliasing artifacts and has superior temporal definition compared to view-sharing and comparable to offline STCR. Impact: We demonstrate low-latency reconstruction for 2D real-time MRI with spatial resolution 1.5/2.25mm2, temporal resolution 25.2/31.8ms, and FOV 240×240/320×320mm, improving visual recognition of rapidly moving boundaries.

  PublisherDOI

• Cartesian MaxGIRF: Model-based EPI reconstruction incorporating gradient nonlinearity and concomitant field effects

  Proceedings on CD-ROM - International Society for Magnetic Resonance in Medicine. Scientific Meeting and Exhibition/Proceedings of the International Society for Magnetic Resonance in Medicine, Scientific Meeting and Exhibition · 2025-09-16

  articleSenior author

  Motivation: Next-generation scanners with strong gradient systems and large-bore low field scanners experience increased gradient nonlinearity and concomitant fields, each of which causes distortions in EPI. Current correction approaches based on image-domain interpolation introduce undesirable spatial blurring. Goal(s): Develop a model-based EPI reconstruction framework that simultaneously corrects distortions from gradient nonlinearity, concomitant fields, and off-resonance during image reconstruction to avoid spatial blurring caused by interpolation-based methods. Approach: Concomitant fields are considered as higher-order encoding and gradient nonlinearity as spatial displacements, which is efficiently handled by the Type-I NUFFT. Results: The proposed method provides better image quality, specifically reduced blurring, compared to conventional correction methods. Impact: Standard corrections for gradient nonlinearity and off-resonance have detrimental effect on image quality and on quantitative diffusion measures. The proposed framework performs all corrections during image reconstruction and provides better image quality by avoiding the smoothing effect of interpolation-based corrections.

  PublisherDOI

• Evaluation of high-amplitude pilot-tone with interference cancellation (HAPTIC) cardiac gating at 0.55T in the challenging case of arrhythmia

  Proceedings on CD-ROM - International Society for Magnetic Resonance in Medicine. Scientific Meeting and Exhibition/Proceedings of the International Society for Magnetic Resonance in Medicine, Scientific Meeting and Exhibition · 2025-09-16

  articleSenior author

  Motivation: Cardiac gating at 0.55T with pilot tone has been elusive because the pilot tone signal scales with B0 field strength, and cardiac motion is a weak modulation. High-amplitude pilot tone with interference cancellation (HAPTIC) addresses this issue, but has not yet been tested in challenging cases. Goal(s): To evaluate HAPTIC cardiac gating at 0.55T in the challenging case of cardiac arrhythmia. Approach: Four subjects with cardiac arrhythmia were scanned, and HAPTIC was compared against ECG recordings. Detected R-wave jitter and false positive/negative rates are reported. Results: Cardiac motion extraction for arrhythmia patients at 0.55T using HAPTIC is possible, and we identify failure cases. Impact: HAPTIC could potentially replace ECG gating, even in patients with cardiac arrhythmia. This would simplify clinical workflow and serve scanners that do not have an integrated ECG.

  PublisherDOI

• A Flexible MRI Array based on Direct-3D-Write Technology at 0.55T

  Proceedings on CD-ROM - International Society for Magnetic Resonance in Medicine. Scientific Meeting and Exhibition/Proceedings of the International Society for Magnetic Resonance in Medicine, Scientific Meeting and Exhibition · 2025-09-16

  article

  Motivation: MR receiver arrays are typically made from rigid copper which is slow to manufacture and cannot fully conform to imaging anatomy or adapt smoothly during movement. Goal(s): To demonstrate a flexible 4-element array that can conform to target anatomy, can perform well with parallel imaging, and is fast and inexpensive to manufacture Approach: We utilize highly conductive silver-based ink and a fast direct-3D-write method that uses easy to modify components. Results: The flexible array provided good targeted coverage, adequate array decoupling (<7% coupling between elements), and satisfactory parallel imaging performance (average g-factors: 1.1 for rate-2 and 1.4 for rate-3) Impact: Flexible MRI receiver arrays, created using 3D-write technology, can be made to conform to the target imaging anatomy while also offering scalability, low cost, and quick manufacturing. They are conductive enough to provide adequate SNR for effective parallel imaging.

  PublisherDOI

• A Feasibility Study of Task-Based fMRI at 0.55 T

  Proceedings on CD-ROM - International Society for Magnetic Resonance in Medicine. Scientific Meeting and Exhibition/Proceedings of the International Society for Magnetic Resonance in Medicine, Scientific Meeting and Exhibition · 2025-09-16

  article

  Motivation: 0.55T MRI offers advantages compared to conventional field strengths, including reduced susceptibility artifacts and better compatibility with simultaneous EEG recordings. However, reliable task-based fMRI at 0.55T has not been significantly demonstrated. Goal(s): To establish a robust task-based fMRI protocol and analysis pipeline at 0.55T that achieves full brain coverage and results comparable to what is expected for activation extent and location. Approach: We attempted fMRI at 0.55T by combining EPI acquisition with custom analysis techniques. Finger-tapping and visual tasks compared 5 and 10-minute runs to enhance activation detection. Results: Significant activations demonstrating that high-quality task-based fMRI is achievable at 0.55T in single subjects. Impact: This study demonstrates that reliable task-based fMRI is feasible on 0.55T scanners, potentially broadening functional neuroimaging access in clinical and research settings where high-field MRI is unavailable or impractical, supporting broader diagnostic and research applications.

  PublisherDOI

• Cartesian <scp>MaxGIRF</scp> : Model‐based <scp>EPI</scp> reconstruction incorporating gradient nonlinearity and concomitant field effects

  Magnetic Resonance in Medicine · 2025-10-03

  articleOpen accessSenior author

  PURPOSE: Lower field strength scanners with large bore size or complex geometries, and scanners with stronger gradient systems experience increased gradient nonlinearity and concomitant fields, each of which causes distortions in EPI. Current correction approaches based on image-domain interpolation introduce undesirable spatial blurring. To avoid spatial blurring, we introduce a model-based EPI reconstruction framework, denoted Cartesian MaxGIRF ("Max"well field correction using "GIRF"-predicted gradients), that simultaneously compensates the effects of concomitant fields, gradient nonlinearity, and off-resonance during image reconstruction. THEORY AND METHODS: Performance of the proposed framework was compared against standard correction methods using phantom datasets at 0.55T: (1) 2D spin-echo EPI (SE-EPI) with reversed phase-encoding directions and (2) accelerated 2D SE-EPI with partial Fourier. Two unique EPI image artifacts induced by concomitant fields ("parabolic shift" and "slice-dependent Nyquist ghost") were demonstrated and mitigated by the proposed framework using long-ETL 3D GRE-EPI and high-resolution 3D GRE-EPI, respectively. Resolution improvements and artifact mitigations by the proposed framework were demonstrated using in-vivo human brain datasets: (1) accelerated 2D diffusion-weighted SE-EPI and (2) high-resolution 3D GRE-EPI at 0.55T. RESULTS: The amount of the parabolic shift for each imaging case was theoretically analyzed. The proposed framework demonstrated the mitigation of both parabolic shifts and slice-dependent Nyquist ghosts and retained better image details than standard correction methods when mitigating geometric distortions for all scenarios. CONCLUSION: The Cartesian MaxGIRF framework simultaneously mitigates the effects of concomitant fields, gradient nonlinearity, and static off-resonance. This approach is particularly useful to mitigate artifacts induced by second-order concomitant fields present in both symmetric and asymmetric gradient systems.

  PublisherOA PDFDOI

• Pathogenic SIV infection is associated with acceleration of epigenetic age in rhesus macaques

  Journal of Clinical Investigation · 2025-07-14 · 1 citations

  articleOpen access

  HIV infection accelerates biological aging, but the contribution of the host's age to this process is unknown. We investigated the influence of SIV infection in macaques (SIVmac) on the risk of comorbidities and aging in young and old rhesus macaques (RMs) by assessing pathogenesis markers, DNA methylation-based epigenetic age (EA), and EA acceleration (EAA) in blood and tissues. Initially, upon SIV infection, the young RMs showed greater resilience to CD4+ T cell depletion, better control of T cell activation, hypercoagulation, and excessive inflammation, yet this resilience was progressively lost in the advanced stages of infection. During the late stages of infection, the young RMs, but not the aged ones, showed an increase in EA in PBMCs; also, EAA in the cerebellum and heart of young RMs was higher compared with old RMs. SIV infection was more pathogenic in aged animals in early stages, leading to a more rapid disease progression; however, accelerated aging mostly affected young animals, so that the levels of multiple key pathogenesis markers in the young RMs converged toward those specific to aged ones in the late stages of infection. We conclude that SIV infection-driven age acceleration is tissue specific, and that host age influences the susceptibility of different tissues to enhanced aging.

  PublisherDOI

• Acceleration of slice encoding for metal artifact correction at 0. <scp>55 T</scp> using hexagonal sampling

  Magnetic Resonance in Medicine · 2025-10-02 · 1 citations

  articleOpen accessSenior author

  PURPOSE: To evaluate a hexagonal sampling approach for accelerated slice encoding for metal artifact correction (SEMAC) at 0.55 T. Contemporary mid-field systems (0.1 T-1.0 T) show tremendous potential for imaging near metal implants. However, the limited parallel-imaging options necessitate alternative methods for scan time reduction. METHODS: We apply retrospective hexagonal undersampling to current state-of-the-art SEMAC with 2-fold generalized autocalibrating partially parallel acquisitions-based parallel imaging at 0.55 T. The hexagonal sampling approach results in an additional 50% scan time reduction. Feasibility is evaluated with phantom experiments involving spinal fixation and total hip arthroplasty hardware, and in vivo experiments involving patients with spinal fusions with varying compositions and 1 volunteer with a total hip arthroplasty. RESULTS: Hexagonal sampling provides an additional 50% scan time reduction with compatible image quality. The two tradeoffs are (i) a small increase in signal void due to the loss of signal from one SEMAC spectral bin during post-acquisition filtering and (ii) position-dependent signal-to-noise-ratio reduction at locations close to the edge of the field of view in the phase-encoding direction. CONCLUSION: We demonstrate that hexagonal sampling can provide 50% scan time reduction in addition to generalized autocalibrating partially parallel acquisitions/parallel imaging for SEMAC at 0.55 T without introducing substantial artifacts. This may be a valuable mechanism for reducing scan time at 0.55 T and other midfield strengths, where parallel-imaging acceleration is limited.

  PublisherOA PDFDOI

• Free-breathing 3D pulmonary ventilation mapping at 0.55T using Stack-of-Spiral Out-in bSSFP

  Proceedings on CD-ROM - International Society for Magnetic Resonance in Medicine. Scientific Meeting and Exhibition/Proceedings of the International Society for Magnetic Resonance in Medicine, Scientific Meeting and Exhibition · 2025-09-16

  articleSenior author

  Motivation: The diagnosis and evaluation of pulmonary dysfunction would benefit from a volumetric approach for ventilation mapping. Goal(s): To demonstrate an efficient and repeatable 3D regional pulmonary ventilation mapping technique, and to compare with existing approaches(2D PREFUL and tidal volume). Approach: We utilize 5-minute free-breathing 3D stack-of-spiral out-in balanced SSFP, constrained reconstruction, and non-rigid registration, to produce 3D maps of regional ventilation. In six healthy volunteers, we compare the proposed method against 2D PREFUL and tidal volume change. Results: The proposed regional ventilation maps show good test-retest repeatability(CoV&lt;5%). Ventilation estimates have strong correlations with PREFUL on a slice-matched basis, and with global tidal volume(R2&gt;0.7). Impact: The proposed approach for 3D regional ventilation mapping requires 5min is feasible and provides consistent measurements and show good agreements with PREFUL in healthy volunteers. This may improve the diagnosis and evaluation of patients with pulmonary dysfunction.

  PublisherDOI

Recent grants

• MRI: Development of a High-Performance Low-Field MRI for Dynamic Imaging

  NSF · $2.5M · 2018–2024

• Improved Myocardial Perfusion Assessment using High-Performance Low-Field MRI

  NIH · $454k · 2022–2025

• Novel Myocardial Perfusion Stress Test using Arterial Spin Labeling

  NIH · $1.3M · 2016–2022

• Volumetric Real-Time MRI at 0.55 Tesla

  NIH · $2.0M · 2023–2026

• Novel Myocardial Perfusion Stress Test using Arterial Spin Labeling

  NIH · $433k · 2016–2020

Frequent coauthors

• Shrikanth Narayanan

  49 shared

• Bob S. Hu

  46 shared

• Yongwan Lim

  40 shared

• Ye Tian

  40 shared

• Dwight G. Nishimura

  37 shared

• Yoon‐Chul Kim

  Yonsei University

  36 shared

• John M. Pauly

  36 shared

• Sophia Cui

  28 shared

Education

• Ph.D., Electrical Engineering

  University of Southern California

  2000

• M.S., Electrical Engineering

  University of Southern California

  1996

• B.S., Electrical Engineering

  Indian Institute of Technology, Kharagpur

  1994

Awards & honors

• 2022 Institute for Electrical and Electronics Engineers (IEE…
• 2020 USC Viterbi School of Engineering Use Inspired Research…
• 2018 Society for Cardiovascular Magnetic Resonance (SCMR) Fe…
• 2017 American Institute for Medical and Biological Engineeri…
• 2014 USC Community Engagement Award