Cardiac self-gating using blind source separation for 2D cine cardiovascular magnetic resonance imaging

PurposeTo develop and validate a new cardiac self-gating algorithm using blind source separation for 2D cine steady-state free precession (SSFP) imaging.MethodsA standard cine SSFP sequence was modified so that the center point of k-space was sampled with each excitation. The center points of k-space were processed by 4 blind source separation methods, and used to detect heartbeats and assign k-space data to appropriate time points in the cardiac cycle. The proposed self-gating technique was prospectively validated in 8 patients against the standard electrocardiogram (ECG)-gating method by comparing the cardiac cycle lengths, image quality metrics, and ventricular volume measurements.ResultsThere was close agreement between the cardiac cycle length using the ECG- and self-gating methods (bias 0.0 bpm, 95% limits of agreement ±2.1 bpm). The image quality metrics were not significantly different between the ECG- and self-gated images. The ventricular volumes, stroke volumes, and mass measured from self-gated images were all comparable with those from ECG-gated images (all biases <5%).ConclusionThe self-gating method yielded comparable cardiac cycle length, image quality, and ventricular measurements compared with standard ECG-gated cine imaging. It may simplify patient preparation, be more robust when there is arrhythmia, and allow cardiac gating at higher field strengths.     

Reproducibility of GABA measurements using 2D J-resolved magnetic resonance spectroscopy

We determined the reproducibility of GABA (gamma-aminobutyric acid) measurements using 2D J-resolved magnetic resonance spectroscopy (MRS) on a clinical 1.5-T MR imaging scanner. Two-dimensional J-resolved spectra were acquired in vitro across five GABA concentrations using a volume head coil and a 5-in. surface coil. Additional spectra using a sixth GABA phantom with a very low concentration and from a healthy volunteer were recorded in the 5-in. surface coil only. In each case, the 3.01-ppm GABA resonance was quantified; for comparison, the peak integrals of choline (3.2 ppm) and creatine (3.03 ppm) were recorded. At a physiological concentration (1.2 mM), in vitro GABA measurement was significantly more reproducible in the surface coil than in the volume coil (P=.005), with coefficients of variation (CVs) being less than 16% with the surface coil and up to 68% with the volume head coil. At the smallest concentration of in vivo GABA reported using other spectroscopy techniques (0.8 mM) and detected only using the surface coil, the CV for GABA was 23% and was less than 10% for choline and creatine, which compare favorably with results from published studies. In vivo, the CV for GABA measurement was 26%, suggesting that 2D J-resolved MRS would be suitable for detecting physiological changes in GABA, similar to those reported using other methods.     

Assessment of normal flow patterns in the pulmonary circulation by using 4D magnetic resonance velocity mapping

Objective

The purpose of this study was to analyze flow patterns in the pulmonary circulation of healthy volunteers by using 4D flow magnetic resonance imaging.

Materials and Methods

The study was approved by the local ethics committee and all subjects gave written informed consent. Eighteen volunteers underwent a 4D flow scan of the whole-heart. Two patients with congenital heart disease were also included to detect possible patterns of flow abnormalities (Patient 1: corrected transposition of great arteries (TGA); Patient 2: partial anomalous pulmonary venous return and atrial septal defect). To analyze flow patterns, 2D planes were placed on the main pulmonary artery (PA), left and right PA. Flow patterns were assessed manually by two independent viewers using vector fields, streamlines and particle traces, and semi-automatically by vorticity quantification.

Results

Two counter-rotating helices were found in the main PA of volunteers. Right-handed helical flow was detected in the right PA of 15 volunteers. Analysis of the helical flow by particles traces revealed that both helices contributed mainly to the flow in the right PA. In the patient with corrected TGA helical flow was not detected. Abnormal vortical flow was visualized in the main PA of patient 2, suggesting elevated mean PA pressure.

Conclusions

Helical flow is normally present in the main PA and right PA. 4D flow is an excellent tool to evaluate noninvasively complex blood flow patterns in the pulmonary circulation. Knowledge of normal and abnormal flow patterns might help to evaluate patients with congenital heart disease adding functional information undetectable with other imaging modalities.     

Three-vessel study of cerebral blood flow using phase-contrast magnetic resonance imaging: Effect of physical characteristics

The development of phase-contrast magnetic resonance imaging (P-C MRI) provides a noninvasive method for measurement of volumetric blood flow (VFR). We performed P-C MRI to study the effects of physical characteristics on cerebral blood flow. VFR of the left and right internal carotid arteries and basilar artery were measured using P-C MRI and total cerebral blood flow (tCBF) was calculated by summing up the VFR values in the three vessels. Moreover, we investigated the changes in these blood flows as influenced by age, head size, height, weight, body surface area, and handedness. The blood flows were 142 ± 58 ml/min (mean ± standard deviation) in the basilar artery; and 229 ± 86 ml/min in the left, and 223 ± 58 ml/min in the right internal carotid artery; and tCBF was 617 ± 128 ml/min. Significant increases were observed in head size-related change of VFR in the basilar artery (p = .028) and height-related change of tCBF (p = .045). The other characteristics did not significantly influence any VFR. The results suggest that head size and height may reflect CBF, and that these effects should be considered when changes of CBF are diagnosed. Phase-contrast MRI is useful for a noninvasive and rapid analysis of cerebral VFR and has potential for clinical use.     

Cardiac magnetic resonance imaging in arrhythmogenic right ventricular cardiomyopathy: correlation to the QRS dispersion

The aim of the study was to evaluate the relationship between the presence of right ventricular abnormalities detected by cardiac magnetic resonance (CMR) and QRS dispersion, the strongest independent predictor of sudden death in ARVC. A consecutive series of 40 patients from a single institution were recruited with a clinical diagnosis of ARVC based on the diagnostic criteria. All patients underwent systematic clinical evaluation, including history and examination, electrocardiography, 24-h Holter monitor, chest radiography, echocardiography and CMR examination and were divided into two groups according to the QRS dispersion: group I, QRS dispersion ≥ 40 ms; group II, QRS dispersion < 40 ms. The relationship between the characteristic parameters of CMR image and QRS dispersion were analyzed in two groups. There were significant differences in QRS dispersion (57±14 ms vs. 26±11 ms), right ventricular end-diastolic diameter (57±10 mm vs. 48±11 mm, P=.012), right ventricular end-systolic diameter (52±10 mm vs. 44±11 mm, P=.010), right ventricular end-diastolic volume (260±105 ml vs. 180±66 ml, P=.006), right ventricular end-systolic volume (222±98 ml vs. 148±61 ml, P=.006) and myocardial fibrosis detection rate (74% vs. 38%, P=.024) between two groups. For all patients with ARVC, QRS dispersion and right ventricular end-diastolic volume (r= 0.66, P<.001), right ventricular end-systolic volume (r= 0.67, P<.001), right ventricular outflow tract area (r= 0.68, P<.001) showed a moderate positive correlation. Right ventricular outflow tract area, right ventricular end-diastolic volume and end-systolic volume detected by CMR in patients with ARVC were positively correlated to the extent of QRS dispersion (≥ 40 ms), the strongest independent predictor of sudden cardiac death.     

A simplified method for the determination of left atrial size and function using cine magnetic resonance imaging

This work aimed at developing a rapid and clinically applicable method for the assessment of left atrial size and function using magnetic resonance imaging (MRI). We studied 17 healthy subjects and 26 cardiac patients. Left atrial cine MRI with 50 ms phases was made in 6–12 contiguous long-axis sections encompassing the entire atrial cavity. A volume-time curve was reconstructed to measure the minimum and maximum volumes as well as the fractional volume change, reservoir function, ejection fraction, and mean filling and emptying rates of the left atrium. The image section with the largest left atrial area was then selected and a comparable area-time curve was reconstructed. The atrial phasic areas and functional indices were determined analogously to the volume-based assessment. The contours of atrial area-time and volume-time curves agreed closely in individual subjects. All area-based left atrial measurements distinguished cardiac patients as a group from healthy persons. The combined specificity of the area-based analyses was 92% and the sensitivity, 65%, in identifying abnormal results in individual patients. The accuracy of the area-based data was best for the atrial minimum size, fractional change, reservoir function, and mean filling rate. The estimated time savings with the simplified method were 5 to 6 h per patient. Left atrial size and function can be studied by reconstructing a phasic atrial area-time curve with cine MRI. Atrial enlargement and abnormalities of filling and reservoir function can be reliably identified, but if data on conduit or stroke function are crucial the three-dimensional MRI technique is still recommended.     

Rapid 3D whole-heart cine imaging using golden ratio stack of spirals

Three-dimensional cine imaging provides a wealth of information about cardiac anatomy and function, but its use in the clinical environment is limited because data acquisition is very time consuming. In this work, a free-breathing 3D whole-heart cine imaging framework was developed using a time-efficient stack of spirals trajectory and accelerated reconstruction. Two suitable view ordering methods are considered with different spacing between k-space readouts in the partition dimension: uniform and tiny golden ratio based. A simulation study suggested the latter did not present any benefits in terms of similarity to the true image. The proposed method was subsequently tested on 10 prospective subjects and compared with conventional multi-slice breath-hold imaging. Image quality was evaluated using objective and subjective scores and ventricular measurements were compared to assess clinical accuracy. Image quality was lower in the proposed technique than in breath-hold images but good agreement was found in clinically relevant ventricular measurements. In addition, the proposed method was fast to acquire, required minimal planning and provided full anatomical coverage with isotropic resolution.     

Validation of non-contrast multiple overlapping thin-slab 4D-flow cardiac magnetic resonance imaging

BackgroundCardiac magnetic resonance (CMR) flow quantification is typically performed using 2D phase-contrast (PC) imaging of a plane perpendicular to flow. 3D-PC imaging (4D-flow) allows offline quantification anywhere in a thick slab, but is often limited by suboptimal signal, potentially alleviated by contrast enhancement. We developed a non-contrast 4D-flow sequence, which acquires multiple overlapping thin slabs (MOTS) to minimize signal loss, and hypothesized that it could improve image quality, diagnostic accuracy, and aortic flow measurements compared to non-contrast single-slab approach.MethodsWe prospectively studied 20 patients referred for transesophageal echocardiography (TEE), who underwent CMR (GE, 3 T). 2D-PC images of the aortic valve and three 4D-flow datasets covering the heart were acquired, including single-slab, pre- and post-contrast, and non-contrast MOTS. Each 4D-flow dataset was interpreted blindly for ≥moderate valve disease and compared to TEE. Flow visualization through each valve was scored (0 to 4), and aortic-valve flow measured on each 4D-flow dataset and compared to 2D-PC reference.ResultsDiagnostic quality visualization was achieved with the pre- and post-contrast 4D-flow acquisitions in 25% and 100% valves, respectively (scores 0.9 ± 1.1 and 3.8 ± 0.5), and in 58% with the non-contrast MOTS (1.6 ± 1.1). Accuracy of detection of valve disease was 75%, 92% and 82%, respectively. Aortic flow measurements were possible in 53%, 95% and in 89% patients, respectively. The correlation between pre-contrast single-slab measurements and 2D-PC reference was weak (r = 0.21), but improved with both contrast enhancement (r = 0.71) and with MOTS (r = 0.67).ConclusionsAlthough non-contrast MOTS 4D-flow improves valve function visualization and diagnostic accuracy, a significant proportion of valves cannot be accurately assessed. However, aortic flow measurements using non-contrast MOTS is feasible and reaches similar accuracy to that of contrast-enhanced 4D-flow.     

3D reconstruction of the brain from magnetic resonance images using a connectivity algorithm

We present high resolution three dimensional (3D) connectivity, surface construction and display algorithms that detect, extract, and display the surface of a brain from contiguous magnetic resonance (MR) images. The algorithms identify the external brain surface and create a 3D image, showing the fissures and surface convolutions of the cerebral hemispheres, cerebellum, and brain stem. Images produced by these algorithms also show the morphology of other soft tissue boundaries such as the cerebral ventricular system and the skin of the patient. For the purposes of 3D reconstruction, our experiments show that T1 weighted images give better contrast between the surface of the brain and the cerebral spinal fluid than T2 weighted images. 3D reconstruction of MR data provides a non-invasive procedure for examination of the brain surface and other anatomical features.     

Simultaneous magnetic resonance gadolinium-enhanced 2D perfusion and 3D angiographic imaging

A method was implemented and tested that allows the simultaneous acquisition of magnetic resonance 2D slice selective perfusion and 3D angiographic data during a single bolus injection of a contrast agent. High quality contrast-enhanced perfusion images and angiograms of the lung, kidney and heart were obtained in healthy volunteers. Combined perfusion and angiography provided additional information with an acceptable increase in acquisition time. No image artifacts were attributed to the technique. The combined information may be useful in detecting, as well as characterizing, vascular abnormalities.     

Simultaneous blood flow and oxygenation measurements using an off-the-shelf spectrometer

Blood oxygenation and flow are both important parameters in a living body. In this Letter, we introduce a simple configuration to simultaneously measure blood flow and oxygenation using an off-the-shelf spectrometer. With the integration time of 10 ms, flow phantom measurements, a liquid blood phantom test, and an arm cuff occlusion paradigm were performed to validate the feasibility of the system. We expect this proof-of-concept study would be widely adopted by other researchers for acquiring both blood flow and oxygenation changes due to its straightforward configuration and the possibility of multimodal measurement.     

Complex 3D blood flow pathways in two cases of aorta to right heart fistulae: a 4D flow MRI study

We present an analysis of 3D blood flow in two cases of Sinus of Valsalva to right heart fistulae based on 4D flow MRI. Despite similar underlying pathology, 3D visualization revealed intricate differences in flow patterns connecting the systemic and pulmonary circulation. The cases illustrates the potential of 4D flow MRI to complement the evaluation of complex structural heart disease by assessing complex flow dynamics and providing quantitative information of flow ratios and flow rates.     

Validation of HNO3 spectroscopic parameters using atmospheric absorption and emission measurements

The improved database of HNO₃ spectroscopic parameters in the 600–950 cm^?1 spectral region presented in [Gomez L, Tran H, Perrin A, Gamache RR, Laraia A, Orphal J, et al. Some improvements of the HNO₃ spectroscopic parameters in the spectral region from 600 to 950 cm^?1. JQSRT 2008, in press] is tested by comparisons between calculations and atmospheric remotely sensed absorption and emission spectra. The line parameters in the 11.3 μm region are validated using ground-based Fourier transform solar absorption measurements, whereas those in the 13.1 μm region are successfully tested using balloon-borne atmospheric emission spectra. In both regions, the quality of the line parameters and the consistency between band intensities is confirmed through comparisons with emission spectra collected by the satellite-borne MIPAS instrument.     

Clinical evaluation of subtracted pointwise encoding time reduction with radial acquisition-based magnetic resonance angiography compared to 3D time-of-flight magnetic resonance angiography for improved flow dephasing at 3 Tesla

ObjectiveFlow dephasing artifacts within intracranial internal carotid artery (ICA) have been problematic for 3D time-of-flight magnetic resonance angiography (3D-TOF-MRA). This study aimed to evaluate pointwise encoding time reduction with radial acquisition subtraction-based MR angiography (PETRA-MRA) for decreasing flow dephasing artifacts compared to 3D-TOF-MRA in intracranial segments of ICA at 3 T.MethodsSixty healthy participants and seven patients with intracranial ICA aneurysms were enrolled to undergo 3D-TOF-MRA and PETRA-MRA. Two radiologists each evaluated the image quality of healthy participants using a 4-point scale (1: the best and 4: the worst). Quantitative analysis of the extent of homogeneity in signal intensity within the ICA and intracranial aneurysms was conducted using a parameter d: the higher the d value, the greater the signal homogeneity. Wilcoxon signed rank test, Chi-square test and the weighted kappa (κ) statistic were used for statistical analyses.ResultsThe image quality of PETRA-MRA with an overall score of 1.35 ± 0.53 was significantly better than that obtained with 3D-TOF-MRA, with an overall score of 3.50 ± 0.62 (Z = -9.56, p < 0.001). The parameter d of PETRA-MRA was higher than that of 3D-TOF-MRA for both 60 healthy participants (0.97 ± 0.05, 0.87 ± 0.11; z = -13.21, p < 0.001) and 7 patients with intracranial aneurysms (0.81 ± 0.18, 0.74 ± 0.16; z = -2.37, p = 0.018).ConclusionCompared with conventional 3D-TOF-MRA, PETRA-MRA remarkably improved the image quality with reduced flow dephasing artifacts in segments of intracranial ICA.     

Accuracy and effectiveness of self-gating signals in free-breathing three-dimensional cardiac cine magnetic resonance imaging

Conventional multiple breath-hold two-dimensional(2D) balanced steady-state free precession(SSFP) presents many difficulties in cardiac cine magnetic resonance imaging(MRI). Recently, a self-gated free-breathing three-dimensional(3D) SSFP technique has been proposed as an alternative in many studies. However, the accuracy and effectiveness of selfgating signals have been barely studied before. Since self-gating signals are crucially important in image reconstruction, a systematic study of self-gating signals and comparison with external monitored signals are needed.Previously developed self-gated free-breathing 3D SSFP techniques are used on twenty-eight healthy volunteers. Both electrocardiographic(ECG) and respiratory bellow signals are also acquired during the scan as external signals. Self-gating signal and external signal are compared by trigger and gating window. Gating window is proposed to evaluate the accuracy and effectiveness of respiratory self-gating signal. Relative deviation of the trigger and root-mean-square-deviation of the cycle duration are calculated. A two-tailed paired t-test is used to identify the difference between self-gating and external signals. A Wilcoxon signed rank test is used to identify the difference between peak and valley self-gating triggers.The results demonstrate an excellent correlation(P = 0, R 0.99) between self-gating and external triggers. Wilcoxon signed rank test shows that there is no significant difference between peak and valley self-gating triggers for both cardiac(H = 0, P 0.10) and respiratory(H = 0, P 0.44) motions. The difference between self-gating and externally monitored signals is not significant(two-tailed paired-sample t-test: H = 0, P 0.90).The self-gating signals could demonstrate cardiac and respiratory motion accurately and effectively as ECG and respiratory bellow. The difference between the two methods is not significant and can be explained. Furthermore, few ECG trigger errors appear in some subjects while these errors are not found in self-gating signals.     

Long-term follow-up of 82 patients with chronic disease of the thoracic aorta using spin-echo and cine gradient magnetic resonance imaging

The objective of this study to examine the clinical impact of magnetic resonance imaging in long-term follow-up of patients (pts) with chronic disease of the thoracic aorta such as coarctation of the aorta, chronic aortic dissection and true aortic aneurysm. A total of 322 magnetic resonance examinations obtained in 82 pts with chronic disease of the thoracic aorta (31 pts with coarctation of the aorta (CoA), 29 pts with chronic aortic dissection and 22 pts with true aneurysm) over a period of 0.25 to 13.5 (mean +/- SD: 6.5 +/- 3.4) years were retrospectively reviewed. Diameters of the thoracic aorta were measured at predefined levels and morphological and functional parameters of special interest were analysed in each patient group. Pts were classified as having constant or progressive disease and clinical end-points were defined as (re-)operation or death. 43 pts (52%) (CoA 15 pts, chronic dissection 16 pts, true aneurysm 12 pts) had constant findings. None of them underwent (re-)operation and seven patients (16%) died, three of them from their aortic disease more than five years later after their last magnetic resonance examination, one from an arrhythmogenic event, and in the remaining 3 pts the cause of death could not be definitely established. 39 pts (48%) (CoA 16 pts, chronic dissection 13 pts, true aneurysm 10 pts) had progressive disease as demonstrated by repetitive magnetic resonance imaging. Of these 39 pts 24 pts underwent (re-)operation, in 15 pts operation was postponed. Four pts died from their aortic disease. Repetitive magnetic resonance imaging is a clinically feasible technique for long-term follow-up of pts with chronic disease of the thoracic aorta because it can detect progressive disease in a large subset of pts requiring elective surgery. The results of magnetic resonance imaging provided the rationale for either (re-)operation or conservative management, thus guiding patient management.     

Simultaneous 2D flow velocity and gas temperature measurements using thermographic phosphors

In this paper a new approach for simultaneous 2D velocity and temperature measurements using phosphoric particles is presented. The phosphoric particles respond to the temperature changes in the flow while acting as tracers for velocity mapping. The temperature sensitive particles were seeded into a heated flow and were excited by a pulsed UV laser. The subsequent red shifted emission was detected and analyzed to infer temperature using calibration procedures for lifetime and emission spectra against temperature. The diameter of the temperature sensitive particles, usually in the range of 1–10 μm, makes them useful for velocity measurements using particle image velocimetry (PIV). As such, simultaneous measurement of temperature and flow velocity of a gaseous flow were performed and presented. PACS  42.62.-b; 47.80.Cb; 47.80.Fg