High-field MRSI of the prostate using a transmit/receive endorectal coil and gradient modulated adiabatic localization

Short symmetric and highly selective asymmetric first and second order gradient modulated offset independent adiabaticity (GOIA) pulses for applications in clinical MRS and MRSI

Gradient modulated RF pulses, especially gradient offset independent adiabaticity (GOIA) pulses, are increasingly gaining attention for high field clinical magnetic resonance spectroscopy and spectroscopic imaging (MRS/MRSI) due to the lower peak B₁ amplitude and associated power demands achievable relative to its non-modulated adiabatic full passage counterparts. In this work we describe the development of two GOIA RF pulses: 1) A power efficient, 3.0 ms wideband uniform rate with smooth truncation (WURST) modulated RF pulse with 15 kHz bandwidth compatible with a clinically feasible peak B₁ amplitude of 0.87 kHz (or 20 µT), and 2) A highly selective asymmetric 6.66 ms RF pulse with 20 kHz bandwidth designed to achieve a single-sided, fractional transition width of only 1.7%. Effects of potential asynchrony between RF and gradient-modulated (GM) waveforms for 3 ms GOIA-WURST RF pulses was evaluated by simulation and experimentally. Results demonstrate that a 20+ µs asynchrony between RF and GM functions substantially degrades inversion performance when using large RF offsets to achieve translation. A projection-based method is presented that allows a quick calibration of RF and GM asynchrony on pre-clinical/clinical MR systems. The asymmetric GOIA pulse was implemented within a multi-pulse OVS sequence to achieve power efficient, highly-selective, and B₁ and T₁-independent signal suppression for extracranial lipid suppression. The developed GOIA pulses were utilized with linear gradient modulation (X, Y, Z gradient fields), and with second-order-field modulations (Z2, X2Y2 gradient fields) to provide elliptically-shaped regions-of-interest for MRS and MRSI acquisitions. Both described GOIA-RF pulses have substantial clinical value; specifically, the 3.0 ms GOIA-WURST pulse is beneficial to realize short TE sLASER localized proton MRS/MRSI sequences, and the asymmetric GOIA RF pulse has applications in highly selective outer volume signal suppression to allow interrogation of tissue proximal to extracranial lipids with full-intensity.

Developments in proton MR spectroscopic imaging of prostate cancer

In this paper, we review the developments of 1H-MR spectroscopic imaging (MRSI) methods designed to investigate prostate cancer, covering key aspects such as specific hardware, dedicated pulse sequences for data acquisition and data processing and quantification techniques. Emphasis is given to recent advancements in MRSI methodologies, as well as future developments, which can lead to overcome difficulties associated with commonly employed MRSI approaches applied in clinical routine. This includes the replacement of standard PRESS sequences for volume selection, which we identified as inadequate for clinical applications, by sLASER sequences and implementation of 1H MRSI without water signal suppression. These may enable a new evaluation of the complementary role and significance of MRSI in prostate cancer management.

ECLIPSE utilizing gradient-modulated offset-independent adiabaticity (GOIA) pulses for highly selective human brain proton MRSI

A multitude of extracranial lipid suppression methods exist for proton MRSI acquisitions. Popular and emerging lipid suppression methods each have their inherent set of advantages and disadvantages related to the achievable level of lipid suppression, RF power deposition, insensitivity to B₁⁺ field and lipid T₁ heterogeneity, brain coverage, spatial selectivity, chemical shift displacement (CSD) errors and the reliability of spectroscopic data spanning the observed 0.9-4.7 ppm band. The utility of elliptical localization with pulsed second order fields (ECLIPSE) was previously demonstrated with a greater than 100-fold in extracranial lipid suppression and low power requirements utilizing 3 kHz bandwidth AFP pulses. Like all gradient-based localization methods, ECLIPSE is sensitive to CSD errors, resulting in a modified metabolic profile in edge-of-ROI voxels. In this work, ECLIPSE is extended with 15 kHz bandwidth second order gradient-modulated RF pulses based on the gradient offset-independent adiabaticity (GOIA) algorithm to greatly reduce CSD and improve spatial selectivity. An adiabatic double spin-echo ECLIPSE inner volume selection (TE = 45 ms) MRSI method and an ECLIPSE outer volume suppression (TE = 3.2 ms) FID-MRSI method were implemented. Both GOIA-ECLIPSE MRSI sequences provided artifact-free metabolite spectra in vivo, with a greater than 100-fold in lipid suppression and less than 2.6 mm in-plane CSD and less than 3.3 mm transition width for edge-of-ROI voxels, representing an ~5-fold improvement compared with the parent, nongradient-modulated method. Despite the 5-fold larger bandwidth, GOIA-ECLIPSE only required a 1.9-fold increase in RF power. The highly robust lipid suppression combined with low CSD and sharp ROI edge transitions make GOIA-ECLIPSE an attractive alternative to commonly employed lipid suppression methods. Furthermore, the low RF power deposition demonstrates that GOIA-ECLIPSE is very well suited for high field (≥3 T) MRSI applications.

Clinical applications of in vivo magnetic resonance spectroscopy in oncology

Magnetic resonance spectroscopy (MRS) can be performed in vivo using commercial MRI systems to obtain biochemical information about tissues and cancers. Applications in brain, prostate and breast aid lesion detection and characterisation (differential diagnosis), treatment planning and response assessment. Multi-centre clinical trials have been performed in all these tissues. Single centre studies have been performed in many other tissues including cervix, uterus, musculoskeletal and liver. While generally MRS is used to study endogenous metabolites it has also been used in drug studies, for example those that include 19F as part of their structure. Recently the hyperpolarisation of compounds enriched with 13C such as [1-13C] pyruvate has been demonstrated in animal models and now in preliminary clinical studies, permitting the monitoring of biochemical processes with unprecedented sensitivity. This review briefly introduces the underlying methods and then discusses the current status of these applications.

Single-shot single-voxel lactate measurements using FOCI-LASER and a multiple-quantum filter

Measurement of tissue lactate using (1) H MRS is often confounded by overlap with intense lipid signals at 1.3 ppm. Single-voxel localization using PRESS is also compromised by the large chemical shift displacement between voxels for the 4.1 ppm (-CH) resonance and the 1.3 ppm -CH3 resonance, leading to subvoxels with signals of opposite phase and hence partial signal cancellation. To reduce the chemical shift displacement to negligible proportions, a modified semi-LASER sequence was written ("FOCI-LASER", abbreviated as fLASER) using FOCI pulses to permit high RF bandwidth even with the limited RF amplitude characteristic of clinical MRI scanners. A further modification, MQF-fLASER, includes a selective multiple-quantum filter to detect lactate and reject lipid signals. The sequences were implemented on a Philips 3 T Achieva TX system. In a solution of brain metabolites fLASER lactate signals were 2.7 times those of PRESS. MQF-fLASER lactate was 47% of fLASER (the theoretical maximum is 50%) but still larger than PRESS lactate. In oil, the main 1.3 ppm lipid peak was suppressed to less than 1%. Enhanced suppression was possible using increased gradient durations. The minimum detectable lactate concentration was approximately 0.5 mM. Coherence selection gradients needed to be at the magic angle to avoid large water signals derived from intermolecular multiple-quantum coherences. In pilot patient measurements, lactate peaks were often observed in brain tumours, but not in cervix tumours; lipids were effectively suppressed. In summary, compared with PRESS, the fLASER sequence yields greatly superior sensitivity for direct detection of lactate (and equivalent sensitivity for other metabolites), while the single-voxel single-shot MQF-fLASER sequence surpasses PRESS for lactate detection while eliminating substantial signals from lipids. This sequence will increase the potential for in vivo lactate measurement as a biomarker in targeted anti-cancer treatments as well as in measurements of tissue hypoxia.

Mapping of prostate cancer by 1H MRSI

In many studies, it has been demonstrated that (1)H MRSI of the human prostate has great potential to aid prostate cancer management, e.g. in the detection and localisation of cancer foci in the prostate or in the assessment of its aggressiveness. It is particularly powerful in combination with T2 -weighted MRI. Nevertheless, the technique is currently mainly used in a research setting. This review provides an overview of the state-of-the-art of three-dimensional MRSI, including the specific hardware required, dedicated data acquisition sequences and information on the spectral content with background on the MR-visible metabolites. In clinical practice, it is important that relevant MRSI results become available rapidly, reliably and in an easy digestible way. However, this functionality is currently not fully available for prostate MRSI, which is a major obstacle for routine use by inexperienced clinicians. Routine use requires more automation in the processing of raw data than is currently available. Therefore, we pay specific attention in this review on the status and prospects of the automated handling of prostate MRSI data, including quality control. The clinical potential of three-dimensional MRSI of the prostate is illustrated with literature examples on prostate cancer detection, its localisation in the prostate, its role in the assessment of cancer aggressiveness and in the selection and monitoring of therapy.

Improved efficiency on editing MRS of lactate and γ-aminobutyric acid by inclusion of frequency offset corrected inversion pulses at high fields

γ-Aminobutyric acid (GABA) and lactate are metabolites which are present in the brain. These metabolites can be indicators of psychiatric disorders or tumor hypoxia, respectively. The measurement of these weakly coupled spin systems can be performed using MRS editing techniques; however, at high field strength, this can be challenging. This is due to the low available B1 (+) field at high fields, which results in narrow-bandwidth refocusing pulses and, consequently, in large chemical shift displacement artifacts. In addition, as a result of the increased chemical shift displacement artifacts and chemical shift dispersion, the efficiency of the MRS method is reduced, even when using adiabatic refocusing pulses. To overcome this limitation, frequency offset corrected inversion (FOCI) pulses have been suggested as a mean to substantially increase the bandwidth of adiabatic pulses. In this study, a Mescher-Garwood semi-localization by adiabatic selection and refocusing (MEGA-sLASER) editing sequence with refocusing FOCI pulses is presented for the measurement of GABA and lactate in the human brain. Metabolite detection efficiencies were improved by 20% and 75% for GABA and lactate, respectively, when compared with editing techniques that employ adiabatic radiofrequency refocusing pulses. The highly efficient MEGA-sLASER sequence with refocusing FOCI pulses is an ideal and robust MRS editing technique for the measurement of weakly coupled metabolites at high field strengths.

Improving SNR and B1 transmit field for an endorectal coil in 7 T MRI and MRS of prostate cancer

Higher magnetic field strengths like 7 T and above are desirable for MR spectroscopy given the increased spectral resolution and signal to noise ratio. At these field strengths, substantial nonuniformities in B(1)(+/-) and radiofrequency power deposition become apparent. In this investigation, we propose an improvement on a conventionally used endorectal coil, through the addition of a second element (stripline). Both elements are used as transceivers. In the center of the prostate, approximately 40% signal to noise ratio increase is achieved. In fact, the signal to noise ratio gain obtained with the quadrature configuration locally can be even greater than 40% when compared to the single loop configuration. This is due to the natural asymmetry of the B(1)(+/-) fields at high frequencies, which causes destructive and constructive interference patterns. Global specific absorption rate is reduced by almost a factor of 2 as expected. Furthermore, approximately a 4-fold decrease in local specific absorption rate is observed when normalized to the B(1) values in the center of the prostate. Because of the 4-fold local specific absorption rate decrease obtained with the dual channel setup for the same reference B(1) value (20 μT at 3.5 cm depth into the prostate) as compared to the single loop, the transmission power B(1) duty cycle can be increased by a factor 4. Consequently, when using the two-element endorectal coil, the radiofrequency power deposition is significantly reduced and radiofrequency intense sequences with adiabatic pulses can be safely applied at 7 T for (1)H magnetic resonance spectroscopy and MRI in the prostate. Altogether, in vivo (1)H magnetic resonance spectroscopic imaging of prostate cancer with a fully adiabatic sequence operated at a minimum B(1)(+) of 20 μT shows insensitivity to the nonuniform transmit field, while remaining within local specific absorption rate guidelines of 10 W/kg.

Amide proton transfer MR imaging of prostate cancer: a preliminary study

PURPOSE

To evaluate the capability of amide proton transfer (APT) MR imaging for detection of prostate cancer that typically shows a higher tumor cell proliferation rate and cellular density leading to an MRI-detectable overall elevated mobile protein level in higher grade tumors.

MATERIALS AND METHODS

Twelve patients with biopsy-proven prostate cancer were imaged on a 3 Tesla MR imaging system before prostatectomy. APT-MR images were acquired by means of a single-slice single-shot turbo spin echo sequence with a saturation prepulse preparation using 33 different frequency offsets (-8 to 8 ppm, interval 0.5 ppm). For quantification we used the APT ratio (APTR) based on the asymmetry of the magnetization transfer ratio at 3.5 ppm in respect to the water signal. Tumor and peripheral zone benign regions of interest (ROIs) were delineated based on whole mount pathology slides after prostatectomy.

RESULTS

APTR in prostate cancer ROIs was 5.8% ± 3.2%, significantly higher than that in the peripheral zone benign regions (0.3% ± 3.2%, P = 0.002).

CONCLUSION

APT-MR imaging is feasible in prostate cancer detection and has the potential to discriminate between cancer and noncancer tissues.

A decade in prostate cancer: from NMR to metabolomics

Over the past 30 years, continuous progress in the application of nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance spectroscopic imaging (MRSI) to the detection, diagnosis and characterization of human prostate cancer has turned what began as scientific curiosity into a useful clinical option. In vivo MRSI technology has been integrated into the daily care of prostate cancer patients, and innovations in ex vivo methods have helped to establish NMR-based prostate cancer metabolomics. Metabolomic and multimodality imaging could be the future of the prostate cancer clinic--particularly given the rationale that more accurate interrogation of a disease as complex as human prostate cancer is most likely to be achieved through paradigms involving multiple, instead of single and isolated, parameters. The research and clinical results achieved through in vivo MRSI and ex vivo NMR investigations during the first 11 years of the 21st century illustrate areas where these technologies can be best translated into clinical practice.

Spectroscopic imaging with improved gradient modulated constant adiabaticity pulses on high-field clinical scanners

The purpose of this work was to design and implement constant adiabaticity gradient modulated pulses that have improved slice profiles and reduced artifacts for spectroscopic imaging on 3T clinical scanners equipped with standard hardware. The newly proposed pulses were designed using the gradient offset independent adiabaticity (GOIA, Tannus and Garwood[13]) method using WURST modulation for RF and gradient waveforms. The GOIA-WURST pulses were compared with GOIA-HSn (GOIA based on nth-order hyperbolic secant) and FOCI (frequency offset corrected inversion) pulses of the same bandwidth and duration. Numerical simulations and experimental measurements in phantoms and healthy volunteers are presented. GOIA-WURST pulses provide improved slice profile that have less slice smearing for off-resonance frequencies compared to GOIA-HSn pulses. The peak RF amplitude of GOIA-WURST is much lower (40% less) than FOCI but slightly higher (14.9% more) to GOIA-HSn. The quality of spectra as shown by the analysis of lineshapes, eddy currents artifacts, subcutaneous lipid contamination and SNR is improved for GOIA-WURST. GOIA-WURST pulse tested in this work shows that reliable spectroscopic imaging could be obtained in routine clinical setup and might facilitate the use of clinical spectroscopy.