MR angiography using steady-state free precession

Equilibrium phase contrast-enhanced magnetic resonance angiography of the thoracic aorta and heart using balanced T1 relaxation-enhanced steady-state

BACKGROUND

Three-dimensional (3D) contrast-enhanced magnetic resonance angiography (CEMRA) is routinely used for vascular evaluation. With existing techniques for CEMRA, diagnostic image quality is only obtained during the first pass of the contrast agent or shortly thereafter, whereas angiographic quality tends to be poor when imaging is delayed to the equilibrium phase. We hypothesized that prolonged blood pool contrast enhancement could be obtained by imaging with a balanced T1 relaxation-enhanced steady-state (bT1RESS) pulse sequence, which combines 3D balanced steady-state free precession (bSSFP) with a saturation recovery magnetization preparation to impart T1 weighting and suppress background tissues. An electrocardiographic-gated, two-dimensional-accelerated version with isotropic 1.1-mm spatial resolution was evaluated for breath-hold equilibrium phase CEMRA of the thoracic aorta and heart.

METHODS

The study was approved by the institutional review board. Twenty-one subjects were imaged using unenhanced 3D bSSFP, time-resolved CEMRA, first-pass gated CEMRA, followed by early and late equilibrium phase gated CEMRA and bT1RESS. Nine additional subjects were imaged using equilibrium phase 3D bSSFP and bT1RESS. Images were evaluated for image quality, aortic root sharpness, and visualization of the coronary artery origins, as well as using standard quantitative measures.

RESULTS

Equilibrium phase bT1RESS provided better image quality, aortic root sharpness, and coronary artery origin visualization than gated CEMRA (P < 0.05), and improved image quality and aortic root sharpness versus unenhanced 3D bSSFP (P < 0.05). It provided significantly larger apparent signal-to-noise and apparent contrast-to-noise ratio values than gated CEMRA and unenhanced 3D bSSFP (P < 0.05) and provided ninefold better fluid suppression than equilibrium phase 3D bSSFP. Aortic diameter and main pulmonary artery diameter measurements obtained with bT1RESS and first-pass gated CEMRA strongly correlated (P < 0.05).

CONCLUSIONS

We found that using bT1RESS greatly prolongs the useful duration of blood pool contrast enhancement while improving angiographic image quality compared with standard CEMRA techniques. Although further study is needed, potential advantages for vascular imaging include eliminating the current requirement for first-pass imaging along with better reliability and accuracy for a wide range of cardiovascular applications.

Multi-Modality Imaging of Atheromatous Plaques in Peripheral Arterial Disease: Integrating Molecular and Imaging Markers

Peripheral artery disease (PAD) is a common and debilitating condition characterized by the narrowing of the limb arteries, primarily due to atherosclerosis. Non-invasive multi-modality imaging approaches using computed tomography (CT), magnetic resonance imaging (MRI), and nuclear imaging have emerged as valuable tools for assessing PAD atheromatous plaques and vessel walls. This review provides an overview of these different imaging techniques, their advantages, limitations, and recent advancements. In addition, this review highlights the importance of molecular markers, including those related to inflammation, endothelial dysfunction, and oxidative stress, in PAD pathophysiology. The potential of integrating molecular and imaging markers for an improved understanding of PAD is also discussed. Despite the promise of this integrative approach, there remain several challenges, including technical limitations in imaging modalities and the need for novel molecular marker discovery and validation. Addressing these challenges and embracing future directions in the field will be essential for maximizing the potential of molecular and imaging markers for improving PAD patient outcomes.

Free-breathing high resolution modified Dixon steady-state angiography with compressed sensing for the assessment of the thoracic vasculature in pediatric patients with congenital heart disease

BACKGROUND

Cardiovascular magnetic resonance angiography (CMRA) is a non-invasive imaging modality of choice in pediatric patients with congenital heart disease (CHD). This study was aimed to evaluate the diagnostic utility of a respiratory- and electrocardiogram-gated steady-state CMRA with modified Dixon (mDixon) fat suppression technique and compressed sensing in comparison to standard first-pass CMRA in pediatric patients with CHD at 3 T.

METHODS

In this retrospective single center study, pediatric CHD patients who underwent CMR with first-pass CMRA followed by mDixon steady-state CMRA at 3 T were analyzed. Image quality using a Likert scale from 5 (excellent) to 1 (non-diagnostic) and quality of fat suppression were assessed in consensus by two readers. Blood-to-tissue contrast and quantitative measurements of the thoracic vasculature were assessed separately by two readers. CMRA images were reevaluated by two readers for additional findings, which could be identified only on either one of the CMRA types. Paired Student t test, Wilcoxon test, and intraclass correlation coefficients (ICCs) were used for statistical analysis.

RESULTS

32 patients with CHD (3.3 ± 1.7 years, 13 female) were included. Overall image quality of steady-state mDixon CMRA was higher compared to first-pass CMRA (4.5 ± 0.5 vs. 3.3 ± 0.5; P < 0.001). Blood-to-tissue contrast ratio of steady-state mDixon CMRA was comparable to first-pass CMRA (7.85 ± 4.75 vs. 6.35 ± 2.23; P = 0.133). Fat suppression of steady-state mDixon CMRA was perfect in 30/32 (94%) cases. Vessel diameters were greater in first-pass CMRA compared to steady-state mDixon CMRA with the greatest differences at the level of pulmonary arteries and veins (e.g., right pulmonary artery for reader 1: 10.4 ± 2.4 vs. 9.9 ± 2.3 mm, P < 0.001). Interobserver agreement was higher for steady-state mDixon CMRA for all measurements compared to first-pass CMRA (ICCs > 0.92). In 9/32 (28%) patients, 10 additional findings were identified on mDixon steady-state CMRA (e.g., partial anomalous venous return, abnormalities of coronary arteries, subclavian artery stenosis), which were not depicted using first-pass CMRA.

CONCLUSIONS

Steady-state mDixon CMRA offers a robust fat suppression, a high image quality, and diagnostic utility for the assessment of the thoracic vasculature in pediatric CHD patients.

Balanced Steady-State Free Precision and Time of Flight Noncontrast Magnetic Resonance Angiography in Peripheral Arterial Disease

Purpose To determine the diagnostic efficacy of balanced steady-state free precession (bSSFP) and time-of-flight (TOF)-based noncontrast magnetic resonance angiography (NC-MRA) in lower limb peripheral arterial disease (PAD).

Material and Methods 10 patients with suspected PAD underwent both NC-MRA (bSSFP and 2D TOF) and contrast-enhanced MR angiography (CE-MRA)/CT angiography (CTA). A total of 170 arterial segments (17 segments in each patient) were analyzed on NC-MRA and compared with CE-MRA/CTA for quality of images and for estimating the degree of stenoses. Image quality was graded as 1—poor, 2—fair, 3—good, and 4—excellent. The degree of stenoses was graded as 0—normal, 1— ≤ 50% narrowing, 2— > 50% narrowing, 3—near complete/100% occlusion. Sensitivity, specificity, positive predictive value, and negative predictive value of NC-MRA in identifying significant stenosis, as compared with CE-MRA/CTA, were estimated.

Results a) Mean grade of the image quality of NC-MRA was 3.10 and the CE-MRA/CTA was 3.64. b) The agreement in the estimation of the degree of stenosis on NC-MRA as compared with CE-MRA/CTA was substantial in aortoiliac segments (weighted kappa 0.646 [95% CI] [0.361–0.931] [p < 0.001]), almost perfect in femoropopliteal segments (weighted kappa 0.911 [95% CI] [0.79–1.032] [p < 0.001]), and poor in infrapopliteal segments (weighted kappa 0.052 [95% CI] [0.189–0.293] [p < 0.33587]).

Conclusion TOF and bSSFP-based NC-MRA was found to be comparable to the CE-MRA/CTA in the evaluation of PAD in lower limbs in the aortoiliac and femoropopliteal regions. NC-MRA was especially helpful in assessing the aortoiliac vessels and femoropopliteal vessels, with the imaging of infrapopliteal arteries being suboptimal.

Subtractive NCE-MRA: Improved background suppression using robust regression-based weighted subtraction

PURPOSE

To correct the intensity difference of static background signal between bright blood images and dark blood images in subtractive non-contrast-enhanced MR angiography using robust regression, thereby improving static background signal suppression on subtracted angiograms.

METHODS

Robust regression (RR), using iteratively reweighted least squares, is used to calculate the regression coefficient of background tissues from a scatter plot showing the voxel intensity of bright blood images versus dark blood images. The weighting function is based on either the Euclidean distance from the estimated regression line or the deviation angle. Results from RR using the deviation angle (RRDA), conventional RR using the Euclidean distance, and ordinary leastsquares regression were compared with reference values determined manually by two observers. Performance was evaluated over studies using different sequences, including 36 thoracic flow-sensitive dephasing data sets, 13 iliac flow-sensitive dephasing data sets, and 26 femoral fresh blood imaging data sets.

RESULTS

RR deviation angle achieved robust and accurate performance in all types of images, with small bias, small mean absolute error, and high-correlation coefficients with reference values. Background tissues, such as muscle, veins, and bladder, were suppressed while the vascular signal was preserved. Euclidean distance gave good performance for thoracic and iliac flow-sensitive dephasing, but could not suppress background tissues in femoral fresh blood imaging. Ordinary least squares regression was sensitive to outliers and overestimated regression coefficients in thoracic flow-sensitive dephasing.

CONCLUSION

Weighted subtraction using RR was able to acquire the regression coefficients of background signal and improve background suppression of subtractive non-contrast-enhanced MR angiography techniques. RR deviation angle has the most robust and accurate overall performance among three regression methods.

Quiescent-Interval Single-Shot Magnetic Resonance Angiography

Lower extremity peripheral arterial disease (PAD) is a chronic, debilitating disease with a significant global burden. A number of diagnostic imaging techniques exist, including computed tomography angiography (CTA) and contrast-enhanced magnetic resonance angiography (CEMRA), to aid in PAD diagnosis and subsequent treatment planning. Due to concerns of renal toxicity or nephrogenic systemic fibrosis (NSF) for iodinated and gadolinium-based contrasts, respectively, a number of non-enhanced MRA (NEMRA) protocols are being increasingly used in PAD diagnosis. These techniques, including time of flight and phase contrast MRA, have previously demonstrated poor image quality, long acquisition times, and/or susceptibility to artifacts when compared to existing contrast-enhanced techniques. In recent years, Quiescent-Interval Single-Shot (QISS) MRA has been developed to overcome these limitations in NEMRA methods, with promising results. Here, we review the various screening and diagnostic tests currently used for PAD. The various NEMRA protocols are discussed, followed by a comprehensive review of the literature on QISS MRA to date. A particular emphasis is placed on QISS MRA feasibility studies and studies comparing the diagnostic accuracy and image quality of QISS MRA versus other diagnostic imaging techniques in PAD.

Recent advances in magnetic resonance imaging for peripheral artery disease

The global burden of peripheral artery disease (PAD) is significant. This has led to numerous recent advances in magnetic resonance imaging (MRI) techniques in PAD. Older techniques such as time of flight MRI or phase contrast MRI are burdened by long acquisition times and significant issues with artifacts. In addition, the most used MRI modality, contrast-enhanced MR angiography (CE-MRA) is limited by the use of gadolinium contrast and its potential toxicity. Novel MRI techniques such as arterial spin labeling (ASL), blood-oxygen-level dependent imaging (BOLD), and first-pass perfusion gadolinium enhancement are advancing the field by providing skeletal muscle perfusion/oxygenation data while maintaining excellent spatial and temporal resolution. Perfusion data can be critical to providing objective clinical data of a visualized stenosis. In addition, there are a number of new MRI sequences assessing plaque composition and lesion severity in the absence of contrast. These approaches used in combination can provide useful clinical and prognostic data and provide critical endpoints in PAD research.

Blood Pool Segmentation Results in Superior Virtual Cardiac Models than Myocardial Segmentation for 3D Printing

The method of cardiac magnetic resonance (CMR) three-dimensional (3D) image acquisition and post-processing which should be used to create optimal virtual models for 3D printing has not been studied systematically. Patients (n = 19) who had undergone CMR including both 3D balanced steady-state free precession (bSSFP) imaging and contrast-enhanced magnetic resonance angiography (MRA) were retrospectively identified. Post-processing for the creation of virtual 3D models involved using both myocardial (MS) and blood pool (BP) segmentation, resulting in four groups: Group 1-bSSFP/MS, Group 2-bSSFP/BP, Group 3-MRA/MS and Group 4-MRA/BP. The models created were assessed by two raters for overall quality (1-poor; 2-good; 3-excellent) and ability to identify predefined vessels (1-5: superior vena cava, inferior vena cava, main pulmonary artery, ascending aorta and at least one pulmonary vein). A total of 76 virtual models were created from 19 patient CMR datasets. The mean overall quality scores for Raters 1/2 were 1.63 ± 0.50/1.26 ± 0.45 for Group 1, 2.12 ± 0.50/2.26 ± 0.73 for Group 2, 1.74 ± 0.56/1.53 ± 0.61 for Group 3 and 2.26 ± 0.65/2.68 ± 0.48 for Group 4. The numbers of identified vessels for Raters 1/2 were 4.11 ± 1.32/4.05 ± 1.31 for Group 1, 4.90 ± 0.46/4.95 ± 0.23 for Group 2, 4.32 ± 1.00/4.47 ± 0.84 for Group 3 and 4.74 ± 0.56/4.63 ± 0.49 for Group 4. Models created using BP segmentation (Groups 2 and 4) received significantly higher ratings than those created using MS for both overall quality and number of vessels visualized (p < 0.05), regardless of the acquisition technique. There were no significant differences between Groups 1 and 3. The ratings for Raters 1 and 2 had good correlation for overall quality (ICC = 0.63) and excellent correlation for the total number of vessels visualized (ICC = 0.77). The intra-rater reliability was good for Rater A (ICC = 0.65). Three models were successfully printed on desktop 3D printers with good quality and accurate representation of the virtual 3D models. We recommend using BP segmentation with either MRA or bSSFP source datasets to create virtual 3D models for 3D printing. Desktop 3D printers can offer good quality printed models with accurate representation of anatomic detail.

Vascular imaging in diabetes

Diabetes is a global epidemic affecting individuals of all socioeconomic backgrounds. Despite intensive efforts, morbidity and mortality secondary to the micro- and macrovascular complications remain unacceptably high. As a result, the use of imaging modalities to determine the underlying pathophysiology, early onset of complications, and disease progression has become an integral component of the management of such individuals. Echocardiography, stress echocardiography, and nuclear imaging have been the mainstay of noninvasive cardiovascular imaging tools to detect myocardial ischemia, but newer modalities such as cardiac MRI, cardiac CT, and PET imaging provide incremental information not available with standard imaging. While vascular imaging to detect cerebrovascular and peripheral arterial disease non-invasively has traditionally used ultrasound, CT- and MRI-based techniques are increasingly being employed. In this review, we will provide an outline of recent studies utilizing non-invasive imaging techniques to assist in disease diagnosis as well as monitoring disease progression. In addition, we will review the evidence for newer modalities such as MR spectroscopy, 3D intravascular ultrasound, and optical coherence tomography that provide exquisite detail of metabolic function and coronary anatomy not available with standard imaging, but that have not yet become mainstream.

Is there a role for free breathing non-contrast steady-state free precession renal MRA imaging for assessing live donors? A preliminary study

OBJECTIVE

Accurate pre-operative evaluation of renal vascular anatomy is essential for successful renal harvest in live donor transplantation. Non-contrast renal MR angiographic (MRA) techniques are potentially well suited to the screening of donors; however, their restricted imaging field of view (FOV) has previously been an important limitation. We sought to assess whether the addition of a large FOV balanced fast field echo (BFFE) steady-state free precession (SSFP) sequence to non-contrast SSFP MRA could overcome this problem. Comparison with contrast-enhanced MRA (CE MRA) and findings at surgery were performed.

METHODS

22 potential renal donors each underwent SSFP and CE MRA. 11 out of 22 potential donors subsequently underwent a donor nephrectomy.

RESULTS

All images were diagnostic. Both SSFP MRA and CE MRA identified an equal number of arteries. Surgery confirmed two accessory renal arteries, both demonstrated with both imaging techniques. A third accessory vessel was identified with both techniques on a kidney contralateral to the donated organ. 6 out of 11 procured kidneys demonstrated early branch arteries at surgery, 5 out of 6 of which had been depicted on both SSFP and CE MRA. The median grading of image quality for main renal arteries was slightly better for CE MRA (p=0.048), but for accessory vessels it was better for SSFP MRA.

CONCLUSION

This pilot study indicates that by combining free-breathing SSFP MRA with large-FOV bFFE images, an accurate depiction of renal vascular anatomy without the need for intravenous contrast administration can be obtained, as compared with surgical findings and CE MRA.

Renovascular imaging in the NSF Era

The detection of the association between nephrogenic systemic fibrosis (NSF), a rare but potentially life-threatening disease only encountered in patients with severely impaired renal function, and the previous administration of some Gd-chelates has cast a shadow on the administration of Gd-chelates in patients with chronic renal failure. So far, contrast-enhanced MR-angiography (MRA) was considered the best diagnostic modality in patients with suspected renal disease. This review explores the most appropriate use of renal MRA with a focus on newly developed nonenhanced MRA techniques. Nonenhanced MRA techniques mainly based on SSFP with ECG-gating allow for acceptable spatial resolution to visualize at least the proximal parts of the renal arteries. In addition functional renal imaging techniques and their current clinical role are critically appreciated. J. Magn. Reson. Imaging 2009;30:1323-1334. (c) 2009 Wiley-Liss, Inc.

Non-contrast-enhanced MR imaging of renal artery stenosis at 1.5 tesla

Balanced steady-state free precession (Bal-SSFP) techniques produce excellent anatomic images of renal arteries without the use of contrast agents and are relatively flow-insensitive. Electrocardiography (ECG)-triggered and non-ECG-triggered sequences have been shown to be quite sensitive for detection of regional arterial stenosis (RAS), and the already high specificity is likely to increase with further refinement of the techniques. Bal-SSFP sequences can be used as a screening tool or as an alternative to contrast-enhanced (CE) magnetic resonance angiography (MRA) when contrast agents are contraindicated. In addition to morphologic imaging of RAS, non-CE techniques can be used in functional assessment of hemodynamic significance. The complimentary tools can be used alone or in combination with CE-MRA for MR imaging of renal vascular hypertension.

Navigator-Gated MR Angiography of the Renal Arteries: A Potential Screening Tool for Renal Artery Stenosis

OBJECTIVE

The purpose of our study was to determine how well unenhanced navigator-gated steady-state free precession (Nav SSFP) MR angiography (MRA) performs as a screening test for the detection of renal artery stenosis.

SUBJECTS AND METHODS

Forty patients referred to rule out renal artery stenosis were imaged using an optimized Nav SSFP MRA sequence before conventional contrast-enhanced MRA (CE-MRA). Two radiologists evaluated Nav SSFP for maximum stenosis measurement, and comparison was made with CE-MRA results.

RESULTS

Fifteen of the 40 patients had greater than 50% renal artery stenosis as determined on CE-MRA. Sensitivity for detecting renal artery stenosis with Nav SSFP was 100%; specificity, 84%; negative predictive value, 100%; and positive predictive value, 79%. The average mean stenosis difference between Nav SSFP and CE-MRA was 10% +/- 9%.

CONCLUSION

Sensitivity and negative predictive value for the detection of renal artery stenosis using Nav SSFP were perfect, with an acceptable specificity of 84%. This suggests Nav SSFP is a promising technique for simple unenhanced screening for the detection of renal artery stenosis.

Parallel imaging technique for the external carotid artery and its branches: Comparison of balanced turbo field echo, phase contrast, and time-of-flight sequences

PURPOSE

To evaluate the parallel imaging technique in the external carotid artery and its branches using 3D balanced turbo field echo (3D bTFE), 3D phase-contrast (3D PC), and 3D time-of-flight (3D TOF) MR angiography (MRA) sequences.

MATERIALS AND METHODS

A total of 26 healthy volunteer subjects underwent 3D bTFE, 3D PC, and 3D TOF MRA with the parallel imaging sensitivity encoding (SENSE) technique. The obtained images were read in a blinded fashion by three radiologists. Interreader and intersequence statistical analyses were performed to compare the visibility of the arteries.

RESULTS

Friedman's ranking test demonstrated that there was no significant difference in visibility between any two pairs of sequences for the external carotid artery and its first branches. However, of the three techniques, 3D PC MRA performed the best for the second-order branches (P < 0.01) and for overall visibility of the external carotid artery and its branches (P < 0.01). The 3D bTFE sequence is superior to 3D TOF; however, an effective means of separating arteries from veins and salivary ducts is needed.

CONCLUSION

The combination of parallel imaging and the 3D PC technique is a promising approach for face and neck MRA.

Prospective intraindividual comparison between respiratory-triggered balanced steady-state free precession and breath-hold gradient-echo and time-of-flight magnetic resonance imaging for assessment of portal and hepatic veins

The purpose of this study was to compare respiratory-triggered balanced steady-state free precession (bSSFP) with breath-hold contrast-enhanced dynamic two-dimensional (2D) gradient-echo (GRE) and time-of-flight (TOF) magnetic resonance imaging (MRI) for portal and hepatic vein visualization and assessment of portal and hepatic venous variants. Sixty patients with liver disease underwent nonenhanced bSSFP and contrast-enhanced GRE, bSSFP, and TOF imaging. Contrast-to-noise ratios (CNRs) for portal and hepatic veins were measured. Two readers rated the quality of portal and hepatic vein visualization on a 5-point Likert scale. The diagnostic performance of each MRI series in the detection of portal and hepatic venous variants was assessed in 40/60 patients who also underwent contrast-enhanced multidetector-row computed tomography (MDCT). CNRs for portal and hepatic veins were highest on contrast-enhanced bSSFP images. Image quality of portal and hepatic veins was rated higher for nonenhanced bSSFP than for contrast-enhanced GRE (p<0.03) and TOF (p<0.003) and higher for contrast-enhanced than for nonenhanced bSSFP (p<0.003). Compared with MDCT, portal and hepatic venous variants were identified with an accuracy of 99% on bSSFP images, with an excellent interobserver agreement (kappa=0.97). Compared with MDCT, presence of surgically important portal and hepatic venous anatomical variants can be predicted with high accuracy on bSSFP images.

Magnetic resonance angiography of abdominal and lower extremity vasculature

Magnetic resonance angiography (MRA) has evolved over the past years from an experimental imaging modality to a technique that is now widely applied in clinical practice. This article reviews the fundamentals of the different magnetic resonance angiographic techniques and how they can be applied for abdominal and peripheral arterial imaging. Currently, contrast-enhanced magnetic resonance angiography (CE-MRA), whereby a luminogram is obtained during initial arterial passage of contrast material, is the most widely used technique. With current hardware and software, high-spatial resolution images of the abdominal aorta and proximal visceral branches can be obtained that are equivalent to intra-arterial digital subtraction angiography (IA-DSA). High-quality imaging of the renal arteries demands isotropic voxels and reformations orthogonal to the vessel axis for evaluation. Contrast-enhanced magnetic resonance angiography of the peripheral vascular tree is now a highly accurate technique and has replaced diagnostic intra-arterial digital subtraction angiography and duplex ultrasonography in many hospitals.

MR Coronary Angiography Using 3D-SSFP With and Without Contrast Application

We compared the performance of a contrast-enhanced with a non-contrast breath-hold 3D-SSFP-sequence for Magnetic Resonance Coronary Angiography in seven healthy subjects and 14 patients. Visibility of coronary segments, vessel length, image quality and the influence of an extracellular contrast agent (Gadolinium-DTPA) were assessed. Overall, the performance of the sequence was better in healthy subjects than in patients. Although the application of Gadolinium-DTPA increased the contrast-to-noise-ratio of the right coronary artery, the overall performance was not significantly improved. We conclude that a 3D-SSFP-technique depicts extensive parts of the coronary arteries and does not require contrast application.

Three-dimensional contrast-enhanced MR angiography of the thoraco-abdominal vessels

With the strategies presented in this article,relevant disease involving the thoraco-abdominal vessels can be well depicted by 3D CEMRA. Aneurysms, dissections, occlusions, congenital lesions, and anatomic anomalies are readily assessed. Time-resolved MRA provides supplemental information in shunts, dissections, aneurysms, and AVMs. Velocity-encoded imaging may help fur-ther characterize lesions and may provide useful functional information to grade and monitor the progression of stenotic disease. 3.0T imaging and recent development in multicoil RF technology will further improve the performance of 3D CEMRAin terms of temporal and spatial resolution.

Water-saturated three-dimensional balanced steady-state free precession for fast abdominal fat quantification

PURPOSE

To compare the performance of a novel water-saturated b-SSFP sequence with that of a conventional T1-weighted turbo spin echo (T1W TSE) sequence for abdominal fat quantification.

MATERIALS AND METHODS

A water-saturated, segmented, three-dimensional balanced steady-state free precession (b-SSFP) sequence and a traditional T1W TSE sequence were both employed on phantom and human studies. For phantom studies, a dual-layered phantom with known internal/external oil volumes was imaged using the two sequences. Images obtained by the two sequences were both processed using a computer-aided semiautomatic program for oil volume quantification. For human studies, six volunteers were scanned axially, centered at L2-L3 levels. Signal-to-noise ratio (SNR)(fat), contrast-to-noise ratio (CNR)(fat-muscle), CNR(fat-large bowel), and CNR(fat-small bowel) were calculated on hand-drawn regions of interest (ROIs), and averaged over all six slices for each subject. Statistical analyses were then performed to determine the SNR and CNR differences between images obtained by the two techniques.

RESULTS

The phantom studies show that water-saturated b-SSFP offers a significantly closer estimation of true oil volumes compared with that of T1W TSE (P < 0.0001), as well as a more accurate internal/external volume ratio (P = 0.0001). In human studies, three-dimensional water-saturated b-SSFP images demonstrated higher CNR than that of T1W TSE (P < 0.0005), and very close SNR(fat) (P = 0.045).

CONCLUSION

The proposed three-dimensional water-saturated b-SSFP sequence can generate high quality fat-only abdominal images with high CNR and SNR in shorter scan duration than the conventional T1W TSE approach. As images generated by this sequence suffer from no flow artifacts, and are less sensitive to bulk, respiratory, and bowel motion, three-dimensional water-saturated b-SSFP is a faster and more robust method for improving abdominal fat quantification using MRI.

Feasibility of Integrating High-Spatial-Resolution 3D Breath-hold Coronary MR Angiography with Myocardial Perfusion and Viability Examinations

The study was institutional review board approved and Health Insurance Portability and Accountability Act compliant. All subjects provided informed consent. Three-dimensional breath-hold coronary magnetic resonance (MR) angiography with use of steady-state free precession was performed in 12 patients up to 20 minutes after 0.2 mmol gadolinium-based contrast material per kilogram of body weight was administered. Within 24 heartbeats, a spatial resolution of up to 1.0 x 1.2 x 2.0 mm was achieved. Sixty-five (82%) of the 79 visualized coronary artery segments had a grade of 3 or 4 on a four-point scale of depiction in which grade 4 indicated excellent depiction. Twenty-seven percent (n = 21) of the 79 segments were assigned a grade of 4; 56% (n = 44), a grade of 3; 16% (n = 13), a grade of 2; and 1% (n = 1), a grade of 1. Coronary MR angiography performed as part of a first-pass myocardial perfusion and viability assessment MR imaging examination is feasible and does not involve additional imaging time.

Frequency-shift based detection of BMS contrast agents using SSFP: potential for MRA

A novel mechanism of MRI contrast enhancement, based on the detection by a balanced steady-state free precession (SSFP) sequence of the proton resonance frequency shift induced by bulk magnetic susceptibility (BMS) contrast agents, was investigated. The potential for this contrast mechanism to image blood vessels was explored. The relaxation time and the frequency shift effects of gadolinium- and dysprosium-DOTA on SSFP signal was first simulated and evaluated on a water phantom at 1.5 T. In vitro, a 5-mM concentration in contrast agent induced a 20-Hz frequency shift, leading to a signal increase of 92% for Dy-DOTA, and a 10-Hz frequency shift, leading to a signal increase of 58% for Gd-DOTA at the reference frequency, taking into account the nonlinear SSFP signal response on frequency offset. The concept was then evaluated in vivo on anesthetized rabbits. Low doses of dysprosium-DOTA were injected in their vascular system, and imaging was performed at the level of neck vessels. Following a bolus injection, mean signal changes of 31%, 20% and 14% were observed in the carotid arteries, the vertebral veins and the jugular veins, respectively. The bolus peak times in arteries and veins were consistent with the rabbit vascular circulation. This frequency-shift based contrast mechanism presents interesting potential for contrast-enhanced MR angiography (CE-MRA) compared to usual relaxation-based contrast, but further investigations on reproducibility will be necessary.

The utility of superparamagnetic contrast agents in MRI: theoretical consideration and applications in the cardiovascular system

This review will discuss the in vivo physical chemical relaxation properties of superparamagnetic iron oxide particles. Various parameters such as size, magnetization, compartmentalization and water exchange effects and how these alter the behavior of the iron oxide particles in an in vitro vs an in vivo situation with special reference to the cardiovascular system will be exemplified. Furthermore, applications using iron oxide particles for vascular, perfusion and viability imaging as well as assessment of the inflammatory status of a given tissue will be discussed.

View ordering for magnetization prepared steady state free precession acquisition: Application in contrast-enhanced MR angiography

Magnetization prepared segmented acquisition requires a view order that maximizes signal contrast during the acquisition of the central portion of k-space. Steady state free precession (SSFP) acquisition further requires a view order that minimizes changes in phase-encoding gradients from one repetition to the next in order to minimize eddy current artifacts. In this article, optimal view ordering schemes satisfying these two requirements are formulated and applied to inversion prepared 3D SSFP contrast-enhanced MR angiography (MRA). Experiments on phantoms and pigs demonstrated improved background suppression and reduced image artifacts.

Magnetic resonance portography using contrast-enhanced fat-saturated three-dimensional steady-state free precession imaging

PURPOSE

To assess the feasibility of contrast-enhanced fat-saturated three-dimensional steady-state free precession (FIESTA) imaging for contrast-enhanced magnetic resonance (MR) portography.

MATERIALS AND METHODS

Contrast-enhanced fat-saturated three-dimensional fast spoiled gradient-echo (SPGR) and FIESTA were performed as MR portography. In 10 cases, fat-saturated three-dimensional FIESTA was first performed and followed by fast SPGR, and the order of post-contrast imaging was reversed in the other 10 cases. Signal-to-noise ratios (SNRs) and contrast-to-noise ratios (CNRs) were estimated for portal and visceral veins on the source images. The visualization of portal vein was scored on three-dimensional MR portography. Portal venous system disorders were assessed using three-dimensional MR portography.

RESULTS

The SNRs, CNRs, and visual assessment of portal and visceral veins were significantly higher in contrast-enhanced fat-saturated three-dimensional FIESTA than contrast-enhanced fat-saturated three-dimensional fast SPGR (P < 0.05). The contrast-enhanced fat-saturated three-dimensional FIESTA provided high venous signals even at 8 minutes after gadolinium injection. The abnormalities of portal venous system were well visualized with MR portography using contrast-enhanced fat-saturated three-dimensional FIESTA.

CONCLUSION

Contrast-enhanced fat-saturated three-dimensional FIESTA was valuable for MR portography, with flexible time window and high vascular signals. This imaging may allow for other post-contrast imaging options before portography and release patients from consecutive breath-holds.

MRA of the thoracic vessels

Magnetic resonance imaging (MRI) is well suited for the noninvasive evaluation of the thoracic vasculature, and with improvements in scanner technology, the ability of MR to illustrate the thoracic vessels has significantly improved. Dedicated vascular software and pulse sequences have become commercially available, and fast imaging, in particular, has facilitated the time-efficient and comprehensive MR evaluation of most thoracic vascular lesions. Over the years, a host of black and bright blood MRI methods have evolved into practical tools for illustration of the thoracic vessels. As with other MR applications, successful vascular depiction relies significantly on the proper selection and prescription of imaging pulse sequences. In this article, these methods with their specific technical and practical pitfalls for thoracic magnetic resonance angiography (MRA) will be discussed. Current clinical indications for thoracic MRA will also be illustrated.

Effects of view ordering and dummy pulse rate on two-dimensional and three-dimensional steady-state free precession imaging

RATIONALE AND OBJECTIVE

To assess the effects of view ordering and dummy pulse rate on the image quality of steady-state free precession (SSFP) imaging.

MATERIAL AND METHODS

Two- and three-dimensional (2D and 3D) SSFP imaging with various view orderings (eg, centric, sequential, reverse centric) and dummy pulse rates were performed in the phantom including several concentrations of gadolinium and patients.

RESULTS

Centric view orderings increased the noise in 3D SSFP images in the phantom. In the patients studied, the centric view ordering increased the signals obtained from abdominal organs in 2D and 3D SSFP and the background signal in 3D images, without changing the vascular signals. An increased dummy pulse rate reduced the background signal in 2D SSFP, but not in 3D SSFP.

CONCLUSION

Sequential ordering is recommended for both 2D and 3D SSFP, and centric view ordering combined with an increase in dummy pulse rate can be used for 2D imaging.