Magnetic resonance angiography (MRA) of the calf station at 3.0 T: intraindividual comparison of non-enhanced ECG-gated flow-dependent MRA, continuous table movement MRA and time-resolved MRA

Non-Contrast Magnetic Resonance Angiography: Techniques, Principles, and Applications

Several non-contrast magnetic resonance angiography (MRA) techniques have been developed, providing an attractive alternative to contrast-enhanced MRA and a radiation-free alternative to computed tomography (CT) CT angiography. This review describes the physical principles, limitations, and clinical applications of bright-blood (BB) non-contrast MRA techniques. The principles of BB MRA techniques can be broadly divided into (a) flow-independent MRA, (b) blood-inflow-based MRA, (c) cardiac phase dependent, flow-based MRA, (d) velocity sensitive MRA, and (e) arterial spin-labeling MRA. The review also includes emerging multi-contrast MRA techniques that provide simultaneous BB and black-blood images for combined luminal and vessel wall evaluation.

Quiescent-Interval Slice-Selective MRA Accurately Estimates Intravascular Stent Dimensions Prior to Intervention in Patients With Peripheral Artery Disease

BACKGROUND

Quiescent-interval slice-selective (QISS) magnetic resonance angiography (MRA) is a non-contrast alternative for the pre-procedural assessment of patients with peripheral artery disease (PAD). However, the feasibility of pre-procedural stent size estimation using QISS MRA would merit investigation.

PURPOSE

To evaluate the feasibility of QISS MRA for pre-procedural stent size estimation in PAD patients compared to computed tomography angiography (CTA).

STUDY TYPE

Retrospective.

SUBJECTS

Thirty-three PAD patients (68 ± 9 years, 18 men, 15 women).

FIELD STRENGTH/SEQUENCE

Two-dimensional balanced steady-state free precession QISS MRA at 1.5 T and 3 T.

ASSESSMENT

All patients received QISS MRA and CTA of the lower extremity run-off followed by interventional digital subtraction angiography (DSA). Stenotic lesion length and diameter were quantified (AMF and AVS with 3 and 13 years of experience in cardiovascular imaging, respectively) to estimate the dimensions of the stent necessary to restore blood flow in the treated arteries. Measured dimensions were adjusted to the closest stent size available.

STATISTICAL TESTS

The Friedman test with subsequent pairwise Wilcoxon signed-rank test was used to compare the estimated stent dimensions between QISS MRA, CTA, and the physical stent size used for intervention. Intra-class correlation (ICC) analysis was performed to assess inter-reader agreement. Significant differences were considered at P < 0.05.

RESULTS

No significant difference was observed between estimated stent diameter by QISS MRA or CTA compared to physical stent diameter (8.9 ± 2.9 mm, 8.8 ± 3.0 mm, and 8.8 ± 3.8 mm, respectively; χ²  = 1.45, P = 0.483). There was a significant underestimation of stent length for both QISS MRA and CTA, compared to physical stent length (45.8 ± 27.8 mm, 46.4 ± 29.3 mm, and 50.4 ± 34.0 mm, respectively; χ²  = 11.96) which could be corrected when measurements were adjusted to the next available stent length (χ²  = 2.38, P = 0.303). Inter-reader assessment showed good to excellent agreement between the readers (all ICC ≥0.81).

DATA CONCLUSION

QISS MRA represents a reliable method for pre-procedural lesion assessment and stent diameter and length estimation in PAD patients.

LEVEL OF EVIDENCE

3 TECHNICAL EFFICACY: Stage 2.

Highly accelerated subtractive femoral non-contrast-enhanced MRA using compressed sensing with k-space subtraction, phase and intensity correction

PURPOSE

To develop an improved reconstruction method, k-space subtraction with phase and intensity correction (KSPIC), for highly accelerated, subtractive, non-contrast-enhanced MRA.

METHODS

The KSPIC method is based on k-space subtraction of complex raw data. It applies a phase-correction procedure to restore the polarity of negative signals caused by subtraction and an intensity-correction procedure to improve background suppression and thereby sparsity. Ten retrospectively undersampled data sets and 10 groups of prospectively undersampled data sets were acquired in 12 healthy volunteers. The performance of KSPIC was compared with another improved reconstruction based on combined magnitude subtraction, as well as with conventional k-space subtraction reconstruction and magnitude subtraction reconstruction, both using quantitative metrics and using subjective quality scoring.

RESULTS

In the quantitative evaluation, KSPIC had the best performance in terms of peak SNR, structural similarity index measure, contrast-to-noise ratio of artery-to-background and sharpness, especially at high acceleration factors. The KSPIC method also had the highest subjective scores for all acceleration factors in terms of vessel delineation, image noise and artifact, and background contamination. The acquisition can be accelerated by a factor of 20 without significant decreases of subjective scores. The optimal size of the phase-correction region was found to be 12-20 pixels in this study.

CONCLUSION

Compared with combined magnitude subtraction and conventional reconstructions, KSPIC has the best performance in all of the quantitative and qualitative measurements, permitting good image quality to be maintained up to higher accelerations. The KSPIC method has the potential to further reduce the acquisition time of subtractive MRA for clinical examinations.

"Push-button" noncontrast MR angiography using balanced T1 relaxation-enhanced steady-state (bT1RESS)

PURPOSE

We introduce a MR imaging technique, balanced T₁ relaxation-enhanced steady-state (bT1RESS), that provides the unique capability to efficiently impart a flexible amount of T₁ weighting to a balanced steady-state free precession acquisition using periodically applied contrast-modifying RF pulses. Leveraging this capability to suppress the signal intensity of background tissues, we implemented a 3D noncontrast MR angiography technique that continuously acquires thin overlapping 3D volumes and tested it for evaluation of the peripheral arteries.

METHODS

bT1RESS used a fast interrupted steady-state readout with a 45° cslab-selective ontrast-modifying RF pulse applied at 262 msec intervals. A series of 16.4-mm thick overlapping 3D volumes was acquired using a radial stack-of-stars k-space trajectory. The combination of slice oversampling, slab overlap, and averaging of edge slices was helpful to reduce venetian blind artifact. Spatial resolution was near isotropic with reconstructed slice thickness = 0.7 mm and in-plane resolution = 0.5 mm.

RESULTS

Pilot studies in the peripheral arteries demonstrated improved vessel sharpness compared with cardiac-gated quiescent interval slice-selective noncontrast MR angiography. bT1RESS noncontrast MR angiography reliably identified stenotic and occlusive arterial disease in a small cohort of patients with peripheral artery disease.

CONCLUSIONS

bT1RESS provides the basis for a simplified, completely "push button" approach for noncontrast MR angiography that obviates the need for contrast agents, electrocardiographic gating, scout imaging, breath holding, or tailoring of imaging parameters for the individual patient. Further work is needed for technical optimization and clinical validation.

[Optimization of Echo Train Length in Non-contrast Enhanced MR Angiography for Clinical Examination of the Calf Arteries]

PURPOSE

Non-contrast magnetic resonanse angiography (MRA) using the three-dimensional electrocardiogram-synchronized fast spin echo method uses systolic and diastolic arterial signal differences. The method relies on the flow void signal of the arterial flow because of dephasing during systole. However, depiction of slow flow such as that in a calf artery was degraded because of insufficient dephasing during systole. In this study, we optimized echo train length (ETL) using a flow phantom and normal volunteers for clinical examination of the calf arteries.

METHODS

Flow phantom and normal volunteer images were obtained with various ETLs (40, 50, 60, and 70). An averaged profile across the tube in the phantom was used for detailed investigation of flow dephasing. Visual evaluation was performed and signal intensity change along vessels was measured using normal volunteer images. Comparison with peak systolic velocity (PSV) measured using ultrasound equipment was also conducted.

RESULTS

Results of the flow phantom and normal volunteer study indicated that the overall depictability was improved with ETL 60 and 70, which was higher than the standard value. Additionally, the visualization of the peroneal artery with low PSV of ETL 70 had better depictability than ETL 60.

CONCLUSION

This study suggested that ETL 70 might be better for clinical examination of the calf arteries.

Modified calcium subtraction in dual-energy CT angiography of the lower extremity runoff: impact on diagnostic accuracy for stenosis detection

OBJECTIVES

To investigate the diagnostic accuracy of a modified three-material decomposition calcium subtraction (CS) algorithm for the detection of arterial stenosis in dual-energy CT angiography (DE-CTA) of the lower extremity runoff compared to standard image reconstruction, using digital subtraction angiography (DSA) as the reference standard.

METHODS

Eighty-eight patients (53 males; mean age, 65.9 ± 11 years) with suspected peripheral arterial disease (PAD) who had undergone a DE-CTA examination of the lower extremity runoff between May 2014 and May 2015 were included in this IRB-approved, HIPAA-compliant retrospective study. Standard linearly blended and CS images were reconstructed and vascular contrast-to-noise ratios (CNR) were calculated. Two independent observers assessed subjective image quality using a 5-point Likert scale. Diagnostic accuracy for ≥ 50% stenosis detection was analyzed in a subgroup of 45 patients who had undergone additional DSA. Diagnostic accuracy parameters were estimated with a random-effects logistic regression analysis and compared using generalized estimating equations.

RESULTS

CS datasets showed higher CNR (15.3 ± 7.3) compared to standard reconstructions (13.5 ± 6.5, p < 0.001). Both reconstructions showed comparable qualitative image quality scores (CS, 4.64; standard, 4.57; p = 0.220). Diagnostic accuracy (sensitivity, specificity, positive and negative predictive values) for CS reconstructions was 96.5% (97.5%, 95.6%, 90.9%, 98.1) and 93.1% (98.8%, 90.4%, 82.3%, 99.1%) for standard images.

CONCLUSIONS

A modified three-material decomposition CS algorithm provides increased vascular CNR, equivalent qualitative image quality, and greater diagnostic accuracy for the detection of significant arterial stenosis of the lower extremity runoff on DE-CTA compared with standard image reconstruction.

KEY POINTS

• Calcified plaques may lead to overestimation of stenosis severity and false positive results, requiring additional invasive digital subtraction angiography (DSA). • A modified three-material decomposition algorithm for calcium subtraction provides greater diagnostic accuracy for the detection of significant arterial stenosis of the lower extremity runoff compared with standard image reconstruction. • The application of this algorithm in patients with heavily calcified vessels may be helpful to potentially reduce inconclusive CT angiography examinations and the need for subsequent invasive DSA.

Noncontrast MR angiography: An update

Both computed tomography (CT) angiography (CTA) and contrast-enhanced MR angiography (CEMRA) have proven to be useful and accurate cross-sectional imaging modalities over a wide range of vascular territories and vascular disorders. A key advantage of MRA is that, unlike CTA, it can be performed without the administration of a contrast agent. In this review article we consider the motivations for using noncontrast MRA, potential contrast mechanisms, imaging techniques, advantages, and drawbacks with respect to CTA and CEMRA, and the level of evidence for using the various MRA techniques. In addition, we explore new developments that promise to expand the reliability and range of clinical applications for noncontrast MRA, along with functional MRA capabilities not available with CTA or CEMRA. Level of Evidence: 1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2019;49:355-373.

Repeatability of Non-Contrast-Enhanced Lower-Extremity Angiography Using the Flow-Spoiled Fresh Blood Imaging

Purpose

The aim of this study was to prospectively evaluate the repeatability of non–contrast-enhanced lower-extremity magnetic resonance angiography using the flow-spoiled fresh blood imaging (FS-FBI).

Methods

Forty-three healthy volunteers and 15 patients with lower-extremity arterial stenosis were recruited in this study and were examined by FS-FBI. Digital subtraction angiography was performed within a week after the FS-FBI in the patient group. Repeatability was assessed by the following parameters: grading of image quality, diameter and area of major arteries, and grading of stenosis of lower-extremity arteries. Two experienced radiologists blinded for patient data independently evaluated the FS-FBI and digital subtraction angiography images. Intraclass correlation coefficients (ICCs), sensitivity, and specificity were used for statistical analysis.

Results

The grading of image quality of most data was satisfactory. The ICCs for the first and second measures were 0.792 and 0.884 in the femoral segment and 0.803 and 0.796 in the tibiofibular segment for healthy volunteer group, 0.873 and 1.000 in the femoral segment, and 0.737 and 0.737 in the tibiofibular segment for the patient group. Intraobserver and interobserver agreements on diameter and area of arteries were excellent, with ICCs mostly greater than 0.75 in the volunteer group. For stenosis grading analysis, intraobserver ICCs range from 0.784 to 0.862 and from 0.778 to 0.854, respectively. Flow-spoiled fresh blood imaging yielded a mean sensitivity and specificity to detect arterial stenosis or occlusion of 90% and 80% for femoral segment and 86.7% and 93.3% for tibiofibular segment at least.

Conclusions

Lower-extremity angiography with FS-FBI is a reliable and reproducible screening tool for lower-extremity atherosclerotic disease, especially for patients with impaired renal function.

When should we use contrast material in cardiac MRI?

At present, most of the cardiac magnetic resonance imaging (MRI) examinations rely on contrast-enhanced protocols, but noncontrast alternatives are emerging. Late gadolinium enhancement (LGE) imaging for the detection of myocardial scar can be considered the main cause for the embedding of cardiac MRI into the clinical routine. The novel noncontrast technique of native T₁ mapping shows promise for tissue characterization in ischemic and nonischemic cardiomyopathy and may provide additional information over conventional LGE imaging. Technical issues, including measurements variability, still need to be resolved to facilitate a wide clinical application. Ischemia detection can be performed with contrast-based stress perfusion and contrast-free stress wall motion imaging. For coronary magnetic resonance angiography (MRA), protocols with and without contrast material have been developed. Research on coronary atherosclerotic plaque characterization has introduced new applications of contrast material. For MRA of the aorta, which traditionally relied on contrast administration, several noncontrast protocols have become available. This review provides an overview of when to use contrast material in cardiac and cardiac-related vascular MRI, summarizes the major imaging building blocks, and describes the diagnostic value of the available contrast-enhanced and noncontrast techniques. Contrast material in cardiac MRI should be used for LGE imaging for tissue characterization in ischemic or nonischemic cardiomyopathy and may be used for stress perfusion imaging for the detection of ischemia. In cardiac-related vascular MRI, use of contrast material should be avoided, unless high-quality angiography is required that cannot be obtained with noncontrast protocols.

LEVEL OF EVIDENCE

5 Technical Efficacy: Stage 3 J. Magn. Reson. Imaging 2017;46:1551-1572.

Image Quality and Stenosis Assessment of Non-Contrast-Enhanced 3-T Magnetic Resonance Angiography in Patients with Peripheral Artery Disease Compared with Contrast-Enhanced Magnetic Resonance Angiography and Digital Subtraction Angiography

PURPOSE

To evaluate the diagnostic performance of flow-sensitive dephasing (FSD)-prepared steady-state free precession (SSFP) magnetic resonance angiography (MRA) at 3 T for imaging infragenual arteries relative to contrast-enhanced MRA (CE-MRA) and digital subtraction angiography (DSA).

MATERIALS AND METHODS

A series of 16 consecutive patients with peripheral arterial disease (PAD) underwent a combined peripheral MRA protocol consisting of FSD-MRA for the calves and large field-of-view CE-MRA. DSA was performed on all patients within 1 week of the MR angiographies. Image quality and degree of stenosis was assessed by two readers with rich experience. Inter-observer agreement was determined using kappa statistics. Receiver operating characteristic (ROC) curve analysis determined the diagnostic value of FSD-MRA, CE-MRA, and CE-MRA combined with FSD-MRA (CE+FSD MRA) in predicting vascular stenosis.

RESULTS

At the calf station, no significantly difference of subjective image quality scores was found between FSD-MRA and CE-MRA. Inter-reader agreement was excellent for both FSD-MRA and CE-MRA. Both of FSD-MRA and CE-MRA carry a stenosis overestimation risk relative to DSA standard. With DSA as the reference standard, ROC curve analysis showed that the area under the curve was largest for CE+FSD MRA. The greatest sensitivity and specificity were obtained when a cut-off stenosis score of 2 was used.

CONCLUSION

In patients with severe PAD,3 T FSD-MRA provides good-quality diagnostic images without a contrast agent and is a good supplement for CE-MRA. CE+FSD MRA can improve the accuracy of vascular stenosis diagnosis.

Effects of gadolinium-based contrast agent concentrations (0.5 M or 1.0 M) on the diagnostic performance of magnetic resonance imaging examinations: systematic review of the literature

Background To date there is no agreement as to what is the optimal concentration for gadolinium-based contrast agents (GBCAs). Purpose To assess whether diagnostic performance differences exist between 0.5 M and 1.0 M GBCAs used for magnetic resonance imaging (MRI). Material and Methods A PubMed literature search identified 21 clinical studies published between 2005 and 2013 which evaluated the diagnostic efficacy of both types of GBCAs. Study design, type of procedure, GBCA administration mode, imaging performances, impact on patient management, study limitations, and biases were analyzed. No statistical test was performed on pooled data. Results Sixteen comparative and five non-comparative studies were analyzed, involving 2183 patients who underwent MRI procedures for various indications. In 67% of the studies, 0.5 M and 1.0 M GBCAs were injected at equimolar gadolinium amounts per kg body weight. Only 33% applied the same molar flow rate for delivery of the GBCAs. No significant differences between GBCAs were reported for 23 out of 27 qualitative endpoints (mainly image quality, lesion, and vessel visualization) and 29 out of 40 quantitative endpoints. Three out of four studies with non-equimolar delivery rates showed better contrast-to-noise and signal-to-noise ratios for 1.0 M gadobutrol, without showing an impact on diagnostic performance. Methodological biases were identified in several studies impairing the interpretation of comparisons. Conclusion Imaging differences between 0.5 M and 1.0 M GBCAs were essentially observed under non-equimolar delivery rates. However, they did not result into greater diagnostic efficacy when performed under equimolar conditions.

Unenhanced 3D turbo spin echo MR angiography of lower extremity arteries: comparison with 128-MDCT angiography

PURPOSE

This study aimed to investigate the feasibility and diagnostic power of unenhanced 3D turbo spin echo MR angiography sequence (a technique based on subtraction of corresponding images acquired in diastole and systole, Syngo Native Space-Siemens healthcare) to identify peripheral artery disease (PAD).

MATERIALS AND METHODS

Thirty patients (619 arterial segments in total) suspected with PAD and who were assessed with lower extremity MDCT angiography were examined starting from the level of aortic bifurcation for both lower extremities by 3D native space MR angiography. Two readers assessed the image quality of native space MR angiography and number of lesions, their degrees of stenosis and localizations. The differences and compliance between the readers in the parameters assessed were investigated.

RESULTS

Out of 619 segments; Reader 1 considered 187 segments (30.2 %) and Reader 2 considered 177 segments (28.6 %) to have poor and inadequate MR image quality. When compared to CTA, sensitivity, specificity and diagnostic accuracy of native space MR angiography were calculated as 81.0, 83.1, 82.6 %, respectively, by Reader 1, while the same parameters were calculated as 69.9, 92.6, 84.9 %, respectively, by Reader 2. When the two readers were examined together, sensitivity, specificity, diagnostic accuracy were found to be 76.3, 88.1, 83.7 %, respectively, and the positive predictive value and negative predictive value was 70.3 and 89.9 %, respectively. A significantly better image quality was acquired with the age group below 50 years (p = 0.002).

CONCLUSION

Native space MR angiography technique can be used as the first-step imaging technique before contrast-enhanced examinations in young and middle age patients with suspected PAD and for patients with the risk of nephrogenic systemic fibrosis in chronic renal failure.

High-permittivity thin dielectric padding improves fresh blood imaging of femoral arteries at 3 T

OBJECTIVES

Fresh blood imaging (FBI) is a useful noncontrast magnetic resonance angiographic (MRA) method for the assessment of peripheral arterial disease, particularly for imaging patients with poor renal function. Compared with 1.5 T, 3 T enables higher signal-to-noise ratio and/or spatiotemporal resolution in FBI. Indeed, previous studies have reported successful FBI of the calf station at 3 T. However, FBI of the thigh station at 3 T has been reported to suffer from signal void in the common femoral artery of 1 thigh only because of the radial symmetry in transmit radiofrequency field (B1+) variation. We sought to increase the signal of femoral artery in FBI at 3 T using high-permittivity dielectric padding.

MATERIALS AND METHODS

We performed FBI and B1+ mapping of the thigh station at 3 T in 13 human subjects to compare the following 3 dielectric padding settings: no padding, commercially available thick (approximately 5 cm) dielectric padding, and high-permittivity thin (approximately 2 cm) dielectric padding. We characterized the radial symmetry in B1+ variation as well as its impact on the FBI signal at baseline and how dielectric padding improves B1+ and FBI. We evaluated the quality of 3 FBI MRA acquisitions using quantitative (ie, contrast-to-noise ratio of femoral arteries) and qualitative (ie, conspicuity of femoral arteries) analyses.

RESULTS

With the subjects positioned on the magnetic resonance table in feet-first, supine orientation, the radial symmetry in B1+ variation attenuates the signal in the right common femoral artery. The signal void can be improved partially with commercial padding and improved further with high-permittivity padding. Averaging the results over the 13 subjects, the mean B1+, contrast-to-noise ratio, and conspicuity scores for the right common femoral artery were significantly higher with high-permittivity padding than with commercial padding and baseline (P < 0.001).

CONCLUSIONS

Our study shows that high-permittivity dielectric padding can be used to increase the signal of femoral artery in FBI at 3 T.

Noncontrast MR angiography (MRA) of infragenual arteries using flow-sensitive dephasing (FSD)-prepared steady-state free precession (SSFP) at 3.0 Tesla: Comparison with contrast-enhanced MRA

BACKGROUND

To evaluate the feasibility and diagnostic performance of flow-sensitive dephasing (FSD)-prepared steady-state free precession (SSFP) MR angiography (MRA) for imaging infragenual arteries at 3.0T, with contrast enhanced MR angiography (CE MRA) as reference.

METHODS

Twenty consecutive patients with suspicion of lower extremity arterial disease undergoing routine CE MRA were recruited. FSD MRA was performed at calf before CE MRA. Image quality and stenosis degree of infragenual arteries from both techniques were independently evaluated and compared. Six patients in this study underwent DSA examination.

RESULTS

Three undiagnostic segments were excluded with severe venous contamination in CE MRA. A total of 197 calf arterial segments images were analyzed. No significant difference existed in the relative signal intensity (rSI) of arterial segments between FSD MRA and CE MRA techniques (0.92 ± 0.09 versus 0.93 ± 0.05; P = 0.207). However, the subjective image quality score was slightly higher in FSD MRA (3.66 ± 0.81 versus 3.49 ± 0.87; P = 0.050). With CE MRA images as reference standard, slight overestimation existed in FSD MRA (2.19 ± 1.24 versus 2.09 ± 1.18; P = 0.019), with total agreement of 84.3% on the basis of all arterial segments. The sensitivity, specificity, negative predictive value, and positive predictive value of FSD MRA was 96.4%, 93.0%, 98.5%, and 84.1%. No significant difference in the stenosis degree score was detected between MRA (FSD MRA and CE MRA) and DSA (P > 0.05).

CONCLUSION

FSD MRA performed on at 3.0T without the use of contrast medium provides diagnostic images allowing for arterial stenosis assessment of calf arteries that was highly comparable with CE MRA. Moreover, venous contamination was less problematic with FSD MRA.

Noncontrast magnetic resonance angiography: concepts and clinical applications

Many noncontrast magnetic resonance angiography techniques have recently been developed in response to concerns about gadolinium in patients with renal impairment. This article describes the theory behind established and recently described techniques and how and where they can be performed in clinical practice.

Non-Contrast Enhanced MR Angiography (NCE-MRA) of the Calf: A Direct Comparison between Flow-Sensitive Dephasing (FSD) Prepared Steady-State Free Precession (SSFP) and Quiescent-Interval Single-Shot (QISS) in Patients with Diabetes

Objectives

To compare the image quality and diagnostic performance of two non-contrast enhanced MR angiography (NCE-MRA) techniques using flow-sensitive dephasing (FSD) prepared steady-state free precession (SSFP) and quiescent-interval single-shot (QISS) for the calf arteries in patients with diabetes.

Materials and Methods

Twenty six patients underwent the two NCE-MRA techniques followed by contrast-enhanced MRA (CE-MRA) of lower extremity on a 1.5T MR system. Image quality scores, arterial stenosis scores, signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), vessel sharpness, and diagnostic accuracy for detecting more than 50% arterial stenosis were evaluated and statistically compared using CE-MRA as the reference standard.

Results

All examinations were performed successfully. Of the total 153 calf arterial segments obtained in the 26 patients, FSD and QISS showed no significant difference in the number of diagnostic arterial segments (151 [98%] vs. 147 [96%], respectively, P>0.05). The image quality of FSD was higher than that of QISS in the peroneal artery and posterior tibial artery (P<0.05), but no significant difference in the anterior tibial artery (P>0.05). SNR and CNR of FSD were higher than those of QISS (P<0.01), while FSD showed comparable vessel sharpness compared with QISS (P>0.05). The time efficiency of SNR and CNR between FSD and QISS showed no significant difference when taking into account the times for FSD-related scout scans. There was no difference in sensitivity (95% vs. 93%, P>0.05) and negative predictive value (98% vs. 97%, P>0.05) between FSD and QISS for detecting stenosis greater than 50%. However, FSD showed higher specificities (99% vs. 92%, P<0.05) and diagnostic accuracy (98% vs. 92%, P<0.05) compared to QISS.

Conclusion

Both FSD and QISS had similar high sensitivity and negative predictive value for detecting calf arteries with over 50% stenosis, but FSD showed slightly higher diagnostic specificity and better depiction of arterial lesions due to its isotropic submillimeter spatial resolution. QISS, being an easier to use and less time-consuming technique, could be a method of choice for rapid screening of arterial disease of the lower extremity.

State-of-the-art MRI techniques in neuroradiology: principles, pitfalls, and clinical applications

This article reviews the most relevant state-of-the-art magnetic resonance (MR) techniques, which are clinically available to investigate brain diseases. MR acquisition techniques addressed include notably diffusion imaging (diffusion-weighted imaging (DWI), diffusion tensor imaging (DTI), and diffusion kurtosis imaging (DKI)) as well as perfusion imaging (dynamic susceptibility contrast (DSC), arterial spin labeling (ASL), and dynamic contrast enhanced (DCE)). The underlying models used to process these images are described, as well as the theoretic underpinnings of quantitative diffusion and perfusion MR imaging-based methods. The technical requirements and how they may help to understand, classify, or follow-up neurological pathologies are briefly summarized. Techniques, principles, advantages but also intrinsic limitations, typical artifacts, and alternative solutions developed to overcome them are discussed. In this article, we also review routinely available three-dimensional (3D) techniques in neuro MRI, including state-of-the-art and emerging angiography sequences, and briefly introduce more recently proposed 3D quantitative neuro-anatomy sequences, and new technology, such as multi-slice and multi-transmit imaging.

Initial evaluation of non-contrast-enhanced magnetic resonance angiography in patients with peripheral arterial occlusive disease at 7 T

OBJECTIVES

The aim of this study was to achieve initial experience with non-contrast-enhanced (ne) magnetic resonance angiography (MRA) of the lower leg arteries in patients with peripheral arterial occlusive disease (PAOD) at 7 T.

MATERIALS AND METHODS

Seven patients with PAOD were examined on a 7-T whole-body magnetic resonance system. A custom-built 16-channel transmit/receive coil and a manually positionable AngioSURF table were used for multistation imaging. For ne-MRA, an axial T1-weighted Turbo-fast low angle shot sequence (repetition time, 700 milliseconds; echo time, 3.84 milliseconds; bandwidth, 930 Hz/pixel; voxel volume, 1 × 1 × 2 mm; matrix, 384 × 288) with phonocardiogram gating was acquired at 7 T. Acquisition time of an entire angiogram covering the vasculature from pelvis to feet amounted to approximately 30 minutes, depending on the patient's heart frequency. All patients underwent a contrast-enhanced MRA (ce-MRA) at 1.5 T as standard of reference. The presence of stenosis and occlusions was evaluated segment based and compared for both MRA techniques. The degree of stenosis was defined as low grade (<50%), high grade (50%-99%), and occlusion (100%). High-grade stenosis and occlusion were considered to be hemodynamically significant stenosis.

RESULTS

The 7-T ne-MRA enabled a homogenous, hyperintense artery signal and nearly total venous suppression with accurate delineation of arterial anatomy both proximal and distal to stenotic disease. A total of 154 artery segments were depicted with ce-MRA at 1.5 T. At 7 T, only 124 segments (80.5%) were displayed and involved for analysis, as the iliacal region was displayed incompletely in 4 patients because of the fact that the fixed coil diameter was too small to contain the lower abdomen and pelvis of these patients. In comparison with ce-MRA at 1.5 T as the reference standard, there was total agreement regarding the characterization of an artery segment as being normal or having any kind of stenosis. Of the 124 included segments, 28 segments (23%) had hemodynamically significant stenosis evaluated with 7-T ne-MRA and 26 segments (21%) assessed with 1.5-T ce-MRA. The sensitivity and specificity values of 7-T ne-MRA for detecting segments with hemodynamically significant stenosis were 93% and 98%, respectively.

CONCLUSIONS

Non-contrast-enhanced MRA by means of T1-weighted Turbo-fast low angle shot imaging at 7 T in patients with PAOD is feasible and allowed for good visualization of stenosis and occlusions in all analyzed artery segments in this small patient group. However, this study also shows the challenges of ultrahigh-field body imaging, and more experience is required to determine the impact of 7-T ne-MRA in clinical practice.

Non-contrast-enhanced MR angiography at 3 Tesla in patients with advanced peripheral arterial occlusive disease

Purpose

The aim of this study was to assess the diagnostic performance of ECG-gated non-contrast-enhanced quiescent interval single-shot (QISS) magnetic resonance angiography at a magnetic field strength of 3 Tesla in patients with advanced peripheral arterial occlusive disease (PAOD).

Method and Materials

A total of 21 consecutive patients with advanced PAOD (Fontaine stage IIb and higher) referred for peripheral magnetic resonance angiography (MRA) were included. Imaging was performed on a 3 T whole body MR. Image quality and stenosis diameter were evaluated in comparison to contrast-enhanced continuous table and TWIST MRA (CE-MRA) as standard of reference. QISS images were acquired with a thickness of 1.5 mm each (high-resolution QISS, HR-QISS). Two blinded readers rated the image quality and the degree of stenosis for both HR-QISS and CE-MRA in 26 predefined arterial vessel segments on 5-point Likert scales.

Results

With CE-MRA as the reference standard, HR-QISS showed high sensitivity (94.1%), specificity (97.8%), positive (95.1%), and negative predictive value (97.2%) for the detection of significant (≥50%) stenosis. Interreader agreement for stenosis assessment of both HR-QISS and CE-MRA was excellent (κ-values of 0.951 and 0.962, respectively). As compared to CR-MRA, image quality of HR-QISS was significantly lower for the distal aorta, the femoral and iliac arteries (each with p<0.01), while no significant difference was found in the popliteal (p = 0.09) and lower leg arteries (p = 0.78).

Conclusion

Non-enhanced ECG-gated HR-QISS performs very well in subjects with severe PAOD and is a good alternative for patients with a high risk of nephrogenic systemic fibrosis.

Nonenhanced magnetic resonance angiography of the lower extremity vessels at 7 tesla: initial experience

OBJECTIVES

The aim of this study was to investigate the feasibility of nonenhanced magnetic resonance angiography (MRA) of the lower extremities at 7 tesla (T).

MATERIALS AND METHODS

Eight healthy volunteers were examined on a 7-T whole-body magnetic resonance system. For image acquisition, a custom-built 16-channel transmit/receive coil and a manually positionable AngioSURF table for multistation imaging were used. A nonenhanced T1-weighted 2-dimensional fast low-angle shot (2D FLASH) sequence was acquired with and without venous saturation radiofrequency pulses in axial orientation, covering the vasculature from the pelvis to the feet. Acquisition time of 1 table position amounted to 2 minutes and 19 seconds (with venous saturation pulse) and 1 minute and 9 seconds (without venous saturation pulse), covering a field of view of approximately 10 cm in the z-axis. Time-interleaved acquisition of modes was integrated to obtain homogeneous image quality of the vasculature. A qualitative image analysis was performed in the iliac, femoral, popliteal, and tibiofibular vessel segments regarding vessel delineation using a 5-point scale (5 to 1, excellent vessel delineation to nondiagnostic). For the quantitative image evaluation, the signal was measured in the specified segments and in the surrounding musculature of both legs to obtain contrast ratios (CR).

RESULTS

T1-weighted 2D FLASH imaging enabled homogeneous, hyperintense delineation of the arteries with saturation of surrounding tissue in almost all analyzed vessel segments. The qualitative image evaluation demonstrated a moderate to good delineation and assessment of the vessel lumen (mean score: iliac, 3.17; femoral, 3.71; popliteal, 4.00; and tibiofibular, 3.31 for 2D FLASH). The quantitative analysis showed similar CRs in all vessel segments, with the best contrast to surrounding tissue achieved in the femoral segments (CRiliac, 0.59; CRfemoral, 0.69; CRpopliteal, 0.74; and CRtibiofibular, 0.57), although a medial signal drop-off in the thigh region could be found in some volunteers. Transformation of the axial images into coronal maximum intensity projection images revealed an artifact characterized by recurrent short declines of vessel signal, most probably because of an interference between the alternating pressure and flow effects during systole and diastole and the image acquisition frequency. Nevertheless, the use of time-interleaved acquisition of modes enabled a homogenous image quality with successful reduction of B1 field inhomogeneities.

CONCLUSIONS

The results of our study demonstrate the feasibility of non-contrast-agent-enhanced MRA of the lower extremity vessels at 7 T. Nonenhanced MRA of the lower extremities at this very high magnetic field can be considered to be in an early but promising stage. Further sequence optimization and the examination of a larger number of participants as well as comparison with contrast-enhanced MRA and nonenhanced techniques at lower field strengths should be pursued in future trials.

Detection of infragenual arterial disease using non-contrast-enhanced MR angiography in patients with diabetes

PURPOSE

To evaluate the diagnostic performance of a newly developed non-contrast-enhanced MR angiography (NCE-MRA) technique using flow-sensitive dephasing (FSD) prepared steady-state free precession (SSFP) for detecting calf arterial disease in patients with diabetes.

MATERIALS AND METHODS

Forty-five patients with diabetes who underwent routine contrast-enhanced MR angiography (CE-MRA) of lower extremities were recruited for NCE-MRA at the calf on a 1.5 Tesla MR system. Image quality evaluated on a 4-point scale and diagnostic performance for detecting more than 50% arterial stenosis were statistically analyzed, using CE-MRA as the standard of reference.

RESULTS

A total of 264 calf arterial segments were obtained in the 45 patients with 88 legs. The percentage of diagnostic arterial segments was all 98% for NCE- and CE-MRA. The image quality, SNR, CNR was 3.3, 177, 138, and 3.5, 103, 99, for NCE-MRA and CE-MRA, respectively. The average sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of NCE-MRA were 97%, 96%, 90%, 99%, and 96%, respectively on a per-segment basis and 90%, 84%, 82%, 91%, and 87%, respectively, on a per-patients basis.

CONCLUSION

The NCE-MRA technique demonstrates adequate image quality in the delineation of calf arteries and consistent diagnostic performance for detecting significant stenosis with CE-MRA in patients with diabetes.

Contrast-enhanced magnetic resonance angiography of the peripheral arteries: technique, tips, pitfalls and problems

Contrast-enhanced magnetic resonance angiography is a reliable way to assess peripheral vascular disease. This article reviews the basic physics behind this technique and discusses our institution's experience with regard to the clinical role, recent advances in image acquisition and use of contrast agents. Problems that can affect image quality and interpretation are also highlighted.

MRI in Lower Extremity Peripheral Arterial Disease: Recent Advancements

Evaluation of peripheral arterial disease by cardiovascular magnetic resonance imaging continues to develop. Of the clinical diagnostics tests currently available, magnetic resonance angiography is well established as one of the preferred techniques for determining areas of arterial occlusive disease affecting the lower extremities. Despite this, there have been new developments in non-gadolinium based contrast-enhanced studies as well as testing done at higher field strength scanners. In the research arena, magnetic resonance spectroscopy, calf muscle perfusion imaging and atherosclerotic plaque evaluation all have made significant advancements over the last year. These techniques are gaining traction as surrogate endpoints in clinical trials of novel therapeutics aimed at alleviating symptoms in patients with peripheral arterial disease.

Sagittal fresh blood imaging with interleaved acquisition of systolic and diastolic data for improved robustness to motion

PURPOSE

To improve robustness to patient motion of "fresh blood imaging" (FBI) for lower extremity noncontrast MR angiography.

METHODS

In FBI, two sets of three-dimensional fast spin echo images are acquired at different cardiac phases and subtracted to generate bright-blood angiograms. Routinely performed with a single coronal slab and sequential acquisition of systolic and diastolic data, FBI is prone to subtraction errors due to patient motion. In this preliminary feasibility study, FBI was implemented with two sagittal imaging slabs, and the systolic and diastolic acquisitions were interleaved to minimize sensitivity to motion. The proposed technique was evaluated in volunteers and patients.

RESULTS

In 10 volunteers, imaged while performing controlled movements, interleaved FBI demonstrated better tolerance to subject motion than sequential FBI. In one patient with peripheral arterial disease, interleaved FBI offered better depiction of collateral flow by reducing sensitivity to inadvertent motion.

CONCLUSIONS

FBI with interleaved acquisition of diastolic and systolic data in two sagittal imaging slabs offers improved tolerance to patient motion.

Peripheral vessel scout imaging based on continuously moving table acquisition of projection data

A fast and spatially seamless peripheral vessel scout is desirable for subsequent planning of magnetic resonance (MR) angiography. We implemented a continuously moving table sequence providing projection data with time-of-flight contrast of the entire lower extremities in less than 2 minutes. Variation of arterial signal during the cardiac cycle and autocorrelation were exploited to enhance vessel-to-background contrast. Subjective image analysis revealed excellent vessel depiction, indicating that the proposed scout allows for seamless expedited visualization of major arterial structures.

Non-contrast enhanced MR angiography: established techniques

Until recently, time-of-flight (TOF) and phase contrast (PC) were the only non-contrast MR angiography (NC-MRA) techniques practically used in clinical. In the decade, NC-MRA have been gained a revival of an interest among the MR researchers and scientists, in part because of safety concerns related to the possible link between gadolinium-based contrast agents and nephrogenic systemic fibrosis (NSF). This article introduces other established NC-MRA techniques, such as ECG-gated partial Fourier fast spin echo (FSE) and balanced steady-state free precession (bSSFP), both with and without arterial spin labeling. Then, the article focuses on two main applications: peripheral run-off and renal MRA. Recently, both applications have achieved remarkable advancements and have become a viable clinical option as an alternative to contrast-enhanced (CE)-MRA. In addition, developments on the horizon including whole body MRA applications and further advancement at 3 Tesla are discussed.

Chronic critical limb ischemia

BACKGROUND

Some 40 000 lower limb amputations are performed in Germany each year, 70% of them in diabetics. About 80% of all major amputations may be preventable with the use of new interventional and vascular surgical procedures, particularly on the arteries of the leg and foot. We present the current state of the art in revascularization techniques and evaluate their usefulness for preservation of the lower limb.

METHODS

This overview is based on the guidelines for the diagnosis and treatment of peripheral artery disease (PAD) and diabetic foot syndrome (DFS) that have been issued by the American Heart Association (AHA), the American College of Cardiology (ACC), the German Society of Angiology (DGA), the Trans-Atlantic Intersociety Consensus (TASC II), the German Society of Vascular Surgery (DGG), and the German Diabetes Society (DDG). A selective search in PubMed for relevant articles that appeared from 2000 to 2011 was conducted with the search terms "pedal bypass," "vascular intervention crural pedal," and "crural-pedal revascularization."

RESULTS

Most of the data on crural and pedal revascularization are derived from small-scale studies. The few comparative studies of interventional treatments and bypass surgery have not revealed any significant differences in outcome, but all studies of revascularization have shown good success rates for lower limb preservation.

CONCLUSION

Though the data are still sparse, the high reported rates of limb preservation imply that peripheral revascularization techniques can play a major role in the treatment of chronic critical limb ischemia (CLI). Therefore, these techniques are recommended without exception by the current guidelines.

Arterial flow characteristics in the presence of vascular disease and implications for fast spin echo-based noncontrast MR angiography

PURPOSE

To investigate arterial flow characteristics in the setting of vascular disease, and examine their effect on the performance of fast spin-echo (FSE)-based noncontrast MR angiography (NC-MRA).

MATERIALS AND METHODS

Seventeen patients were recruited from among those scheduled for routine contrast-enhanced MR angiography (CE-MRA) of the lower extremities at 1.5 Tesla. The research portion of the exam was performed before the clinically-indicated protocol and included phase-contrast imaging at multiple levels in the legs and FSE-based NC-MRA in the calf and thigh, using a three-dimensional ECG-gated technique that exploits differences in arterial flow velocity between diastole and systole.

RESULTS

Vascular occlusions were associated with reduced systolic velocity, a delayed systolic peak, and, in two middle-aged patients, an increase in diastolic velocity. Elevated systolic and diastolic velocities were observed in a subject with a nonhealing ulcer. NC-MRA allowed visualization of arteries with systolic velocities as low as 3 cm/s, and exhibited comparable depiction to CE-MRA for diastolic velocities as high as 6 cm/s. At the highest diastolic velocities observed (15 cm/s) arterial depiction was severely degraded.

CONCLUSION

FSE-based NC-MRA as presently implemented performs successfully over a wide range of flow patterns, but does not accommodate extremely low systolic velocities or very high diastolic velocities.