Dr. Pipe's lab has a long history of developing clinically impactful technology for MRI. A few examples are given here.
PROPELLER MRI for motion correction
PROPELLER MRI images are blurry when the patient moves, like traditional magnetic resonance images, as seen on the left. The unique properties of PROPELLER MRI allow the scanner to remove the effects of that motion so that the same image can be corrected, as seen on the right.
Periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) is a method that collects MRI data in an overlapping fashion. Where the data overlap, data consistency can be used to determine whether the patient was moving and can also be used to remove the effects of that motion. PROPELLER MRI (also called MultiVane XD and BLADE) is available on nearly every modern scanner.
- Pipe JG. Motion correction with PROPELLER MRI: Application to head motion and free-breathing cardiac imaging. Magnetic Resonance in Medicine. 1999;42:963.
PROPELLER MRI for diffusion-weighted imaging
The diffusion tensor imaging data on top are from a five-minute scan at 1.5 T, with (left to right) B0, B1000, color FA maps. The DWI data on the bottom are B1000 images of a patient after a stroke, from a five-minute scan at 3 T.
The same properties that allow PROPELLER MRI to correct for motion make it suitable for multishot diffusion-weighted imaging (DWI). Multishot PROPELLER MRI allows the operator to collect high-resolution, distortion-free DWI data.
- Pipe JG, Farthing VG, Forbes KPN. Multishot diffusion-weighted FSE using PROPELLER MRI. Magnetic Resonance in Medicine. 2002;47:42.
Localized quadratic encoding
Localized quadratic encoding
Localized quadratic encoding
Sections through a drinking straw stenosis phantom (flow coming from top left of image) for (left) conventional 2D ToF MRA, and (right) spiral MRI with localized quadratic encoding. The high SNR and better depiction of the intralumenal space of the right image reflect the SNR efficiencies and low motion sensitivities (gradient moments) of both localized quadratic encoding and spiral MRI.
By applying a low flip-angle, frequency-swept radiofrequency (RF) pulse to 2D MRI, one can effectively phase encode through the width of each excited slice as it becomes wider. Subsequent reconstruction can then decompose those excited slabs into smaller, resolved slices with the same signal-to-noise benefit of 3D MRI, but with greater flexibility and efficiency, and without slab boundaries.
This approach also has important flow properties — such as reduced gradient moments and directional sensitivity — that make it particularly suited for time-of-flight magnetic resonance angiography (ToF MRA). The research team is now adding this technology to spiral MRI for rapid, robust ToF MRA.
- Pipe JG. Spatial encoding and reconstruction in MRI with quadratic phase profiles. Magnetic Resonance in Medicine. 1995;33:24.
- Pipe JG. Analysis of localized quadratic encoding and reconstruction. Magnetic Resonance in Medicine. 1996;36:137.
- Pipe JG. Asymmetric sampling along k(slice-select) in two-dimensional multislice MRI. Magnetic Resonance in Medicine 1998;39:625.
- Pipe JG. Flow effects in localized quadratic, partial fourier MRA. Magnetic Resonance in Medicine. 1999;41:309.
Sampling density correction for non-Cartesian imaging
Dr. Pipe's research team created part of a mathematical and computational framework for reconstructing images from data that are sampled in nonuniform patterns. The theory gives mechanisms both for handling undersampled data and for preferentially weighting preferred data in areas of data oversampling. This is an important area in many modern MRI methods, including PROPELLER MRI and spiral MRI.
- Pipe JG. Sampling density compensation in MRI: Rationale and an iterative numerical solution. Magnetic Resonance in Medicine 1999;41:179.
- Pipe JG. Reconstructing MR images from undersampled data: Data-weighting considerations. Magnetic Resonance in Medicine. 2000;43:867.
- Zwart NR, Johnson KO, Pipe JG. Efficient sample density estimation by combining gridding and an optimized kernel. Magnetic Resonance in Medicine. 2012;67:701.
- Pipe JG, Zwart NR, Aboussouan EA, Robison RK, Devaraj A, Johnson KO. A new design and rationale for 3D orthogonally oversampled k-space trajectories. Magnetic Resonance in Medicine. 2011;66:1303.
- Johnson KO, Pipe JG. Convolution kernel design and efficient algorithm for sampling density correction. Magnetic Resonance in Medicine. 2009;61:439.
Robust spiral deblurring with built-in fat and water separation
Dr. Pipe's lab built a mathematical and computational framework for removing the blur that occurs in spiral magnetic resonance images in areas where the magnetic field B0 is nonuniform. Additionally, this method can be adapted to simultaneously separate the magnetic resonance signal from both water and fat components. It is computationally efficient and robust and has been an important part of moving spiral MRI toward clinical adoption.
- Wang D, Zwart NR, Pipe JG. Joint water-fat separation and deblurring for spiral imaging. Magnetic Resonance in Medicine. 2018; doi:10.1002/mrm.26950.
Methods for phase-contrast magnetic resonance angiography
Dr. Pipe's lab has developed unique methods involving phase-contrast magnetic resonance angiography (PC-MRA), including the use of signal loss to infer the distribution of flow within each voxel in an image and a novel approach for highly SNR-efficient collection of PC-MRA data. Because the latter approach also increases signal loss due to intravoxel flow, these two methods are complementary.
The lab is now exploring a combination of these methods with spiral MRI for rapid measurement of high-SNR, robust, information-rich flow data.
- Pipe JG. A simple measure of flow disorder and wall shear stress in phase-contrast MRI. Magnetic Resonance in Medicine. 2003;49:543.
- Zwart NR, Pipe JG. Multidirectional high-moment encoding in phase contrast MRI. Magnetic Resonance in Medicine. 2013;69:1553.