Profile order and time-dependent artifacts in contrast-enhanced coronary MR angiography at 3T: origin and prevention

Cardiovascular MR imaging at 3 T: opportunities, challenges, and solutions

Although 3-T magnetic resonance (MR) imaging is well established in neuroradiology and musculoskeletal imaging, it is in the nascent stages in cardiovascular imaging applications, and there is limited literature on this topic. The primary advantage of 3 T over 1.5 T is a higher signal-to-noise ratio (SNR), which can be used as such or traded off to improve spatial or temporal resolution and decrease acquisition time. However, the actual gain in SNR is limited by other factors and modifications in sequences adapted for use at 3 T. Higher resonance frequencies result in improved spectral resolution, which is beneficial for fat suppression and spectroscopy. The higher T1 values of tissues at 3 T aid in myocardial tagging, angiography, and perfusion and delayed-enhancement sequences. However, there are substantial challenges with 3-T cardiac MR imaging, including higher magnetic field and radiofrequency inhomogeneities and susceptibility effects, which diminish image quality. Off-resonance artifacts are particularly challenging, especially with steady-state free precession sequences. These artifacts can be managed by using higher-order shimming, frequency scouts, or low repetition times. B1 inhomogeneities can be managed by using radiofrequency shimming, multitransmit coils, or adiabatic pulses. Chemical shifts are also increased at 3 T. The higher radiofrequency results in higher radiofrequency deposition power and a higher specific absorption rate. MR angiography, dynamic first-pass perfusion sequences, myocardial tagging, and MR spectroscopy are more effective at 3 T, whereas delayed-enhancement, flow quantification, and black-blood sequences are comparable at 1.5 T and 3 T. Knowledge of the relevant physics helps in identifying artifacts and modifying sequences to optimize image quality. Online supplemental material is available for this article.

Tortuosity of coronary bifurcation as a potential local risk factor for atherosclerosis: CFD steady state study based on in vivo dynamic CT measurements

The purpose of the present study was to determine whether in vivo bifurcation geometric factors would permit prediction of the risk of atherosclerosis. It is worldwide accepted that low or oscillatory wall shear stress (WSS) is a robust hemodynamic factor in the development of atherosclerotic plaque and has a strong correlation with the local site of plaque deposition. However, it still remains unclear how coronary bifurcation geometries are correlated with such hemodynamic forces. Computational fluid dynamics simulations were performed on left main (LM) coronary bifurcation geometries derived from CT of eight patients without significant atherosclerosis. WSS amplitudes were accurately quantified at two high risk zones of atherosclerosis, namely at proximal left anterior descending artery (LAD) and at proximal left circumflex artery (LCx), and also at three high WSS concentration sites near the bifurcation. Statistical analysis was used to highlight relationships between WSS amplitudes calculated at these five zones of interest and various geometric factors. The tortuosity index of the LM-LAD segment appears to be an emergent geometric factor in determining the low WSS amplitude at proximal LAD. Strong correlations were found between the high WSS amplitudes calculated at the endothelial regions close to the flow divider. This study not only demonstrated that CT imaging studies of local risk factor for atherosclerosis could be clinically performed, but also showed that tortuosity of LM-LAD coronary branch could be used as a surrogate marker for the onset of atherosclerosis.

3.0-T whole-heart coronary magnetic resonance angiography: comparison of gadobenate dimeglumine and gadofosveset trisodium

Gadolinium enhanced coronary magnetic resonance angiography (MRA) at 3 T appears to be superior to non-contrast methods. Gadofosveset is an intravascular contrast agent that may be well suited to this application. The purpose of this study was to perform an intra-individual comparison of gadofosveset and gadobenate for coronary MRA at 3 T. In this prospective randomized study, 22 study subjects [8 (36%) male; 27.9 ± 6 years; BMI = 22.8 ± 2 kg/m(2)] underwent two studies using a contrast-enhanced inversion recovery three-dimensional fast low angle shot MRA at 3 T. The order of contrast agent administration was varied randomly, separated by an average of 30 ± 5 days, using either gadobenate dimeglumine (Gd-BOPTA; Bracco, 0.1 mmol/Kg) or gadofosveset trisodium (MS-325; Lantheus Med, 0.03 mmol/Kg). Acquisition time, signal-to-noise ratio (SNR) of coronary vessels and contrast-to-noise ratio (CNR) were evaluated. Of 308 coronary arteries and veins segment analyzed, overall SNR of coronary arteries and veins segments were not different for the two contrast agents (132 ± 79 for gadofosveset vs. 135 ± 78 for gadobenate, p = 0.69). Coronary artery CNR was greater for gadofosveset in comparison to gadobenate (73.5 ± 46.9 vs. 59.3 ± 75.7 respectively, p = 0.03). Gadofosveset-enhanced MRA images displayed better image quality than gadobenate-enhanced MRA images (2.77 ± 0.61 for gadofosveset vs. 2.11 ± 0.51, p < .001). Inter- and intra-reader variability was excellent (ICC > 0.90) for both contrast agents. Gadofosveset trisodium appears to show slightly better performance for coronary MRA at 3 T compared to gadobenate.

Pulmonary vein morphology by free-breathing whole heart magnetic resonance imaging at 3 Tesla versus breathhold multi-detector computed tomography

PURPOSE

To compare pulmonary vein and left atrial anatomy using three-dimensional free-breathing whole-heart magnetic resonance imaging (MR) at 3 Tesla (T) and multi-detector computed tomography (MDCT).

MATERIALS AND METHODS

Thirty-three subjects (19 male, age 49 ± 12 years) underwent free-breathing 3T MR and contrast-enhanced MDCT during inspiratory breath hold. Pulmonary vein parameters (ostial areas, diameters, angles) were measured.

RESULTS

All pulmonary veins and anomalies were identified by 3T MR and by MDCT. The right-sided pulmonary veins were directed more posteriorly, the right superior pulmonary vein more inferiorly, and the right inferior pulmonary vein more superiorly by 3T MR when compared with MDCT. The cross-sectional area, perimeters and minimum diameters of right-sided pulmonary vein ostia were significantly larger by MR, as were the maximum diameters of right and left inferior pulmonary veins. There were no significant differences between techniques in distance to first pulmonary vein branch.

CONCLUSION

Pulmonary vein measurements demonstrated significant differences in angulations and dimensions when 3T MR is compared with MDCT. These differences likely represent hemodynamic and respiratory variation during free-breathing with MR versus breath-holding with MDCT. MR imaging at 3T during free-breathing offers an alternate method to define pulmonary vein and left atrial anatomy without exposure to radiation.

Use of 2D sensitivity encoding for slow-infusion contrast-enhanced isotropic 3-T whole-heart coronary MR angiography

OBJECTIVE

The purpose of this study was to improve the blood-pool signal-to-noise ratio (SNR) and blood-myocardium contrast-to-noise ratio (CNR) of slow-infusion 3-T whole-heart coronary MR angiography (MRA).

SUBJECTS AND METHODS

In 2D sensitivity encoding (SENSE), the number of acquired k-space lines is reduced, allowing less radiofrequency excitation per cardiac cycle and a longer TR. The former can be exploited for signal enhancement with a higher radiofrequency excitation angle, and the latter leads to noise reduction due to lower data-sampling bandwidth. Both effects contribute to SNR gain in coronary MRA when spatial and temporal resolution and acquisition time remain identical. Numeric simulation was performed to select the optimal 2D SENSE pulse sequence parameters and predict the SNR gain. Eleven patients underwent conventional unenhanced and the proposed 2D SENSE contrast-enhanced coronary MRA acquisition. Blood-pool SNR, blood-myocardium CNR, visible vessel length, vessel sharpness, and number of side branches were evaluated.

RESULTS

Consistent with the numeric simulation, using 2D SENSE in contrast-enhanced coronary MRA resulted in significant improvement in aortic blood-pool SNR (unenhanced vs contrast-enhanced, 37.5 ± 14.7 vs 121.3 ± 44.0; p < 0.05) and CNR (14.4 ± 6.9 vs 101.5 ± 40.8; p < 0.05) in the patient sample. A longer length of left anterior descending coronary artery was visualized, but vessel sharpness, coronary artery coverage, and image quality score were not improved with the proposed approach.

CONCLUSION

In combination with contrast administration, 2D SENSE was found effective in improving SNR and CNR in 3-T whole-heart coronary MRA. Further investigation of cardiac motion compensation is necessary to exploit the SNR and CNR advantages and to achieve submillimeter spatial resolution.

Contrast-enhanced whole-heart coronary MRI with bolus infusion of gadobenate dimeglumine at 1.5 T

We sought to investigate the T(1) kinetics of blood and myocardium after three infusion schemes of gadobenate dimeglumine (Gd-BOPTA) and subsequently compared contrast-enhanced whole-heart coronary MRI after a bolus Gd-BOPTA infusion with nonenhanced coronary MRI at 1.5 T. Blood and myocardium T(1) was measured in seven healthy adults, after each underwent three Gd-BOPTA infusion schemes (bolus: 0.2 mmol/kg at 2 mL/sec, hybrid: 0.1 mmol/kg at 2 mL/sec followed by 0.1 mmol/kg at 0.1 mL/sec, and slow: 0.2 mmol/kg at 0.3 mL/sec). Fourteen additional subjects underwent contrast-enhanced coronary MRI with an inversion-recovery steady-state free precession sequence after bolus Gd-BOPTA infusion. Images were compared with nonenhanced T(2) -prepared steady-state free precision whole-heart coronary MRI in signal-to-noise ratio, contrast-to-noise ratio, depicted vessel length, vessel sharpness, and subjective image quality. Bolus and slow infusion schemes resulted in similar T(1) during coronary MRI, whereas the hybrid infusion method yielded higher T(1) values. A bolus infusion of Gd-BOPTA significantly improved signal-to-noise ratio, contrast-to-noise ratio, depicted coronary artery length, and subjective image quality, when all segments were collectively compared but not when compared segment by segment. In conclusion, whole-heart steady-state free precision coronary MRI at 1.5 T can benefit from a bolus infusion of 0.2 mmol/kg Gd-BOPTA.

Cardiovascular magnetic resonance at 3.0 T: current state of the art

There are advantages to conducting cardiovascular magnetic resonance (CMR) studies at a field strength of 3.0 Telsa, including the increase in bulk magnetization, the increase in frequency separation of off-resonance spins, and the increase in T1 of many tissues. However, there are significant challenges to routinely performing CMR at 3.0 T, including the reduction in main magnetic field homogeneity, the increase in RF power deposition, and the increase in susceptibility-based artifacts.In this review, we outline the underlying physical effects that occur when imaging at higher fields, examine the practical results these effects have on the CMR applications, and examine methods used to compensate for these effects. Specifically, we will review cine imaging, MR coronary angiography, myocardial perfusion imaging, late gadolinium enhancement, and vascular wall imaging.