Nikolai Mickevicius headshot

Nikolai Mickevicius, PhD

Bentson Translational Research Fellow

Department of Human Oncology

I am a Bentson Translational Research Fellow in the Department of Human Oncology. Throughout my undergraduate studies at the Milwaukee School of Engineering, I accumulated a desire to understand the underlying physics of medical imaging devices and the signal processing required to produce images of diagnostic quality. This inclination culminated in undergraduate research and industry experiences working in MRI and CT which solidified my decision to pursue medical imaging graduate studies. At the Medical College of Wisconsin (MCW), I earned my PhD in Biophysics while developing impactful motion management solutions for MRI-guided radiation therapy applications. Following graduation, I continued my research in this area for three years as a research scientist at MCW with a strong focus on technology development for the Elekta Unity MR-Linac. I now have the well-defined career goal of leading a research lab developing novel MRI data acquisition and image reconstruction methods to extract quantitative tissue properties with high degrees of precision, repeatability, and reproducibility. Under the mentorship of Dr. Carri Glide-Hurst, I am learning what it takes to achieve this objective while tackling challenging problems in the field of MR-guided radiotherapy.


PhD, Medical College of Wisconsin, Biophysics (2018)

BS, Milwaukee School of Engineering, Biomedical Engineering (2013)

Academic Appointments

Bentson Translational Research Fellow, Department of Human Oncology (2021)

Research Scientist, Department of Radiation Oncology, Medical College of Wisconsin (2018)

Selected Honors and Awards

1st Place Research Poster, ISMRM Workshop on Data Sampling and Image Reconstruction (2020)

Young Investigator Award, 3rd International Symposium on MRI in Radiation Therapy (2015)

Boards, Advisory Committees and Professional Organizations

International Society for Magnetic Resonance in Medicine (ISMRM), 2014-Pres.

American Association of Physicists in Medicine (AAPM), 2021-Pres.

Research Focus

• Magnetic Resonance Imaging Physics

Magnetic resonance imaging (MRI) solutions for image-guided therapy applications.

MRI is no longer just a diagnostic tool. The versatile capabilities of MRI make it suitable for therapeutic planning and guiding treatment deliveries, particularly for radiation therapy applications. From motion management to biologically adaptive radiotherapy, many areas of focus require tailored MRI technology developments. A few projects that I have worked on in this field are shown below.   

Project 1: Generalized Simultaneous Multi-Orientation MRI

Flexibility in slice prescription is critical for precise real-time motion monitoring during MR-guided therapies. Adding more slices to improve spatial coverage during rapid 2D cine imaging often hampers temporal resolution. This work describes a framework to simultaneously acquire multiple arbitrarily oriented slices that share one common axis. This framework allows for higher frame rates for a given number of slices compared to conventional interleaved-slice multi-orientation cine imaging. A novel pulse sequence capable of measuring data from several arbitrarily oriented slices was developed and tested in vivo. Example images are shown in Figure 1. The proposed technique demonstrated promise for continued development for motion monitoring during MR-guided therapies.

Four abdominal imaging slices at different orientations between Coronal and Sagittal
Figure 1. Four abdominal imaging slices at different orientations between Coronal (left) and Sagittal (right). These data for each of these slices were simultaneously acquired using the proposed novel method.





Respiratory Phase Resolved MRI

Creating radiotherapy treatment plans for abdominal and thoracic tumors is challenging due to the potential for respiration-induced motion of the tumor. To compensate for this motion during planning, margins will be increased around the target volume resulting in a larger volume being irradiated. In an effort to reduce these margins while sparing healthy tissue outside of the tumor, respiratory phase resolved (i.e., 4D) imaging may be utilized. In the context of online adaptive 4D-MRI guided radiotherapy, the acquisition and reconstruction of the 4D images must be carried out in a very narrow time window. To help meet these criteria, we developed a subspace-constrained image reconstruction method to reduce the computation complexity of reconstructing the 4D-MRI images while also improving image quality. Example images using this approach are shown in Figure 2. Compared with conventional methods, the subspace-constrained reconstruction method reduced processing times by nearly three minutes. These methods were tested as part of a retrospective study from five patients on a 1.5T clinical MR-Linac.

4D-MRI images
Figure 2. 4D-MRI images reconstructed using the proposed subspace-constrained reconstruction algorithm. Four contrasts are shown: Turbo Field Echo (TFE), TFE with fat suppression (SPAIR TFE), balanced Turbo Field Echo (bTFE), and bTFE with fat suppression (SPAIR bTFE). The bottom row shows a projection along the superior/inferior direction as a function of 10 respiratory phases between inspiration and expiration.






















Project 3: Dose Accumulation in the Presence of Respiratory Motion

The ability to accumulate radiation dose in the presence of respiratory motion during the treatment of abdominal and thoracic tumors is highly sought after. In an ideal world, 3D tomographic images would be acquired at a sufficient frame rate to resolve respiratory motion. However, the fulcrum upon which signal-to-noise ratio, spatial resolution, and temporal resolution balances in MRI is delicate; Acquiring high frame rate 3D cine imaging is not feasible. Thus, efforts to use combined 2D cine imaging along with a pre-beam 4D-MRI dataset have been proposed. Due to motion differences that could occur between the start of pre-treatment planning and treatment delivery, this is not easily accomplished. Thus, we sought to develop a method that not only provides rapid 2D cine imaging for real time positional verification during treatment, but also simultaneously acquires 4D images. Using this approach, an initial experiment irradiated a target filled with dose sensitive gel while moving with respiratory-like motion driven by a dynamic phantom. The accumulated dose matches closely with that measured experimentally by gel dosimetry (See Figure 3).

Measured 3D dose distributions
Figure 3. (Top row) Measured 3D dose distributions obtained via R1 mapping of FXG gel in a treated 3D-printed target of a dynamic motion phantom. (Bottom row) Theoretical dose distribution calculated using the proposed MR-guided dose accumulation method. Focal dose hot spots due to the electron return effect are observable at air tissue interfaces.
  • On the use of low-dimensional temporal subspace constraints to reduce reconstruction time and improve image quality of accelerated 4D-MRI
  • Split-slice training and hyperparameter tuning of RAKI networks for simultaneous multi-slice reconstruction
  • A Patient-Specific Autosegmentation Strategy Using Multi-Input Deformable Image Registration for Magnetic Resonance Imaging-Guided Online Adaptive Radiation Therapy: A Feasibility Study
  • 4D-MRI driven MR-guided online adaptive radiotherapy for abdominal stereotactic body radiation therapy on a high field MR-Linac: Implementation and initial clinical experience
  • Generalized simultaneous multi-orientation 2D imaging
  • Simultaneous acquisition of orthogonal plane cine imaging and isotropic 4D-MRI using super-resolution
  • Simultaneous motion monitoring and truth-in-delivery analysis imaging framework for MR-guided radiotherapy
  • Simultaneous orthogonal plane cine imaging with balanced steady-state free-precession contrast using k-t GRAPPA
  • Investigation of undersampling and reconstruction algorithm dependence on respiratory correlated 4D-MRI for online MR-guided radiation therapy
  • Simultaneous orthogonal plane imaging
  • Wavelet Domain Radiofrequency Pulse Design Applied to Magnetic Resonance Imaging
  • Location of brain tumor intersecting white matter tracts predicts patient prognosis
  • Precise ex vivo histological validation of heightened cellularity and diffusion-restricted necrosis in regions of dark apparent diffusion coefficient in 7 cases of high-grade glioma

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