On the use of steady-state signal equations for 2D TrueFISP imaging

Near-isotropic noncontrast MRA of the renal and peripheral arteries using a thin-slab stack-of-stars quiescent interval slice-selective acquisition

PURPOSE

Noncontrast MRA avoids potential risks from gadolinium-based contrast agents. A 2D noncontrast technique, quiescent interval slice-selective (QISS), accurately evaluates the peripheral arteries but has limited spatial resolution along the slice direction. We therefore implemented a prototype thin-slab stack-of-stars version (tsSOS-QISS) with nearly isotropic spatial resolution and tested it in the renal and peripheral arteries of healthy subjects and patients with vascular disease.

METHODS

The study was approved by the hospital institutional review board. A total of 16 subjects were scanned at 1.5 T: 7 for imaging of the renal arteries and 9 for imaging of the peripheral arteries. For tsSOS-QISS of the renal arteries, each slab consisted of about sixteen 1.3-mm-thick or 2.0-mm-thick slices (interpolated to thirty-two 0.65-mm-thick or 1.0-mm-thick 3D partitions) oriented in an oblique axial or oblique coronal view along the length of the target vessel and was acquired in a breath-hold. For tsSOS-QISS of the peripheral arteries, 20 axial overlapping thin slabs were typically acquired, each with twelve 1.3-mm-thick slices (interpolated to twenty-four 0.65-mm-thick 3D partitions). Image quality, vessel sharpness in multiplanar reconstructions, and normalized SNR were measured.

RESULTS

Image quality and normalized SNR in the renal and peripheral arteries were significantly better compared with 2D QISS acquired at the same spatial resolution, while vessel sharpness was improved in multiplanar reconstructions of the renal arteries.

CONCLUSION

The tsSOS-QISS technique overcomes a significant limitation of 2D QISS by providing nearly isotropic spatial resolution with improved image quality, normalized SNR, and vessel sharpness in multiplanar reconstructions.

Influence of longitudinal position on the evolution of steady-state signal in cardiac cine balanced steady-state free precession imaging

Background

Emerging quantitative cardiac magnetic resonance imaging (CMRI) techniques use cine balanced steady-state free precession (bSSFP) to measure myocardial signal intensity and probe underlying physiological parameters. This correlation assumes that steady-state is maintained uniformly throughout the heart in space and time.

Purpose

To determine the effects of longitudinal cardiac motion and initial slice position on signal deviation in cine bSSFP imaging by comparing two-dimensional (2D) and three-dimensional (3D) acquisitions.

Material and Methods

Nine healthy volunteers completed cardiac MRI on a 1.5-T scanner. Short axis images were taken at six slice locations using both 2D and 3D cine bSSFP. 3D acquisitions spanned two slices above and below selected slice locations. Changes in myocardial signal intensity were measured across the cardiac cycle and compared to longitudinal shortening.

Results

For 2D cine bSSFP, 46% ± 9% of all frames and 84% ± 13% of end-diastolic frames remained within 10% of initial signal intensity. For 3D cine bSSFP the proportions increased to 87% ± 8% and 97% ± 5%. There was no correlation between longitudinal shortening and peak changes in myocardial signal. The initial slice position significantly impacted peak changes in signal intensity for 2D sequences (P < 0.001).

Conclusion

The initial longitudinal slice location significantly impacts the magnitude of deviation from steady-state in 2D cine bSSFP that is only restored at the center of a 3D excitation volume. During diastole, a transient steady-state is established similar to that achieved with 3D cine bSSFP regardless of slice location.

Effect of slice excitation profile on ungated steady state cardiac perfusion imaging

In cardiac perfusion imaging, choice of flip angle is an important factor for steady state acquisition. This work focuses on presenting an analytical framework for understanding how non-ideal slice excitation profiles affect contrast in ungated 2D steady state cardiac perfusion studies, and to study a technique for estimating flip angle that maximizes enhanced/unenhanced myocardial contrast-to-noise ratio (CNR) in single slice and multi-slice acquisitions. A numerical simulation of ungated 2D golden ratio radial spoiled gradient echo (SPGR) was created that takes into consideration the actual (Bloch simulated) slice excitation profile. The effect of slice excitation profile on myocardial CNR as a function of flip angle was assessed in phantoms and in-vivo. For fast RF pulses, the flip angle that yields maximum CNR (considering the actual slice excitation profile) was considerably higher than expected, assuming an ideal excitation. The simulation framework presented accurately predicts the flip angle yielding maximum CNR when the actual slice excitation profile is taken into consideration. The prescribed flip angle for optimal contrast in ungated 2D steady-state SPGR cardiac perfusion studies can vary significantly from that calculated when an ideal slice excitation profile is assumed. Consideration of the actual slice excitation can yield a more optimal flip angle estimate in both the single slice and multi-slice cases.

Flip-angle profile of slice-selective excitation and the measurement of the MR longitudinal relaxation time with steady-state magnetization

In MRI, the flip angle (FA) of slice-selective excitation is not uniform across the slice-thickness dimension. This work investigates the effect of the non-uniform FA profile on the accuracy of a commonly-used method for the measurement, in which the T1 value, i.e., the longitudinal relaxation time, is determined from the steady-state signals of an equally-spaced RF pulse train. By using the numerical solutions of the Bloch equation, it is shown that, because of the non-uniform FA profile, the outcome of the T1 measurement depends significantly on T1 of the specimen and on the FA and the inter-pulse spacing τ of the pulse train. A new method to restore the accuracy of the T1 measurement is described. Different from the existing approaches, the new method also removes the FA profile effect for the measurement of the FA, which is normally a part of the T1 measurement. In addition, the new method does not involve theoretical modeling, approximation, or modification to the underlying principle of the T1 measurement. An imaging experiment is performed, which shows that the new method can remove the FA-, the τ-, and the T1-dependence and produce T1 measurements in excellent agreement with the ones obtained from a gold standard method (the inversion-recovery method).

Fast monitoring of T(1) , T(2) , and relative proton density (M(0) ) changes in skeletal muscles using an IR-TrueFISP sequence

PURPOSE

To investigate the feasibility of fast and simultaneous assessment of T(1) , T(2) , and M(0) (relative proton density) changes in skeletal muscle studies using an inversion recovery true fast imaging with steady-state precession (TrueFISP) sequence.

MATERIALS AND METHODS

NMR signal dynamics in calf muscles were analyzed under four different conditions: intravenous injection of a low-molecular weight Gd contrast agent (CA), postarterial occlusion reactive hyperemia, local cooling, and an exercise bout. Experiments were conducted on a clinical 3T whole-body scanner.

RESULTS

At rest, average muscle T(1) and T(2) values obtained from the IR-TrueFISP experiments were 1.34 ± 0.13 seconds and 45 ± 5 msec, respectively (median ± standard deviation). 1) Noticeable T(1) decreases (ΔT(1) max ≈-30%) were measured in the calf muscles after CA injection, while no significant changes were observed for T(2) and M(0) . 2) T(2) increased rapidly during reactive hyperemia and reached a peak value (+6%) at about 1 minute postischemia. During ischemia, a significant decrease was observed only in the soleus muscle. No significant paradigm-related changes in M(0) and T(1) were noted in all muscle groups, except in the m. soleus (ΔT(1) ≈+1% during reactive hyperemia). 3) Opposite variations in muscle T(1) (ΔT(1) max ≈-30%) and M(0) (ΔM(0) max ≈+25%) associated with local cooling were detected. 4) Concomitant changes in T(1) (ΔT(1) max ≈+15%), T(2) (ΔT(2) max ≈+35%), and M(0) (ΔM(0) max ≈+16%) were observed in the activated muscles following the exercise bout.

CONCLUSION

IR-TrueFISP was sufficiently fast and sensitive to detect small and transient T(1) , T(2) , and M(0) changes in the calf muscles under different experimental conditions. The sequence offers a time-resolution adequate to track rapid physiological adaptations in skeletal muscle.

Prediction of myocardial signal during CINE balanced SSFP imaging

OBJECT

To develop a signal model for accurate prediction of myocardial signal during cine-balanced steady-state free precession (bSSFP) imaging.

METHODS

We present a signal model that takes into account the effects of non-ideal slice profile, off-resonance, and radio-frequency transmit variation on myocardial signal behavior. Each of the three factors was examined over the range of imaging parameters routinely used in cine bSSFP cardiac imaging at 3 Tesla.

RESULTS

In five healthy volunteers and over a wide range of prescribed flip angles, the conventional on-resonance signal model exhibited 28.9 +/- 3.9% error, while the proposed model exhibited only 2.9 +/- 1.4% error, and therefore more accurate predictions of myocardial signal behavior. Slice profile effects were found to be significant and accounted for most of the improvement. Off-resonance and RF transmit inhomogeneity effects were less significant but did produce more accurate signal prediction.

CONCLUSIONS

The proposed signal model produced more accurate predictions of myocardial signal compared to existing models and can be used for the optimization of pulse sequences and protocols.