On the mechanisms enabling myocardial edema contrast in bSSFP-based imaging approaches

A dual flip angle 3D bSSFP magnetization transfer-like method to differentiate between recent and old myocardial infarction

BACKGROUND

Magnetic resonance imaging (MRI) tissue signal is modulated by magnetization transfer (MT) phenomena, intrinsically induced by balanced steady-state free precession (bSSFP) imaging.

PURPOSE

To investigate the possible value of such a MT-like bSSFP approach in two clinical settings involving focal myocardial lesions highligthed by late gadolinium enhancement (LGE+): edema induced by recent myocardial infarction (MI) and fibrotic scar related to chronic infarction.

MATERIALS AND METHODS

Population: 48 LGE + patients were studied: 26 with recent MI, 22 with chronic MI. 20 LGE-normal subjects were considered the control group. Field strength/sequence: Navigator-based short axis 3D-bSSFP sequences with 20° and 90° excitation flip angles were acquired (1.5T).

ASSESSMENT

Pixel-wise normalized MT Ratio (nMTR) parametric images were calculated according to: nMTR = 100*(S₂₀ -S₉₀ *k)/S₂₀ , with S₂₀ and S₉₀ signal intensity in 20° and 90° flip angle images and k = Blood₂₀ /Blood₉₀ as a normalization ratio. Statistical tests: analysis of variance (ANOVA), receiver operating characteristic (ROC) analysis.

RESULTS

Overall normal myocardial nMTR was 50.2 ± 3.6%. In recent MI, nMTR values were significantly reduced in LGE + regions (-22.3 ± 9.9%, P < 0.0001). In cases of chronic infarct, nMTR was significantly increased in LGE + regions (14.2 ± 11.4%, P < 0.0001). Comparison between observed results and theoretical values obtained with the Freeman-Hill formula showed that most variations observed in MI are related to MT effects instead of relaxation effects.

CONCLUSION

In contrast to LGE imaging, which may show a similar hyperenhancement in recent and old infarctions, nMTR imaging demonstrates an opposite pattern: decreased values for recent infarction and increased values for old infarction, thus allowing to discriminate between these two clinical conditions without gadolinium injection.

LEVEL OF EVIDENCE

3 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:798-808.

Gadolinium free cardiovascular magnetic resonance with 2-point Cine balanced steady state free precession

BACKGROUND

Cardiovascular magnetic resonance (CMR) of ventricular structure and function is widely performed using cine balanced steady state free precession (bSSFP) MRI. The bSSFP signal of myocardium is weighted by magnetization transfer (MT) and T1/T2-relaxation times. In edematous and fibrotic tissues, increased T2 and reduced MT lead to increased signal intensity on images acquired with high excitation flip angles. We hypothesized that acquisition of two differentially MT-weighted bSSFP images (termed 2-point bSSFP) can identify tissue that would enhance with gadolinium similar to standard of care late gadolinium enhancement (LGE).

METHODS

Cine bSSFP images (flip angles of 5° and 45°) and native-T1 and T2 maps were acquired in one mid-ventricular slice in 47 patients referred for CMR and 10 healthy controls. Afterwards, LGE images and post-contrast T1 maps were acquired and gadolinium partition coefficient (GPC) was calculated. Maps of ΔS/So were calculated as (S45-S5)/S5*100 (%), where Sflip_angle is the voxel signal intensity.

RESULTS

Twenty three patients demonstrated areas of myocardial hyper-enhancement with LGE. In enhanced regions, ΔS/So, native-T1, T2, and GPC were heightened (p < 0.05 vs. non-enhanced tissues). ΔS/So, native-T1, and T2 all demonstrated association with GPC, however the association was strongest for ΔS/So. Bland-Altman analysis revealed a slight bias towards larger volume of enhancement with ΔS/So compared to LGE, and similar transmurality. Subjective analysis with 2-blinded expert readers revealed agreement between ΔS/So and LGE of 73.4 %, with false positive detection of 16.7 % and false negative detection of 15.2 %.

CONCLUSIONS

Gadolinium free 2-point bSSFP identified tissue that enhances at LGE with strong association to GPC. Our results suggest that with further development, MT-weighted CMR could be used similar to LGE for diagnostic imaging.

Contrast-enhanced Steady-state Free Precession in the Assessment of Hypertrophic Obstructive Cardiomyopathy after Alcohol Septal Ablation

PURPOSE

We evaluated the feasibility of contrast-enhanced steady-state free precession (ceSSFP) in the assessment of myocardial injury and obstruction of the left ventricular outflow tract (LVOT) in patients with hypertrophic obstructive cardiomyopathy (HOCM) after alcohol septal ablation (ASA).

METHODS

Twelve patients with HOCM underwent 16 magnetic resonance (MR) examinations following ASA. Precontrast SSFP, ceSSFP and late gadolinium enhancement (LGE) imaging were performed with a 1.5-tesla imager. ceSSFP was performed 3 to 7 min after gadolinium injection. We visually and quantitatively evaluated the signal patterns of the myocardium after ASA on SSFP and LGE MR imaging. We observed the LVOT using ceSSFP in the 3-chamber view.

RESULTS

We could visualize ASA-induced myocardial infarction (MI) in all 16 studies by LGE and ceSSFP but in only 6 studies (37.5%) by precontrast SSFP. Contrast was higher between MI and remote myocardium with LGE than ceSSFP (P < 0.01). ASA-induced hypointense regions were well visualized by the 2 sequences after contrast in the 7 patients who underwent MR imaging within 7 weeks of ASA and in a few patients after 80 weeks from ASA. The ceSSFP allowed comparable visualization of the jet flow crossing the LVOT to that derived from echocardiographic data.

CONCLUSION

Contrast-enhanced steady-state free precession allows assessment of myocardial injury as well as of the left ventricular outflow tract after alcohol septal ablation in a single scan without penalty in scan time and cine imaging contrast.

Comparison between balanced steady-state free precession and standard spoiled gradient echo magnetization transfer ratio imaging in multiple sclerosis: methodical and clinical considerations

Different pathological processes like demyelination and axonal loss can alter the magnetisation transfer ratio (MTR) in brain tissue. The standard method to measure this effect is to scan the respective tissue twice, one with and one without a specific saturation pulse. A major drawback of this technique based on spoiled gradient echo (GRE) sequences relates to its long acquisition time due to the saturation pulses. Recently, an alternative concept for MT imaging based on balanced steady state free precession (bSSFP) has been proposed. Modification of the duration of the radiofrequency pulses for imaging allows scanning MT sensitive and non-sensitive images. The steady-state character of bSSFP with high intrinsic signal-to-noise ratio (SNR) allows three-dimensional (3D) whole brain MTR at high spatial resolution within short and thus clinically feasible acquisition times. In the present study, both bSSFP-MT and 2D GRE-MT imaging were used in a cohort of 31 patients with multiple sclerosis (MS) to characterize different normal appearing (NA) and pathological brain structures. Under the constraint of identical SNR and scan time, a 3.4 times higher voxel size could be achieved with bSSFP. This increased resolution allowed a more accurate delineation of the different brain structures, especially of cortex, hippocampus and MS lesions. In a multiple linear regression model, we found an association between MTR of cortical lesions and a clinical measure of disability (r= -0.407, p=0.035) in the bSSFP dataset only. The different relaxation weighting of the base images (T2/T1 in bSSFP, proton density in GRE) had no effects besides a larger spreading of the MTR values of the different NA structures. This was demonstrated by the nearly perfect linearity between the NA matter MTR of both techniques as well as in the absolute MTR differences between NA matter and the respective lesions.