Novel T1 Analysis Method to Address Reduced Measurement Accuracy Due to Irregular Heart Rate Variability in Myocardial T1 Mapping Using Polarity-corrected Inversion Time Preparation.

PURPOSE: Polarity-corrected inversion time preparation (PCTIP), a myocardial T1 mapping technique, is expected to reduce measurement underestimation in the modified Look-Locker inversion recover method. However, measurement precision is reduced, especially for heart rate variability. We devised an analysis using a recurrence formula to overcome this problem and showed that it improved the measurement accuracy, especially at high heart rates. Therefore, this study aimed to determine the effect of this analysis on the accuracy and precision of T1 measurements for irregular heart rate variability. METHODS: A PCTIP scan using a 3T MRI scanner was performed in phantom experiment. We generated the simulated R-waves required for electrocardiogram (ECG)-gated acquisition using a signal generator set to 30 combinations. T1 map was generated using the signal train of the PCTIP images by nonlinear curve fitting using conventional and recurrence formulas. Accuracy against reference T1 and precision of heart rate variability were evaluated. To evaluate the fitting accuracy of both analyses, the relative fitting error was calculated. RESULTS: For the longer T1, the fitting error was larger than the short T1, with the conventional analysis showing 10.1±2.0%. The recurrence formula analysis showed a small fitting error less than 1%, which was consistent for all heart rate variability patterns. In the conventional analysis, the accuracy, especially for longer T1, showed a large underestimation of the measurements and poor linearity. However, in the recurrence formula analysis, the accuracy improved at a long T1, and linearity also improved. The Bland-Altman plot showed that it varied greatly depending on the heart rate variability pattern for the longer T1 in the conventional analysis, whereas the recurrence formula analysis suppressed this variation. CONCLUSION: T1 analysis of PCTIP using the recurrence formula analysis achieved accurate and precise T1 measurements, even for irregular heart rate variability.

Using Dictionary Matching to Improve the Accuracy of MOLLI Myocardial T1 Analysis and Measurements of Heart Rate Variability.

We analyzed modified Look-Locker inversion recovery (MOLLI) T1 measurements by applying a dictionary matching strategy and aimed to acquire T1 measurements more accurately than those acquired by the conventional three-parameter matching analysis. We particularly clarified the robustness of this method for measuring heart rate (HR) variability. A phantom experiment using a 3T MRI system was performed for various HRs. The ideal MOLLI signal corresponding to the scan parameter in the MRI experiment was simulated over a wide range of T1 values according to the dictionary. The unknown T1 values were determined by finding the simulated signals in the dictionary corresponding to the measured signals using pattern matching. The measured T1 values showed that the proposed analysis improved the accuracy of T1 measurements compared to those acquired by traditional analysis by up to 10%. In addition, the variability of measurements at several HRs was reduced by up to 100 ms.

A novel myocardial T1 analysis method robust to fluctuations in longitudinal magnetization recovery due to heart rate variability in polarity-corrected inversion time preparation.

Myocardial T1 mapping is useful for characterizing the myocardial tissues. Polarity-corrected inversion time preparation (PCTIP), one of the T1 mapping techniques, was expected to reduce measurement underestimation versus the MOLLI method. However, measurement accuracy is reportedly reduced, especially at high heart rates (HR), owing to the shorter time interval of inversion recovery (IR) pulses. This phantom-based experiment aimed to evaluate the dependence of T1 mapping with PCTIP on HR. Here we proposed and evaluated the effectiveness of a novel HR-independent analysis method for T1 mapping. A PCTIP scan using a 3-T magnetic resonance imaging scanner was performed on a T1 measurement phantom. The virtual HR were set at 50, 60, 75, and 100 bpm. The T1 of the phantom was estimated by a least-squares fit of the PCTIP data for each obtained inversion time and a theoretical longitudinal relaxation formula. This analysis was performed for the conventional and proposed formulas. The proposed formula was derived for adapting to the transient state of longitudinal magnetization recovery caused by the trigger interval as a recurrence formula. The estimated T1 measurements using the conventional formula varied widely with HR and the accuracy decreased, especially at a high HR. However, the proposed analysis showed good accuracy versus the conventional method independent of HR. T1 mapping using the PCTIP method combined with the novel method proposed here showed good accuracy.

Evaluation of the homogeneity of native T1 myocardial mapping using the polarity corrected inversion time preparation method in a myocardial phantom and healthy volunteers.

Myocardial T1 mapping is a useful technique for the diagnosis of diffuse fibrosis. Although modified look-locker inversion recovery is a widely used T1 mapping method, variation in T1 values has been reported. Non-uniform T1 maps may hinder differentiation between healthy and diseased myocardial tissue. The purpose of this study was to investigate the uniformity of T1 mapping using polarity corrected inversion time preparation (PC TI prep) in a myocardial phantom and healthy volunteers. The myocardial phantom was scanned between polyvinyl alcohol (PVA) and air. T1 values were measured using inversion recovery fast spin-echo (IR-FSE) and PC TI prep in areas adjacent to PVA and air. For the volunteer study, the short-axis plane was imaged using the PC TI prep to compare T1 values in the myocardium of the septal and lateral walls. The T1 value of the phantom using the IR-FSE was not significantly different in the area between PVA and air, whereas the T1 value using the PC TI prep in the air area was significantly lower than that in the PVA area. T1 mapping of the healthy myocardium exhibited no significant difference between the septal and lateral walls. The T1 value using the PC TI prep in the air area was 6.3% lower than that using IR-FSE. In this study, T1 mapping using the PC TI prep exhibited high uniformity of T1 values.

Estimation of Spatiotemporal Sensitivity Using Band-limited Signals with No Additional Acquisitions for k-t Parallel Imaging.

PURPOSE: Dynamic MR techniques, such as cardiac cine imaging, benefit from shorter acquisition times. The goal of the present study was to develop a method that achieves short acquisition times, while maintaining a cost-effective reconstruction, for dynamic MRI. k - t sensitivity encoding (SENSE) was identified as the base method to be enhanced meeting these two requirements. METHODS: The proposed method achieves a reduction in acquisition time by estimating the spatiotemporal (x - f) sensitivity without requiring the acquisition of the alias-free signals, typical of the k - t SENSE technique. The cost-effective reconstruction, in turn, is achieved by a computationally efficient estimation of the x - f sensitivity from the band-limited signals of the aliased inputs. Such band-limited signals are suitable for sensitivity estimation because the strongly aliased signals have been removed. RESULTS: For the same reduction factor 4, the net reduction factor 4 for the proposed method was significantly higher than the factor 2.29 achieved by k - t SENSE. The processing time is reduced from 4.1 s for k - t SENSE to 1.7 s for the proposed method. The image quality obtained using the proposed method proved to be superior (mean squared error [MSE] ± standard deviation [SD] = 6.85 ± 2.73) compared to the k - t SENSE case (MSE ± SD = 12.73 ± 3.60) for the vertical long-axis (VLA) view, as well as other views. CONCLUSION: In the present study, k - t SENSE was identified as a suitable base method to be improved achieving both short acquisition times and a cost-effective reconstruction. To enhance these characteristics of base method, a novel implementation is proposed, estimating the x - f sensitivity without the need for an explicit scan of the reference signals. Experimental results showed that the acquisition, computational times and image quality for the proposed method were improved compared to the standard k - t SENSE method.

[The Measurement Precision and Accuracy of T1 Mapping Using Polarity Corrected (PC) TI Prep Tool].

The aim of this study was to evaluate the measurement precision and accuracy of T1 mapping using a polarity corrected (PC) TI prep tool, which was based on fast field echo (FFE) and obtained one data point with one inversion recovery (IR) pulse. A phantom was used consisting of eight materials with different Gd concentrations. T1 mappings were measured by changing the trigger interval and the inversion time (TI) interval. The T1 mapping measurement precision using the PC TI prep tool increased as the trigger interval was made longer. The measurement precision didn't depend on the interval of TI. On the other hand, when the trigger intervals are more than 1000 ms, the measurement accuracy was less than approximately 8%. By setting the optimal end of TI, the T1 mapping using a PC TI prep tool could measure the T1 value precisely and accurately.

Coronary Whole-heart MR Angiography: Feasibility of Breath-hold Chasing with Diaphragmatic Navigation.

We propose a simple but novel data acquisition technique for whole-heart coronary magnetic resonance angiography (CMRA). In this technique, the breath-hold chasing MRA, data are collected during breath-hold intervals, with the navigation window manually adjusted to the diaphragmatic level. Compared with the conventional free breathing MRA, this method provided 33% reduction of acquisition time and improved visibility of right coronary artery in 18 normal subjects without any additional software or hardware requirements.

Clinical application of an automatic slice-alignment method for cardiac MR imaging.

PURPOSE: We evaluated the usefulness of an automatic slice-alignment method to simplify planning of cardiac magnetic resonance (MR) scans with a 3-tesla scanner. METHODS: We obtained 2-dimensional (2D) axial multislice images using steady-state free precession (SSFP) sequences covering the whole heart at the end-diastole phase with electrocardiography (ECG) gating in 38 patients. We detected several anatomical feature points of the heart and calculated all planes required for cardiac imaging based on those points. We visually evaluated the acceptability of an acquired imaging plane and measured the angular differences of each view between the results obtained by this method and by a conventional manual pointing approach. RESULTS: The average visual scores were 3.4 ± 1.0 for short-axis images, 3.2 ± 0.9 for 4-chamber images, 3.2 ± 0.8 for 2-chamber images, and 3.3 ± 0.8 for 3-chamber images; average angular differences were 5.8 ± 5.1 (short axis), 7.7 ± 5.7 (4-chamber), 11.5 ± 6.7 (2-chamber), and 9.1 ± 4.6 degrees (3-chamber). Processing time was within 1.8 s in all subjects. CONCLUSION: The proposed method can provide planes within the clinically acceptable range and within a short time in cardiac imaging of patients with various cardiac shapes and diseases without the need for high level operator proficiency in performing the examination and interpreting results.

Automatic slice alignment method for cardiac magnetic resonance imaging.

OBJECTIVES: Automatic slice alignment is important for easier operation and shorter examination times in cardiac magnetic resonance imaging (MRI) examinations. We propose a new automatic slice alignment method for six cardiac planes (short-axis, vertical long-axis, horizontal long-axis, 4-chamber, 2-chamber, and 3-chamber views). MATERIALS AND METHODS: ECG-gated 2D steady-state free precession axial multislice images were acquired using a 1.5-T MRI scanner during a single breath-hold. The scanning time was set to <20 s in 23 volumes from 23 healthy volunteers. In this method, the positions of the mitral valve, cardiac apex, left ventricular outflow tract, tricuspid valve, anterior wall of the heart, and right ventricular corner are detected to determine the positions of six reference planes by combining knowledge-based recognition and image processing techniques. In order to evaluate the results of automatic slice alignment for the short-axis, 4-chamber, 2-chamber, and 3-chamber views, the angular and positional errors between the results obtained by our proposed method and by manual annotation were measured. RESULTS: The average angular errors for the short-axis, 4-chamber, 2-chamber, and 3-chamber views were 3.05°, 4.52°, 7.28°, and 5.79°, respectively. The average positional errors for the short-axis (base), short-axis (apex), 4-chamber, 2-chamber, and 3-chamber views were 6.61°, 3.80°, 1.55°, 1.52°, and 1.48°, respectively. CONCLUSION: The experimental results showed that our proposed method can detect the cardiac planes quickly and accurately. Our method is therefore beneficial to both patients and operators.

Whole-heart 3D late gadolinium-enhanced MR imaging: investigation of optimal scan parameters and clinical usefulness.

PURPOSE: Whole-heart 3-dimensional (3D) late-gadolinium-enhanced magnetic resonance (MR) imaging (WH-LGE) uses respiratory gating combined with acquisition of 3D data for the entire heart in a single scan, which permits reconstruction of any plane with high resolution. We investigated the optimal scan parameters and compared WH-LGE with the conventional scanning method. MATERIALS AND METHODS: We employed inversion recovery 3D fast field echo using a 1.5-tesla system and scan parameters: repetition time (TR), 6.6 ms; echo time (TE), 2.5 ms; number of segments, 2; parallel imaging factor, 1.8; matrix size, 128 × 256; field of view (FOV), 320 × 320 mm; and acquisition slice thickness, 3 mm (reconstruction slice thickness, 1.5 mm). Five healthy volunteers underwent scanning during free breathing with real-time motion correction, from which we determined optimal scan parameters. We then used those parameters to scan 25 patients with myocardial infarction to compare scan time and image quality between the WH-LGE and conventional 3D breath-holding methods (slice thickness, 10 mm; matrix size, 128 × 256). RESULTS: Results in volunteers showed optimal scan parameters of 12° flip angle, fat suppression turned off in combination, and interleaved ordering. In clinical cases, scan times did not differ significantly. Sharpness of the margins of normal myocardium at the apex of the heart and contrast between enhanced and nonenhanced myocardium improved significantly with WH-LGE. CONCLUSION: WH-LGE yields high resolution images during free breathing and is considered useful for accurately estimating the area and transmural extent of myocardial infarction.

Radiologist model for cardiac rest period determination based on fuzzy rule.

Image data acquisition for the coronary arteries is generally implemented during the diastole rest period, in order to suppress blurring due to cardiac movement. The purpose of this study is to improve the semi-automated application to determine the cardiac rest period based on fuzzy logic. The cardiac rest period from 25 subjects were determined based on their normalized cross-correlation of consecutive frame images as well as normalized frame number as the measured variables. The fuzzy set and membership are generated based on the measured variables from the radiologist's visual assessment. That visual assessment is also regarded as a gold standard for verification. The distance difference between the proposed method and visual assessment was analyzed. The fuzzy logic approach for cardiac rest period determination has no significant difference compared to the visual assessment (p>0.05) in terms of start frame and end frame. The algorithm could be extended easily in case of there are some necessary variables should be added to accommodate rest period definition from different radiologist.

Whole-heart coronary magnetic resonance angiography with parallel imaging: comparison of acceleration in one-dimension vs. two-dimensions.

PURPOSE: To evaluate visualization of the whole-heart coronary arteries accelerated with parallel imaging (PI) applied in two-dimension (2D) in comparison with one-dimension (1D). MATERIALS AND METHODS: Seventeen healthy subjects were studied with a 1.5-T scanner equipped with a whole body phased array coil system and 16-channel receivers. Using 16 coil elements, whole-heart coronary magnetic resonance angiography (CMRA) was acquired in two conditions of 1D-PI and 2D-PI. The former scan was accelerated in phase direction by factor of 2 and the latter in phase and slice directions by factors of 2.5 and 2, respectively. Visualized length of right coronary artery (RCA), left anterior descending artery (LAD), and left circumflex artery (LCX) was measured. Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) was also measured. The CMRA quality was assessed in segment-wise with a five-point scale. RESULTS: The average scan time decreased to 5.3+/-2.2 min in 2D-PI from 11.6+/-3.5 min in 1D-PI, reducing the scan time to 45%. The visualized length, SNR, and CNR in average were smaller for images of 2D-PI compared with those of 1D-PI, however, statistically significant results were observed only in RCA (P<0.05). Score reduction of 2D-PI image quality was limited to 0.34 in average, and only two out of fifteen segments (#2, 6) showed significant score deterioration (P<0.05). CONCLUSIONS: Compared with the relatively limited degree of image degradation, 2D-PI offered a large reduction of the acquisition time, which is of large benefit in clinical situations.

Facilitated acquisition of whole-heart coronary magnetic resonance angiography with visual feedback of respiration status.

Three dimensional (3D) whole-heart (WH) coronary MR angiography (CMRA) requires an extended imaging time, but it may be reduced by providing a subject with visual feedback (VFB). Thirteen healthy volunteers were scanned and quality of 3D WH-CMRA images was compared among three scan conditions: free breathing (FB) with and without VFB (FB + VFB and FB - VFB, respectively) and multiple breath-holds with VFB (MBH + VFB). All but two subjects were able to complete all scans. The average scan times were 10.0 +/- 2.2, 10.0 +/- 2.5, and 8.2 +/- 1.3 min for FB - VFB, FB + VFB, and MBH + VFB, respectively. In the MBH + VFB condition, scan time was significantly reduced by 18% compared with both FB scans. No significant difference in image quality was observed between the FB - VFB and MBH + VFB conditions, but scores were significantly deteriorated at some segments in the FB + VFB condition. The MBH + VFB scan can be performed with a shorter scan time without failure or impairment of image quality.

[Clinical evaluation of 3D delayed-enhancement MRI using parallel imaging in the assessment of myocardial viability].

This study aimed to evaluate the efficacy of breath-hold three-dimensional (3D)delayed-enhancement MRI using parallel imaging in terms of the effect of parallel imaging on the image quality and visualization of myocardial infarction. Twenty-two patients (17 men and 5 women) with suspected myocardial infarction underwent breath-hold 3D late-enhanced viability examination at least 30 days after occurrence. All patients underwent a Tl-scintigraphy examination. First, 10 patients were examined without applying parallel imaging, then the next 12 patients were studied using parallel imaging. All 3D late-enhanced images at the short axis were acquired 10, 15, and 20 min after an injection of contrast agent, and both the long axis and the four-chamber views were acquired after 15 min. In quantitative analysis, the late-enhanced myocardial images at 10, 15, and 20 min showed higher contrast-to-noise ratios (CNR) in parallel imaging than those with no parallel imaging. During the time-intensity curve of the myocardium, no significant change was observed at 10 or 15 min; however, marked signal reduction was observed at 20 min. In diagnostic evaluation, images obtained with parallel imaging were superior to those without parallel imaging. In general, the application of parallel imaging reduces acquisition time with an expense of reduction in SNR. However, the breath-hold 3D late-enhanced images with parallel imaging showed no apparent SNR reduction. Furthermore, parallel imaging provided clear edge definition between the infarction and the normal region. The reduction of acquisition time with parallel imaging may be less susceptible to fast cardiac motion. In conclusion, breath-hold 3D delayed-enhancement MRI using parallel imaging was highly evaluated in our study and may show promise in clinical application.

PET imaging of ischemic neuronal death in the hippocampus of living monkeys.

The aim of the present study was to visualize postischemic hippocampal neuronal death in the living monkey brain, using a high-resolution positron emission tomography (PET) and novel radioligands. In preceding papers, we reported on postischemic hippocampal neuronal death in a model of Japanese monkeys (Macaca fuscata) undergoing a 20-min complete whole-brain ischemia. Using the same model here, we investigated the in vivo bindings of two radiotracers, [11C]Ro15-4513 (a type II benzodiazepine receptor ligand) and [11C](+)3-MPB (a muscarinic cholinergic receptor ligand), in the hippocampus on day 7 after ischemia, as compared to the normal hippocampus. A significant decrease in the in vivo binding of [11C]Ro154513 and [11C(+)3-MPB was observed in the postischemic monkey hippocampus on day 7 after ischemia compared to controls. Light and electron microscopic analyses of postischemic CA1 neurons showed typical features of coagulation necrosis, as associated with a marked reduction of postsynaptic densities and presynaptic vesicles. These results suggest that semiquantification of hippocampal neuronal death is possible in the living primate brain using PET, and that the same procedures can be applied for evaluating neuronal cell loss in patients with ischemic injuries and/or dementia.