Dualband spectral‐spatial RF pulses for prostate MR spectroscopic imaging

Abstract

Although MR spectroscopic imaging (MRSI) of the prostate has demonstrated clinical utility for the staging and monitoring of cancer extent, current acquisition methods are often inadequate in several aspects. Conventional 180°pulses can suffer from chemical shift misregistration, and have high peak-power requirements that can exceed hardware limits in many prostate MRSI studies. Optimal water and lipid suppression are also critical to obtain interpretable spectra. While complete suppression of the periprostatic lipid resonance is desired, controlled partial suppression of water can provide a valuable phase and frequency reference for data analysis and an assessment of experimental success in cases in which all other resonances are undetectable following treatment. In this study, new spectral-spatial RF pulses were developed to negate chemical shift misregistration errors and to provide dualband excitation with partial excitation of the water resonance and full excitation of the metabolites of interest. Optimal phase modulation was also included in the pulse design to provide 40% reduction in peak RF power. Patient studies using the new pulses demon- Recently, 3D magnetic resonance spectroscopic imaging (MRSI) studies have demonstrated clinical utility for determining the location and spatial extent of cancer within the prostate (1-3) and its spread outside the gland (4). Prostate cancer can be metabolically identified with high specificity by an elevation in the choline-to-citrate peak area ratio relative to normal prostatic tissues. The high specificity of MRSI complements the high sensitivity of MRI for identifying cancer, and MRSI is now used routinely in clinical/research MR staging exams at our institution to provide a combined anatomic/metabolic assessment of prostate cancer extent. For the combined MRI/ MRSI prostate exam to be accepted in general clinical practice, robust acquisition techniques must be developed and implemented. The conventional point-resolved spectroscopy (PRESS) (5-7) technique is often limited by inadequate water or lipid suppression and chemical shift misregistration.Spectral-spatial excitation pulses (8 -11) have demonstrated the ability to address these problems (8,12). This type of RF pulse excites a slice, but only at a specific range of frequencies. They are also known as echo-planar spinecho (EPSE) pulses, because they use a gradient waveform that is essentially the same as echo-planar imaging (EPI). The spectral-spatial PRESS sequence uses two EPSE 180°p ulses to define both spatial and spectral profiles (8). However, the complete suppression of the water resonance provided by these pulses is undesirable for clinical MRSI since residual water signals can serve as valuable references for phase and frequency correction. The detection of residual water in each voxel is also critical for monitoring treatment response. Following hormone ablation or radiation therapy, choline, creatine, and citrate levels can decrease to noise levels (1,2). In these cases, the observation of residua...

Prostate Cancer: Prediction of Biochemical Failure after External-Beam Radiation Therapy—Kattan Nomogram and Endorectal MR Imaging Estimation of Tumor Volume

“…The following data were extracted: age, pretreatment PSA level, clinical stage, Gleason score, date of radiation treatment, treatment dose, use of hormonal therapy, PSA level nadir, date of biochemical failure lipid resonance. The infl uence of chemical shift on the apparent location of the selected volume was also reduced by the higher spectral bandwidth of the spectral-spatial pulses ( 34,35 ). Outer voxel saturation pulses were also used to further sharpen volume selection and conform the selected volume to the shape of the prostate to eliminate susceptibility artifacts from periprostatic fat and rectal air ( 36 ).…”

Prostate Cancer Managed with Active Surveillance: Role of Anatomic MR Imaging and MR Spectroscopic Imaging

“…Threedimensional MR spectroscopic imaging data were acquired by using a waterand lipid-suppressed double spin-echo point-resolved spectroscopy sequence, with spectral-spatial pulses for the two 180° excitation pulses, that was optimized for the quantitative detection of choline, creatine, polyamines, and citrate. The spectral-spatial pulses enabled both sharp volume selection and frequency selection to reduce the water resonance and suppress lipid resonances ( 11,12 ). The infl uence of chemical shift on the apparent location of the selected volume was reduced by the high spectral bandwidth of the spectral-spatial pulses ( 11,12 ).…”

Clinical Stage T1c Prostate Cancer: Evaluation with Endorectal MR Imaging and MR Spectroscopic Imaging

“…The point-resolved spatial selection box was positioned to maximize coverage of the prostate while minimizing the inclusion of periprostatic fat. Water and lipid suppression within the point-resolved spatial selection volume was achieved by using the spectral or spatial pulses (15,16), and selective outer-voxel suppression pulses were used to reduce contamination from surrounding tissues (17). The MR spectroscopic imaging data were processed by using the Functool package with the GE Advantage workstation (GE Medical Systems), which enables one to align the spectral data with the MR images and archive arrays of spectral data with the corresponding images in Digital Imaging and Communications in Medicine format.…”