Quantification of myocardial infarct area based on TRAFFn relaxation time maps:comparison with cardiovascular magnetic resonance late gadolinium enhancement, T1ρ and T2 in vivo

Abstract Background: Two days after myocardial infarction (MI), the infarct consists mostly on necrotic tissue, and the myocardium is transformed through granulation tissue to scar in two weeks after the onset of ischemia in mice. In the current work, we determined and optimized cardiovascular magnetic resonance (CMR) methods for the detection of MI size during the scar formation without contrast agents in mice. Methods: We characterized MI and remote areas with rotating frame relaxation time mapping including relaxation along fictitious field in nth rotating frame (RAFFn), T1ρ and T2 relaxation time mappings at 1, 3, 7, and 21 days after MI. These results were compared to late gadolinium enhancement (LGE) and Sirius Red-stained histology sections, which were obtained at day 21 after MI. Results: All relaxation time maps showed significant differences in relaxation time between the MI and remote area. Areas of increased signal intensities after gadolinium injection and areas with increased TRAFF2 relaxation time were highly correlated with the MI area determined from Sirius Red-stained histology sections (LGE: R² = 0.92, P < 0.01, TRAFF2: R² = 0.95, P < 0.001). Infarct area determined based on T1ρ relaxation time correlated highly with Sirius Red histology sections (R² = 0.97, P < 0.01). The smallest overestimation of the LGE-defined MI area was obtained for TRAFF2 (5.6 ± 4.2%) while for T1ρ overestimation percentage was > 9% depending on T1ρ pulse power. Conclusion: T1ρ and TRAFF2 relaxation time maps can be used to determine accurately MI area at various time points in the mouse heart. Determination of MI size based on TRAFF2 relaxation time maps could be performed without contrast agents, unlike LGE, and with lower specific absorption rate compared to on-resonance T1ρ relaxation time mapping

Relaxation Along a Fictitious Field (RAFF) and Z-spectroscopy using Alternating-Phase Irradiation (ZAPI) in Permanent Focal Cerebral Ischemia in Rat

Cerebral ischemia alters the molecular dynamics and content of water in brain tissue, which is reflected in NMR relaxation, diffusion and magnetization transfer (MT) parameters. In this study, the behavior of two new MRI contrasts, Relaxation Along a Fictitious Field (RAFF) and Z-spectroscopy using Alternating-Phase Irradiation (ZAPI), were quantified together with conventional relaxation parameters (T1, T2 and T1ρ) and MT ratios in acute cerebral ischemia in rat. The right middle cerebral artery was permanently occluded and quantitative MRI data was acquired sequentially for the above parameters for up to 6 hours. The following conclusions were drawn: 1) Time-dependent changes in RAFF and T1ρ relaxation are not coupled to those in MT. 2) RAFF relaxation evolves more like transverse, rather than longitudinal relaxation. 3) MT measured with ZAPI is less sensitive to ischemia than conventional MT. 4) ZAPI data suggest alterations in the T2 distribution of macromolecules in acute cerebral ischemia. It was shown that both RAFF and ZAPI provide complementary MRI information from acute ischemic brain tissue. The presented multiparametric MRI data may aid in the assessment of brain tissue status early in ischemic stroke

A dose–neutral image quality comparison of different CBCT and CT systems using paranasal sinus imaging protocols and phantoms

Abstract Purpose: To compare the image quality produced by equivalent low-dose and default sinus imaging protocols of a conventional dental cone-beam computed tomography (CBCT) scanner, an extremity CBCT scanner and a clinical multidetector computed tomography (MDCT) scanner. Methods: Three different phantoms were scanned using dose–neutral ultra-low-dose and low-dose sinus imaging protocols, as well as default sinus protocols of each device. Quantified parameters of image quality included modulation transfer function (MTF) to characterize the spatial response of the imaging system, contrast-to-noise ratio, low contrast visibility, image uniformity and Hounsfield unit accuracy. MTF was calculated using the line spread and edge spread functions (LSF and ESF). Results: The dental CBCT had superior performance over the extremity CBCT in each studied parameter at similar dose levels. The MDCT had better contrast-to-noise ratio, low contrast visibility and image uniformity than the CBCT scanners. However, the CBCT scanners had better resolution compared to the MDCT. Accuracy of HU values for different materials was on the same level between the dental CBCT and MDCT, but substantially poorer performance was observed with the extremity CBCT. Conclusions: The studied dental CBCT scanner showed superior performance over the studied extremity CBCT scanner when using dose–neutral imaging protocols. In case a dental CBCT is not available, the given extremity CBCT is still a viable option as it provides the benefit of high resolution over a conventional MDCT

Capturing exchange using periodic radiofrequency irradiation

Abstract The dynamics of spin system coupled by chemical exchange between two sites with different chemical shifts during periodic radiofrequency (RF) irradiation was here investigated. When the instantaneous π-flip of effective frequency during the course of frequency sweep was applied, a significant increase of exchange-induced relaxation rate constants was observed for small tip angle of magnetization in the laboratory frame of reference. This increase of the rate constants corresponds to the side bands generated by the periodic irradiation during the RF pulses. The exchange-induced relaxation rate constants depend on the exchange conditions, the RF power and the irradiation period. The described phenomenon promises applications for studying protein dynamics and for generating exchange specific relaxation contrasts in MRI

Molecular imaging to monitor left ventricular remodeling in heart failure

Abstract Purpose of Review: Cardiovascular diseases are the leading cause of deaths worldwide. Many complex cellular and molecular pathways lead to myocardial remodeling after ischemic insults. Anatomy, function, and viability of the myocardium can be assessed by modern medical imaging techniques by both visualizing and quantifying damages. Novel imaging techniques aim for a precise and accurate visualization of the myocardium and for the detection of alternations at the molecular level. Recent Findings: Magnetic resonance imaging assesses anatomy, function, and tissue characterization of the myocardium non-invasively with high spatial resolution, sensitivity, and specificity. Using hyperpolarized magnetic resonance imaging, molecular and metabolic conditions can be assessed non-invasively. Single photon-emission tomography and positron-emission tomography are the most sensitive techniques to detect biological processes in the myocardium. Cardiac perfusion, metabolism, and viability are the most common clinical targets. In addition, molecular-targeted imaging of biological processes involved in heart failure, such as myocardial innervation, inflammation, and extracellular matrix remodeling, is feasible. Summary: Novel imaging techniques can provide a precise and accurate visualization of the myocardium and for the detection of alternations at molecular level

Excess of visceral adipose tissue with or without aortic elongation leads to a steeper heart position

Abstract Background: The heart’s position determined as the heart–aorta angle (HAA) has been demonstrated to associate with ascending aortic (AA) dilatation. Visceral adipose tissue (VAT) and aortic elongation may shift the heart to the steeper position. Purpose: To investigate whether VAT and aortic length influence the HAA. Material and methods: We examined 346 consecutive patients (58.4% men; mean age = 67.0 ± 14.1 years) who underwent aortic computed tomography angiography (CTA). HAA was measured as the angle between the long axis of the heart and AA midline. The amount of VAT was measured at the level of middle L4 vertebra from a single axial CT slice. Aortic length was measured by combining four anatomical segments in different CTA images. The amount of VAT and aortic length were determined as mild with values in the lowest quartile and as excessive with values in the other three quartiles. Results: A total of 191 patients (55.2%) had no history of aortic diseases, 134 (38.7%) displayed AA dilatation, 8 (2.3%) had abdominal aortic aneurysm (AAA), and 13 (3.8%) had both AA dilatation and AAA. There was a strong nonlinear regression between smaller HAA and VAT/height, and HAA and aortic length/height. Median HAA was 124.2° (interquartile range 119.0°–130.8°) in patients with a mild amount of VAT versus 120.5° (interquartile range 115.4°–124.7°) in patients with excessive VAT (P < 0.001). Conclusion: An excessive amount of VAT and aortic elongation led to a steeper heart position. These aspects may possess clinical value when evaluating aortic diseases in obese patients

Lymphatic insufficiency leads to distinct myocardial infarct content assessed by magnetic resonance TRAFFn, T1ρ and T₂ relaxation times

Abstract The role of cardiac lymphatics in the pathogenesis of myocardial infarction (MI) is unclear. Lymphatic system regulates cardiac physiological processes such as edema and tissue fluid balance, which affect MI pathogenesis. Recently, MI and fibrosis have been assessed using endogenous contrast in magnetic resonance imaging (MRI) based on the relaxation along a fictitious field with rank n (RAFFn). We extended the RAFFn applications to evaluate the effects of lymphatic insufficiency on MI with comparison to longitudinal rotating frame (T1ρ) and T₂ relaxation times. MI was induced in transgenic (TG) mice expressing soluble decoy VEGF receptor 3 that reduces lymphatic vessel formation and their wild-type (WT) control littermates for comparison. The RAFFn relaxation times with rank 2 (TRAFF2), and rank 4 (TRAFF4), T1ρ and T₂ were acquired at time points 0, 3, 7, 21 and 42 days after the MI at 9.4 T. Infarct sizes were determined based on TRAFF2, TRAFF4, T1ρ and T₂ relaxation time maps. The area of differences (AOD) was calculated based on the MI areas determined on T₂ and TRAFF2, TRAFF4 or T1ρ relaxation time maps. Hematoxylin–eosin and Sirius red stained histology sections were prepared to confirm MI locations and sizes. MI was detected as increased TRAFF2, TRAFF4, T1ρ and T₂ relaxation times. Infarct sizes were similar on all relaxation time maps during the experimental period. Significantly larger AOD values were found together with increased AOD values in the TG group compared to the WT group. Histology confirmed these findings. The lymphatic deficiency was found to increase cardiac edema in MI. The combination of TRAFF2 (or TRAFF4) and T₂ characterizes MI and edema in the myocardium in both lymphatic insufficiency and normal mice without any contrast agents

New methods for robust continuous wave T1ρ relaxation preparation

Abstract Measurement of the longitudinal relaxation time in the rotating frame of reference (T₁ₚ) is sensitive to the fidelity of the main imaging magnetic field (B₀) and that of the RF pulse (B₁). The purpose of this study was to introduce methods for producing continuous wave (CW) T₁ₚ contrast with improved robustness against field inhomogeneities and to compare the sensitivities of several existing and the novel T₁ₚ contrast generation methods with the B₀ and B₁ field inhomogeneities. Four hard-pulse and four adiabatic CW-T₁ₚ magnetization preparations were investigated. Bloch simulations and experimental measurements at different spin-lock amplitudes under ideal and non-ideal conditions, as well as theoretical analysis of the hard-pulse preparations, were conducted to assess the sensitivity of the methods to field inhomogeneities, at low (ω₁ << ΔB₀) and high (ω₁ >> ΔB₀) spin-locking field strengths. In simulations, previously reported single-refocus and new triple-refocus hard-pulse and double-refocus adiabatic preparation schemes were found to be the most robust. The mean normalized absolute deviation between the experimentally measured relaxation times under ideal and non-ideal conditions was found to be smallest for the refocused preparation schemes and broadly in agreement with the sensitivities observed in simulations. Experimentally, all refocused preparations performed better than those that were non-refocused. The findings promote the use of the previously reported hard-pulse single-refocus ΔB0 and B₁ insensitive T₁ₚ as a robust method with minimal RF energy deposition. The double-refocus adiabatic B₁ insensitive rotation-4 CW-T₁ₚ preparation offers further improved insensitivity to field variations, but because of the extra RF deposition, may be preferred for ex vivo applications

Myocardium assessment by relaxation along fictitious field, extracellular volume, feature tracking, and myocardial strain in hypertensive patients with left ventricular hypertrophy

Abstract Background: Previous research has shown impaired global longitudinal strain (GLS) and slightly elevated extracellular volume fraction (ECV) in hypertensive patients with left ventricular hypertrophy (HTN LVH). Up to now, only little attention has been paid to interactions between macromolecules and free water in hypertrophied myocardium. Purpose: To evaluate the feasibility of relaxation along a fictitious field with rank 2 (RAFF2) in HTN LVH patients. Study Type: Single institutional case control. Subjects: 9 HTN LVH (age, 69 ± 10 years) and 11 control subjects (age, 54 ± 12 years). Field Strength/Sequence: Relaxation time mapping (T1, T1p, and TRAFF2 with 11.8 μT maximum radio frequency field amplitude) was performed at 1.5 T using a Siemens Aera (Erlangen, Germany) scanner equipped with an 18-channel body array coil. Assessment: ECV was calculated using pre- and postcontrast T1, and global strains parameters were assessed by Segment CMR (Medviso AB Co, Sweden). The parametric maps of T1p and TRAFF2 were computed using a monoexponential model, while the Bloch-McConnell equations were solved numerically to model effect of the chemical exchange during radio frequency pulses. Statistical Tests: Parametric maps were averaged over myocardium for each subject to be used in statistical analysis. Kolmogorov-Smirnov was used as the normality test followed by Student’s t-test and Pearson’s correlation to determine the difference between the HTN LVH patients and controls along with Hedges’ g effect size and the association between variables, respectively. Results: TRAFF2 decreased statistically (83 ± 2 ms vs 88 ± 6 ms, P < 0.031), and global longitudinal strain was impaired (GLS, −14 ± 3 vs −18 ± 2, P < 0.002) in HTN LVH patients compared to the controls, respectively. Also, significant negative correlation was found between TRAFF2 and GLS (r = −0.53, P < 0.05). Data conclusion: Our results suggest that TRAFF2 decrease in HTN LVH patients may be explained by gradual collagen accumulation which can be reflected in GLS changes. Most likely, it increases the water proton interactions and consequently decreases TRAFF2 before myocardial scarring