Volumetric and multispectral DWI near metallic implants using a non-linear phase Carr-Purcell-Meiboom-Gill diffusion preparation

Abstract

Purpose: DWI near metal implants has not been widely explored due to substantial challenges associated with through-slice and in-plane distortions, the increased encoding requirement of different spectral bins, and limited SNR. There is no widely adopted clinical protocol for DWI near metal since the commonly used EPI trajectory fails completely due to distortion from extreme off-resonance ranging from 2 to 20 kHz. We present a sequence that achieves DWI near metal with moderate b-values (400-500 s/mm² ) and volumetric coverage in clinically feasible scan times.

Theory and methods: Multispectral excitation with Cartesian sampling, view angle tilting, and kz phase encoding reduce in-plane and through-plane off-resonance artifacts, and Carr-Purcell-Meiboom-Gill (CPMG) spin-echo refocusing trains counteract T2* effects. The effect of random phase on the refocusing train is eliminated using a stimulated echo diffusion preparation. Root-flipped Shinnar-Le Roux refocusing pulses permits preparation of a high spectral bandwidth, which improves imaging times by reducing the number of excitations required to cover the desired spectral range. B₁ sensitivity is reduced by using an excitation that satisfies the CPMG condition in the preparation. A method for ADC quantification insensitive to background gradients is presented.

Results: Non-linear phase refocusing pulses reduces the peak B₁ by 46% which allows RF bandwidth to be doubled. Simulations and phantom experiments show that a non-linear phase CPMG pulse pair reduces B₁ sensitivity. Application in vivo demonstrates complementary contrast to conventional multispectral acquisitions and improved visualization compared to DW-EPI.

Conclusion: Volumetric and multispectral DW imaging near metal can be achieved with a 3D encoded sequence.

Keywords: diffusion preparation; diffusion weighted imaging; distortionless diffusion; multispectral imaging; root-flipped SLR pulses.

FIGURE 1:

Stimulated echo diffusion preparation that corrects non-CPMG artifacts. Dephasing prior to tipup ensures uniform signal is returned to the longitudinal axis, regardless of phase from bulk motion. View Angle Tilting (VAT) gradients are applied during readout to reduce in-plane distortion as in most selective MSI methods.

FIGURE 2:

Magnetization path for different pulse pairs at separate locations in the slice profile after excitation and refocusing. During excitation, the magnetization rotates about the orange arrow and follows the path denoted by the orange dotted line. During refocusing, the magnetization vector rotates about the blue arrow. A) Linear phase pulses have a rotation axis that is constant in the slice direction. The refocusing axis of rotation is offset 90° from the excitation to satisfy the CPMG condition. B) Excitation and refocusing rotation axes are matched so that the magnetization is aligned along the x direction at the end of the pulse pair. The gray arrow represents the magnetization after excitation. The rotation axes vary across the slice. C) With an identical phase pulse pair, magnetization at the end of the 90–180 pair does not have linear phase but the CPMG condition is satisfied within the slice.

FIGURE 3:

Top: Beta magnitude and phase profile of a minimum-phase, TBW 5 SLR pulse (A) compared to the beta profile of the minimum peak B1 RF obtained using root-flipping (B). The phase of the excitation beta profile (C) is identical to that of the refocusing pulse. Bottom: RF pulse obtained using the SLR transform and a minimum-phase alpha polynomial. The root-flipping algorithm reduces the peak B1 from 0.34 G to 0.19 G. The root-flipped SLR approach enables the use of a short and high TBW refocusing pulse that remains within the maximum B1 limit.

FIGURE 4:

A-C) Simulated slice profiles of the transverse m_x demonstrate the improvement of a twice-refocused, non-linear phase CPMG preparation (C) over single-refocused (A) and twice-refocused with linear phase excitation and tipup (B). RF waveforms are shown on the left, where subscripts x, y, denote linear phase slice profiles, and θ(z) a non-linear phase profile. The dotted line shows the ideal cos² 2π(2z) modulation resulting from rephasing the stimulated echo after 2 cycles of spoiling prior to the tipup. Negative m_x values add destructively. D) The total signal, determined by the area in the shaded regions of the simulated slice profiles, shows the reduced B1 sensitivity of the twice-refocused scheme with identical phase 90s. The maximum value is 0.5 due to 2x signal loss from the stimulated echo pathway. The theoretical maximum signal assumes an ideal twice-refocused CPMG preparation.

FIGURE 5:

Effect of excitation beta profile on B1 sensitivity of preparations using root-flipped refocusing pulses in a uniform agar ball phantom. A) Center slice of the linear phase refocusing and excitation preparation with dotted yellow line showing location of extracted 1D signal profile. B) A linear phase excitation with root-flipped 180 has evident B1 shading, corresponding to the B1 map in D. C) An identical phase excitation with root-flipped 180 has similar shading to the linear phase preparation. The signal profiles of B and C normalized to the linear phase preparation in A are shown in E.

FIGURE 6:

Three of the 16 acquired bin images at a fixed slice location from different preparation schemes and the sum-of-squares bin combination. The difference is calculated using the low bandwidth bin combined image as reference. Top row: Preparation with only linear phase pulses reduces the bin bandwidth, resulting in unexcited spins close to the head of the implant and above the phantom (yellow arrow). Middle row: A root-flipped refocusing pulse enables greater bin bandwidth, exciting spins that are not excited with the low bandwidth preparation (yellow, red arrows). Signal differences adjacent to the shaft (blue arrow) are likely due to B1 affecting the stimulated echo pathway. B1 inhomogeneity also causes band-like signal loss above the head. Bottom row: An excitation with identical phase to the root-flipped refocusing pulse reduces band-like signal loss above the implant (green arrow).

FIGURE 7:

A-F) DW-MSI b = 0, 500 s/mm², and calculated ADC maps obtained with and without metal using a twice-refocused M0-nulled sequence. ADC maps are unaffected by the static background gradient caused by metal-induced off-resonance and exhibit no spatial distortion due to B0 inhomogeneity. The femoral head lies in-plane, denoted by the circle in (D). G) Mean ADC values in each vial with the corresponding H₂O/acetone concentrations by volume.

FIGURE 8:

Comparison of ADC maps overlaid on b = 0 s/mm² images acquired with DW-MSI and DW-EPI in subject with ACL reconstruction surgery, with reference PD and STIR MAVIRC-SL. The metal screw (green arrow) causes severe distortion in DW-EPI. DW-MSI corrects these distortions and permits ADC calculation in tissue near the screw. Cartilage (purple arrow), and joint fluid (blue arrow) are visible on both DW-EPI and DW-MSI. The popliteal artery (yellow arrow) is bright on b0 DW-EPI but not b0 DW-MSI due to first order moments in the CPMG refocusing train and a small minimum refocusing flip angle.

FIGURE 9:

A) Patient with unilateral hip replacement. Fluid that is bright on STIR MAVRIC-SL (dashed white arrows), appears on the ADC map as an elevated ADC. Soft tissue edema is indicated by the dotted white arrows. DW-MSI has reduced spectral coverage compared to MAVRIC-SL due to limited scan time, resulting in signal appearing on MAVRIC-SL but not in DW-MSI (purple arrow). B) The green arrow indicates an area where the phase navigator resolution was too low, resulting in loss of signal and ADC overestimation. A signal void due to reduced spectral coverage relative to MAVRIC-SL is present on DW-MSI (orange arrow). C) Patient with bilateral hip replacement has a small amount of fluid above the right femur (solid white arrow) which can be visualized with DW-MSI and appears on axial STIR. D) Coronal radiographs for implant visualization.