Optimization of alternating TR-SSFP for fat-suppression in abdominal images at 3T

Optimization of flip angle and radiofrequency pulse phase to maximize steady-state magnetization in three-dimensional missing pulse steady-state free precession

Missing pulse (MP) steady-state free precession (SSFP) is a magnetic resonance imaging (MRI) pulse sequence that is highly tolerant to the magnetic field inhomogeneity. In this study, optimal flip angle and radiofrequency (RF) phase scheduling in three-dimensional (3D) MP-SSFP is introduced to maximize the steady-state magnetization while keeping broadband excitation to cover widely distributed frequencies generated by inhomogeneous magnetic fields. Numerical optimization based on extended phase graph (EPG) simulation was performed to maximize the MP-SSFP steady-state magnetization. To limit the specific absorption rate (SAR) associated with the broadband excitation in 3D MP-SSFP, SAR constraint was introduced in the numerical optimization. Optimized flip angle and RF phase settings were experimentally tested by introducing a linear inhomogeneous magnetic field in a range of 10-20 mT/m and using a phantom with known T1/T2 relaxation and diffusion parameters at 3 T. The experimental results were validated through comparisons with EPG simulation. Image contrasts and molecular diffusion effects were investigated in in vivo human brain imaging with 3D MP-SSFP with the optimal flip angle and RF phase settings. In the phantom measurements, the optimal flip angle and RF phase settings improved the MP-SSFP steady-state magnetization/signal-to-noise ratio by up to 41% under the fixed SAR conditions, which matched well with EPG simulation results. In vivo brain imaging with the optimal RF pulse settings provided T2-like image contrasts. Diffusion effects were relatively minor with the linear inhomogeneous field of 10-20 mT/m for white and gray matter, but cerebrospinal fluid showed conspicuous signal intensity attenuation as the linear inhomogeneous field increased. Numerical optimization achieved significant improvement in the steady-state magnetization in MP-SSFP compared with the RF pulse settings used in previous studies. The proposed flip angle and RF phase optimization is promising to improve 3D MP-SSFP image quality for MRI in inhomogeneous magnetic fields.

Real-time 4D MRI using MR signature matching (MRSIGMA) on a 1.5T MR-Linac system

Objective. To develop real-time 4D MRI using MR signature matching (MRSIGMA) for volumetric motion imaging in patients with pancreatic cancer on a 1.5T MR-Linac system.Approach. Two consecutive MRI scans with 3D golden-angle radial stack-of-stars acquisitions were performed on ten patients with inoperable pancreatic cancer. The complete first scan (905 angles) was used to compute a 4D motion dictionary including ten pairs of 3D motion images and signatures. The second scan was used for real-time imaging, where each angle (275 ms) was processed separately to match it to one of the dictionary entries. The complete second scan was also used to compute a 4D reference to assess motion tracking performance.Dicecoefficients of the gross tumor volume (GTV) and two organs-at-risk (duodenum-stomach and small bowel) were calculated between signature matching and reference. In addition, volume changes, displacements, center of mass shifts, andDicescores over time were calculated to characterize motion.Main results. Total imaging latency of MRSIGMA (acquisition + matching) was less than 300 ms. TheDicecoefficients were 0.87 ± 0.06 (GTV), 0.86 ± 0.05 (duodenum-stomach), and 0.85 ± 0.05 (small bowel), which indicate high accuracy (high mean value) and low uncertainty (low standard deviation) of MRSIGMA for real-time motion tracking. The center of mass shift was 3.1 ± 2.0 mm (GTV), 5.3 ± 3.0 mm (duodenum-stomach), and 3.4 ± 1.5 mm (small bowel). TheDicescores over time (0.97 ± [0.01-0.03]) were similarly high for MRSIGMA and reference scans in all the three contours.Significance. This work demonstrates the feasibility of real-time 4D MRI using MRSIGMA for volumetric motion tracking on a 1.5T MR-Linac system. The high accuracy and low uncertainty of real-time MRSIGMA is an essential step towards continuous treatment adaptation of tumors affected by real-time respiratory motion and could ultimately improve treatment safety by optimizing ablative dose delivery near gastrointestinal organs.

Magnetic resonance imaging with gradient sound respiration guide

Respiratory motion management is crucial for high-resolution MRI of the heart, lung, liver and kidney. In this article, respiration guide using acoustic sound generated by pulsed gradient waveforms was introduced in the pulmonary ultrashort echo time (UTE) sequence and validated by comparing with retrospective respiratory gating techniques. The validated sound-guided respiration was implemented in non-contrast enhanced renal angiography. In the sound-guided respiration, breathe−in and–out instruction sounds were generated with sinusoidal gradient waveforms with two different frequencies (602 and 321 Hz). Performance of the sound-guided respiration was evaluated by measuring sharpness of the lung-liver interface with a 10–90% rise distance, w_10-90, and compared with three respiratory motion managements in a free-breathing UTE scan: without respiratory gating (w/o gating), 0-dimensional k-space navigator (k-point navigator), and image-based self-gating (Img-SG). The sound-guided respiration was implemented in stack-of-stars balanced steady-state free precession with inversion recovery preparation for renal angiography. No subjects reported any discomfort or inconvenience with the sound-guided respiration in pulmonary or renal MRI scans. The lung-liver interface of the UTE images for sound-guided respiration (w_10-90 = 6.99 ± 2.90 mm), k-point navigator (8.51 ± 2.71 mm), and Img-SG (7.01 ± 2.06 mm) was significantly sharper than that for w/o gating (17.13 ± 2.91 mm; p < 0.0001 for all of sound-guided respiration, k-point navigator and Img-SG). Sharpness of the lung-liver interface was comparable between sound-guided respiration and Img-SG (p = 0.99), but sound-guided respiration achieved better visualization of pulmonary vasculature. Renal angiography with the sound-guided respiration clearly delineated renal, segmental and interlobar arteries. In conclusion, the gradient sound guided respiration can facilitate a consistent diaphragm position in every breath and achieve performance of respiratory motion management comparable to image-based self-gating.

Usefulness of 3D balanced turbo-field-echo MR sequence evaluating the branching pattern of the intrahepatic bile ducts: comparison with drip infusion CT cholangiography

PURPOSE

For liver surgery, it is crucial to preoperatively examine the course of the right posterior bile duct. While MR cholangiopancreatography (MRCP) can only visualize the bile ducts, 3D balanced turbo-field-echo (BTFE) sequence clearly depicts the bile ducts and portal veins as well as drip infusion CT cholangiography (DIC-CT), without contrast media. We evaluated whether BTFE could substitute for DIC-CT.

MATERIALS AND METHODS

Thirty patients undergoing MRCP and BTFE on 1.5-T MR and DIC-CT were evaluated. Two readers retrospectively evaluated the branching pattern (supra-type: A-C or infra-type: D-E) and scored the degree of confidence and motion artifacts using a 3-point scale for the three 2-mm-thick reconstructed images.

RESULTS

The bile duct diameter did not differ between DIC-CT and MRCP (p = 0.07). Five patients (17%) had intrahepatic biliary dilatation (>3 mm). The A, B, C, D, and E types were diagnosed in 21, 6, 1, 1, and 1 patient, respectively (28 supra-types and 2 infra-types) on DIC-CT. For DIC-CT, MRCP, and BTFE, the mean motion artifact scores were 3.0/3.0, 2.7/2.6, and 2.9/2.8, respectively. The mean diagnostic confidence scores were 2.9/2.9, 2.4/2.4, and 2.9/2.8, respectively, with no difference between DIC-CT and BTFE. The concordance between DIC-CT and BTFE was high (infra- or supra-type: κ = 1.00/1.00, A-E: κ = 0.86/0.66), but it was poor between DIC-CT and MRCP (infra- or supra-type: κ = 0.35/-0.05, A-E: κ = 0.33/0.41) for both readers.

CONCLUSIONS

Similar to DIC-CT, the BTFE MR sequence had high diagnostic accuracy regarding the branching pattern of the intrahepatic bile duct, especially for the supra/infraportal type.

Self-gated fat-suppressed cardiac cine MRI

PURPOSE

To develop a self-gated alternating repetition time balanced steady-state free precession (ATR-SSFP) pulse sequence for fat-suppressed cardiac cine imaging.

METHODS

Cardiac gating is computed retrospectively using acquired magnetic resonance self-gating data, enabling cine imaging without the need for electrocardiogram (ECG) gating. Modification of the slice-select rephasing gradients of an ATR-SSFP sequence enables the acquisition of a one-dimensional self-gating readout during the unused short repetition time (TR). Self-gating readouts are acquired during every TR of segmented, breath-held cardiac scans. A template-matching algorithm is designed to compute cardiac trigger points from the self-gating signals, and these trigger points are used for retrospective cine reconstruction. The proposed approach is compared with ECG-gated ATR-SSFP and balanced steady-state free precession in 10 volunteers and five patients.

RESULTS

The difference of ECG and self-gating trigger times has a variability of 13 ± 11 ms (mean ± SD). Qualitative reviewer scoring and ranking indicate no statistically significant differences (P > 0.05) between self-gated and ECG-gated ATR-SSFP images. Quantitative blood-myocardial border sharpness is not significantly different among self-gated ATR-SSFP ( 0.61±0.15 mm -1), ECG-gated ATR-SSFP ( 0.61±0.15 mm -1), or conventional ECG-gated balanced steady-state free precession cine MRI ( 0.59±0.15 mm -1).

CONCLUSION

The proposed self-gated ATR-SSFP sequence enables fat-suppressed cardiac cine imaging at 1.5 T without the need for ECG gating and without decreasing the imaging efficiency of ATR-SSFP.

Evaluation of the small bowel in inflammatory bowel disease

Crohn's disease is a chronic inflammatory bowel disease (IBD) that can affect the entire GI tract, and adequate visualization of the small bowel is imperative for both diagnosis and management. Magnetic resonance and computed tomography enterography have gradually replaced barium-based studies. Magnetic resonance enterography has the distinct advantage of avoiding ionizing radiation to which many patients with IBD are overexposed. Endoscopy-based techniques, including capsule endoscopy and device-assisted enteroscopy, allow direct visualization of the small bowel mucosa. Deep enteroscopy has the additional benefit of allowing sampling of the mucosa for histological analysis. Small intestine contrast ultrasound is rapidly gaining credence as an excellent, radiation-free imaging technique, but is not available in all countries. Other imaging modalities, such as positron emission tomography and leucocyte scintigraphy, continue to be studied and may have a role in specific circumstances. This review summarizes the evidence for the various techniques for evaluating the small bowel in IBD.