Comparison of k-t SENSE/k-t BLAST with conventional SENSE applied to BOLD fMRI.

PURPOSE: To compare k-t BLAST (broad-use linear-acquisition speedup technique)/k-t SENSE (sensitivity encoding) with conventional SENSE applied to a simple fMRI paradigm. MATERIALS AND METHODS: Blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) was performed at 3 T using a displaced ultra-fast low-angle refocused echo (UFLARE) pulse sequence with a visual stimulus in a block paradigm. Conventional SENSE and k-t BLAST/k-t SENSE data were acquired. Also, k-t BLAST/k-t SENSE was simulated at different undersampling factors from fully sampled data after removal of lines of k-space data. Analysis was performed using SPM5. RESULTS: Sensitivity to the BOLD response in k-t BLAST/k-t SENSE was comparable with that of SENSE in images acquired at an undersampling factor of 2.3. Simulated k-t BLAST/k-t SENSE yielded reliable detection of activation-induced BOLD contrast at undersampling factors of 5 or less. Sensitivity increased significantly when training data were included in k-space before Fourier transformation (known as "plug-in"). CONCLUSION: k-t BLAST/k-t SENSE performs at least as well as conventional SENSE for BOLD fMRI at a modest undersampling factor. Results suggest that sufficient sensitivity to BOLD contrast may be achievable at higher undersampling factors with k-t BLAST/k-t SENSE than with conventional parallel imaging approaches, offering particular advantages at the highest magnetic field strengths.

Comparison of left ventricular function assessment using phonocardiogram- and electrocardiogram-triggered 2D SSFP CINE MR imaging at 1.5 T and 3.0 T.

OBJECTIVE: As high-field cardiac MRI (CMR) becomes more widespread the propensity of ECG to interference from electromagnetic fields (EMF) and to magneto-hydrodynamic (MHD) effects increases and with it the motivation for a CMR triggering alternative. This study explores the suitability of acoustic cardiac triggering (ACT) for left ventricular (LV) function assessment in healthy subjects (n = 14). METHODS: Quantitative analysis of 2D CINE steady-state free precession (SSFP) images was conducted to compare ACT's performance with vector ECG (VCG). Endocardial border sharpness (EBS) was examined paralleled by quantitative LV function assessment. RESULTS: Unlike VCG, ACT provided signal traces free of interference from EMF or MHD effects. In the case of correct R-wave recognition, VCG-triggered 2D CINE SSFP was immune to cardiac motion effects-even at 3.0 T. However, VCG-triggered 2D SSFP CINE imaging was prone to cardiac motion and EBS degradation if R-wave misregistration occurred. ACT-triggered acquisitions yielded LV parameters (end-diastolic volume (EDV), end-systolic volume (ESV), stroke volume (SV), ejection fraction (EF) and left ventricular mass (LVM)) comparable with those derived from VCG-triggered acquisitions (1.5 T: ESV(VCG) = (56 +/- 17) ml, EDV(VCG) = (151 +/- 32) ml, LVM(VCG) = (97 +/- 27) g, SV(VCG) = (94 +/- 19) ml, EF(VCG) = (63 +/- 5)% cf. ESV(ACT) = (56 +/- 18) ml, EDV(ACT) = (147 +/- 36) ml, LVM(ACT) = (102 +/- 29) g, SV(ACT) = (91 +/- 22) ml, EF(ACT) = (62 +/- 6)%; 3.0 T: ESV(VCG) = (55 +/- 21) ml, EDV(VCG) = (151 +/- 32) ml, LVM(VCG) = (101 +/- 27) g, SV(VCG) = (96 +/- 15) ml, EF(VCG) = (65 +/- 7)% cf. ESV(ACT) = (54 +/- 20) ml, EDV(ACT) = (146 +/- 35) ml, LVM(ACT) = (101 +/- 30) g, SV(ACT) = (92 +/- 17) ml, EF(ACT) = (64 +/- 6)%). CONCLUSIONS: ACT's intrinsic insensitivity to interference from electromagnetic fields renders it suitable for clinical CMR.

Feasibility of k-t BLAST for BOLD fMRI with a spin-echo based acquisition at 3 T and 7 T.

OBJECTIVES: This study tested the feasibility of applying k-t BLAST to blood oxygen level dependent functional MRI of the brain at 3 Tesla (T) and at 7 T. Shorter echo train lengths, achieved through the application of k-t BLAST, are expected to counteract increased sensitivity to inhomogeneities in B0 at higher magnetic field strengths, especially in echo planar images, and reduce the relatively long acquisition times and high RF power deposition in spin-echo based methods. MATERIALS AND METHODS: k-t BLAST was combined with displaced UFLARE at 3 T and 7 T. Temporal and spatial fidelity of k-t BLAST were investigated using a test object, in which localized variations in signal intensity mimic activation-induced signal changes. fMRI was performed using typical box-car design finger tapping. In a separate analysis full k-space data were decimated to simulate k-t BLAST acquisitions and compare results with the fully sampled data, thereby avoiding physiological and noise differences between acquisitions. RESULTS: Activation can be detected at under-sampling factors as high as 16, whereas appropriately reconstructed data, under-sampled at factors below 8 entail insignificant loss of sensitivity and considerable reductions in acquisition times and RF power deposition. CONCLUSIONS: k-t BLAST is compatible with fMRI acquisitions and opens up possibilities including distortion-free T2*-weighted blood oxygen level dependent fMRI with displaced UFLARE at high magnetic field strengths.

Feasibility of cardiac gating free of interference with electro-magnetic fields at 1.5 Tesla, 3.0 Tesla and 7.0 Tesla using an MR-stethoscope.

OBJECTIVES: To circumvent the challenges of conventional electrocardiographic (ECG)-gating by examining the efficacy of an MR stethoscope, which offers (i) no risk of high voltage induction or patient burns, (ii) immunity to electromagnetic interference, (iii) suitability for all magnetic field strengths, and (iv) patient comfort together with ease of use for the pursuit of reliable and safe (ultra)high field cardiac gated magnetic resonance imaging (MRI). MATERIALS AND METHODS: The acoustic gating device consists of 3 main components: an acoustic sensor, a signal processing unit, and a coupler unit to the MRI system. Signal conditioning and conversion are conducted outside the 0.5 mT line using dedicated electronic circuits. The final waveform is delivered to the internal physiological signal controller circuitry of a clinical MR scanner. Cardiovascular MRI was performed of normal volunteers (n = 17) on 1.5 T, 3.0 T and 7.0 T whole body MR systems. Black blood imaging, 2D CINE imaging, 3D phase contrast MR angiography, and myocardial T2* mapping were carried out. RESULTS: The MR-stethoscope provided cardiograms at 1.5 T, 3.0 T and 7.0 T free of interference from electromagnetic fields and magneto-hydrodynamic effects. In comparison, ECG waveforms were susceptible to T-wave elevation and other distortions, which were more pronounced at higher fields. Acoustically gated black blood imaging at 1.5 T and 3.0 T provided image quality comparable with or even superior to that obtained from the ECG-gated approach. In the case of correct R-wave recognition, ECG-gated 2D CINE SSFP imaging was found to be immune to cardiac motion effects -even at 3.0 T. However, ECG-gated 2D SSFP CINE imaging was prone to cardiac motion artifacts if R-wave mis-registration occurred because of T-wave elevation. Acoustically gated 3D PCMRA at 1.5 T, 3.0 T and 7.0 T resulted in images free of blood pulsation artifacts because the acoustic gating approach provided cardiac signal traces free of interference with electromagnetic fields or magneto-hydrodynamic effects even at 7.0 Tesla. Severe ECG-trace distortions and T-wave elevations occurred at 3.0 T and 7.0 T. Acoustically cardiac gated T2* mapping at 3.0 T yielded a T2* value of 22.3 +/- 4.8 ms for the inferoseptal myocardium. CONCLUSIONS: The proposed MR-stethoscope presents a promising alternative to currently available techniques for cardiac gating of (ultra)high field MRI. Its intrinsic insensitivity to interference from electromagnetic fields renders it suitable for clinical imaging because of its excellent trigger reliability, even at 7.0 Tesla.

Myocardial T2* mapping free of distortion using susceptibility-weighted fast spin-echo imaging: a feasibility study at 1.5 T and 3.0 T.

This study demonstrates the feasibility of applying free-breathing, cardiac-gated, susceptibility-weighted fast spin-echo imaging together with black blood preparation and navigator-gated respiratory motion compensation for anatomically accurate T2* mapping of the heart. First, T2* maps are presented for oil phantoms without and with respiratory motion emulation T2* = (22.1 +/- 1.7) ms at 1.5 T and T2* = (22.65 +/- 0.89) ms at 3.0 T). T2* relaxometry of a ferrofluid revealed relaxivities of R2* = (477.9 +/- 17) mM(-1)s(-1) and R2* = (449.6 +/- 13) mM(-1)s(-1) for UFLARE and multiecho gradient-echo imaging at 1.5 T. For inferoseptal myocardial regions mean T2* values of 29.9 +/- 6.6 ms (1.5 T) and 22.3 +/- 4.8 ms (3.0 T) were estimated. For posterior myocardial areas close to the vena cava T2*-values of 24.0 +/- 6.4 ms (1.5 T) and 15.4 +/- 1.8 ms (3.0 T) were observed. The merits and limitations of the proposed approach are discussed and its implications for cardiac and vascular T2*-mapping are considered.

Understanding and optimizing the amplitude modulated control for multiple-slice continuous arterial spin labeling.

Multiple-slice perfusion imaging by continuous arterial spin labeling (CASL) is made possible by amplitude modulation (AM) of the labeling RF pulse, but perfusion sensitivity is reduced relative to the single-slice technique. A computer model of the Bloch equations for velocity driven adiabatic fast passage was developed to elucidate the compromised sensitivity to perfusion of the AM control technique for CASL. Calculations were performed over ranges of RF pulse amplitude, B1; magnetic field gradient, G; phase, phi, and frequency, f, of the modulation function; velocity, v, and relaxation times, T1 and T2, of blood. It was found that unless f>2piB1, phi determines the performance of the AM control; excessively high B1 or v reduces the efficiency of the AM control; and T1 relaxation dominates if f is too great. In vivo, in rat brain (n=5) at 2.35 T, the sensitivity of the AM technique to perfusion was 70% of the sensitivity of single-slice CASL.

Velocity-driven adiabatic fast passage for arterial spin labeling: results from a computer model.

Velocity-driven adiabatic fast passage (AFP) is commonly employed for perfusion imaging by continuous arterial spin labeling (CASL). The degree of inversion of protons in blood determines the sensitivity of CASL to perfusion. For this study, a computer model of the modified Bloch equations was developed to establish the optimum conditions for velocity-driven AFP. Natural variations in blood velocity over the course of the cardiac cycle were found to result in significant variations in the degree of inversion. However, the mean degree of inversion was similar to that for blood moving at a constant velocity, equal to the time-averaged mean, at peak velocities and heart rates within normal ranges. A train of RF pulses instead of a continuous RF pulse for labeling was found to result in a highly nonlinear dependence of the degree of inversion on RF duty cycle. This may have serious implications for the quantification of perfusion.