Cardiovascular magnetization transfer ratio imaging compared with histology: a postmortem study

Diagnostic value of T1- and T2-weighted 3-Tesla MRI for postmortem detection and age stage classification of myocardial infarction

The aims of this study are to retrospectively evaluate the diagnostic value of T1- and T2-weighted 3-T magnetic resonance imaging (MRI) for postmortem detection of myocardial infarction (MI) in terms of sensitivity and specificity and to compare the MRI appearance of the infarct area with age stages. Postmortem MRI examinations (n = 88) were retrospectively reviewed for the presence or absence of MI by two raters blinded to the autopsy results. The sensitivity and specificity were calculated using the autopsy results as the gold standard. A third rater, who was not blinded to the autopsy findings, reviewed all cases in which MI was detected at autopsy for MRI appearance (hypointensity, isointensity, hyperintensity) of the infarct area and the surrounding zone. Age stages (peracute, acute, subacute, chronic) were assigned based on the literature and compared with the age stages reported in the autopsy reports. The interrater reliability between the two raters was substantial (κ = 0.78). Sensitivity was 52.94% (both raters). Specificity was 85.19% and 92.59%. In 34 decedents, autopsy identified an MI (peracute: n = 7, acute: n = 25, chronic: n = 2). Of 25 MI classified as acute at autopsy, MRI classified peracute in four cases and subacute in nine cases. In two cases, MRI suggested peracute MI, which was not detected at autopsy. MRI could help to classify the age stage and may indicate the area for sampling for further microscopic examination. However, the low sensitivity requires further additional MRI techniques to increase the diagnostic value.

Quantitative magnetization transfer imaging for non-contrast enhanced detection of myocardial fibrosis

PURPOSE

To develop a novel gadolinium-free model-based quantitative magnetization transfer (qMT) technique to assess macromolecular changes associated with myocardial fibrosis.

METHODS

The proposed sequence consists of a two-dimensional breath-held dual shot interleaved acquisition of five MT-weighted (MTw) spoiled gradient echo images, with variable MT flip angles (FAs) and off-resonance frequencies. A two-pool exchange model and dictionary matching were used to quantify the pool size ratio (PSR) and bound pool T2 relaxation ( T 2 B ). The signal model was developed and validated using 25 MTw images on a bovine serum albumin (BSA) phantom and in vivo human thigh muscle. A protocol with five MTw images was optimized for single breath-hold cardiac qMT imaging. The proposed sequence was tested in 10 healthy subjects and 5 patients with myocardial fibrosis and compared to late gadolinium enhancement (LGE).

RESULTS

PSR values in the BSA phantom were within the confidence interval of previously reported values (concentration 10% BSA = 5.9 ± 0.1%, 15% BSA = 9.4 ± 0.2%). PSR and T 2 B in thigh muscle were also in agreement with literature (PSR = 10.9 ± 0.3%, T 2 B = 6.4 ± 0.4 us). In 10 healthy subjects, global left ventricular PSR was 4.30 ± 0.65%. In patients, PSR was reduced in areas associated with LGE (remote: 4.68 ± 0.70% vs. fibrotic: 3.12 ± 0.78 %, n = 5, P < .002).

CONCLUSION

In vivo model-based qMT mapping of the heart was performed for the first time, with promising results for non-contrast enhanced assessment of myocardial fibrosis.

Contrast-free high-resolution 3D magnetization transfer imaging for simultaneous myocardial scar and cardiac vein visualization

OBJECTIVE

To develop a three-dimensional (3D) high-resolution free-breathing magnetization transfer ratio (MTR) sequence for contrast-free assessment of myocardial infarct and coronary vein anatomy.

MATERIALS AND METHODS

Two datasets with and without off-resonance magnetization transfer preparation were sequentially acquired to compute MTR. 2D image navigators enabled beat-to-beat translational and bin-to-bin non-rigid motion correction. Two different imaging sequences were explored. MTR scar localization was compared against 3D late gadolinium enhancement (LGE) in a porcine model of myocardial infarction. MTR variability across the left ventricle and vessel sharpness in the coronary veins were evaluated in healthy human subjects.

RESULTS

A decrease in MTR was observed in areas with LGE in all pigs (non-infarct: 25.1 ± 1.7% vs infarct: 16.8 ± 1.9%). The average infarct volume overlap on MTR and LGE was 62.5 ± 19.2%. In humans, mean MTR in myocardium was between 37 and 40%. Spatial variability was between 15 and 20% of the mean value. 3D whole heart MT-prepared datasets enabled coronary vein visualization with up to 8% improved vessel sharpness for non-rigid compared to translational motion correction.

DISCUSSION

MTR and LGE showed agreement in infarct detection and localization in a swine model. Free-breathing 3D MTR maps are feasible in humans but high spatial variability was observed. Further clinical studies are warranted.

Multiparametric mapping in post-mortem perinatal MRI: a feasibility study

OBJECTIVES

To demonstrate feasibility of a 3 T multiparametric mapping (MPM) quantitative pipeline for perinatal post-mortem MR (PMMR) imaging.

METHODS

Whole body quantitative PMMR imaging was acquired in four cases, mean gestational age 34 weeks, range (29-38 weeks) on a 3 T Siemens Prisma scanner. A multicontrast protocol yielded proton density, T₁ and magnetic transfer (MT) weighted multi-echo images obtained from variable flip angle (FA) 3D fast low angle single-shot (FLASH) acquisitions, radiofrequency transmit field map and one B₀ field map alongside four MT weighted acquisitions with saturation pulses of 180, 220, 260 and 300 degrees were acquired, all at 1 mm isotropic resolution.

RESULTS

Whole body MPM was achievable in all four foetuses, with R₁, R₂*, PD and MT maps reconstructed from a single protocol. Multiparametric maps were of high quality and show good tissue contrast, especially the MT maps.

CONCLUSION

MPM is a feasible technique in a perinatal post-mortem setting, which may allow quantification of post-mortem change, prior to being evaluated in a clinical setting.

ADVANCES IN KNOWLEDGE

We have shown that the MPM sequence is feasible in PMMR imaging and shown the potential of MT imaging in this setting.

Total-body CT and MR features of postmortem change in in-hospital deaths

OBJECTIVES

To evaluate the frequency of total-body CT and MR features of postmortem change in in-hospital deaths.

MATERIALS AND METHODS

In this prospective blinded cross-sectional study, in-hospital deceased adult patients underwent total-body postmortem CT and MR followed by image-guided biopsies. The presence of PMCT and PMMR features related to postmortem change was scored retrospectively and correlated with postmortem time interval, post-resuscitation status and intensive care unit (ICU) admittance.

RESULTS

Intravascular air, pleural effusion, periportal edema, and distended intestines occurred more frequently in patients who were resuscitated compared to those who were not. Postmortem clotting was seen less often in resuscitated patients (p = 0.002). Distended intestines and loss of grey-white matter differentiation in the brain showed a significant correlation with postmortem time interval (p = 0.001, p<0.001). Hyperdense cerebral vessels, intravenous clotting, subcutaneous edema, fluid in the abdomen and internal livores of the liver were seen more in ICU patients. Longer postmortem time interval led to a significant increase in decomposition related changes (p = 0.026).

CONCLUSIONS

There is a wide variety of imaging features of postmortem change in in-hospital deaths. These imaging features vary among clinical conditions, increase with longer postmortem time interval and must be distinguished from pathologic changes.

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.