Bolus-chasing contrast-enhanced 3D MRA of the lower extremity. Comparison with intraarterial DSA

Evaluation of the Abdominal Aorta and External Iliac Arteries Using Three-Dimensional Time-of-Flight, Three Dimensional Electrocardiograph-Gated Fast Spin-Echo, and Contrast-Enhanced Magnetic Resonance Angiography in Clinically Healthy Cats

Arterial thromboembolism is associated with high morbidity and mortality rates in cats. Definitive diagnosis requires advanced imaging modalities, such as computed tomography angiography (CTA) and contrast-enhanced (CE) magnetic resonance angiography (MRA). However, CTA involves exposure to a large amount of ionized radiation, and CE-MRA can cause systemic nephrogenic fibrosis. Non-contrast-enhanced (NE) MRA can help accurately diagnose vascular lesions without such limitations. In this study, we evaluated the ability of NE-MRA using three-dimensional electrocardiograph-gated fast spin-echo (3D ECG-FSE) and 3D time-of-flight (3D TOF) imaging to visualize the aorta and external iliac arteries in clinically healthy cats and compared the results with those obtained using CE-MRA. All 11 cats underwent 3D ECG-FSE, 3D TOF, and CE-MRA sequences. Relative signal intensity (rSI) for quantitative image analysis and image quality scores (IQS) for qualitative image analysis were assessed; the rSI values based on the 3D TOF evaluations were significantly lower than those obtained using 3D ECG-FSE (aorta 3D TOF: 0.57 ± 0.06, aorta 3D ECG-FSE: 0.83 ± 0.06, P < 0.001; external iliac arteries 3D TOF: 0.45 ± 0.06, external iliac arteries 3D ECG-FSE:0.80 ± 0.05, P < 0.001) and similar to those obtained using CE-MRA (aorta: 0.58 ± 0.05, external iliac arteries: 0.57 ± 0.03). Moreover, IQS obtained using 3D TOF were significantly higher than those obtained using 3D ECG-FSE (aorta 3D TOF: 3.95 ± 0.15, aorta 3D ECG-FSE: 2.32 ± 0.60, P < 0.001; external iliac arteries 3D ECG-FSE: 3.98 ± 0.08, external iliac arteries 3D ECG-FSE: 2.23 ± 0.56, P < 0.001) and similar to those obtained using CE-MRA (aorta: 3.61 ± 0.41, external iliac arteries: 3.57 ± 0.41). Thus, 3D TOF is more suitable and produces consistent image quality for visualizing the aorta and external iliac arteries in clinically healthy cats and this will be of great help in the diagnosis of FATE.

Comparative evaluation of the geometrical accuracy of intravascular magnetic resonance imaging: a phantom study

RATIONALE AND OBJECTIVES

To evaluate and compare the accuracy of cross-sectional imaging using an intravascular antenna in the context of vascular morphological measurements performed during a magnetic resonance imaging (MRI)-guided vascular intervention.

MATERIALS AND METHODS

Cross-sectional imaging of a multimodality vascular phantom was performed using intravascular and surface MRI, multidetector computed tomography, and intravascular ultrasound (IVUS). Using a balanced steady-state free-precession sequence, 18 sequences parameters sets were investigated (12 for intravascular MRI and 6 for surface MRI). Vessel diameters for all images and modalities were computed using an automated vessel segmentation algorithm.

RESULTS

Using IVUS as a gold standard, imaging using an intravascular antenna leads to an increase in geometrical accuracy in comparison to traditional surface MRI. This level of accuracy appears to follow a significant inverse proportionality relation in respect to vessel wall signal-to-noise ratio (SNR). Taking into account the rapid decrease in SNR as a function of the distance to the intravascular antenna, these results imply that, for a given level of geometrical accuracy, faster sequences can be used for the imaging of smaller vessels.

CONCLUSION

Imaging using an intravascular antenna appears as a valuable assistance to increase the accuracy of vascular morphological measurements. This increase in geometrical accuracy would be beneficial during the realization of an MRI-guided intervention, either to perform pretreatment measurements or to assess the outcome of the procedure. Acquisition parameters should be tailored to vessel size and procedural time constraints.

Intra-arterial MR-angiography on an open-bore MR-scanner compared to digital-subtraction angiography of the infra-popliteal runoff in patients with peripheral arterial occlusive disease

PURPOSE

To evaluate the diagnostic value of contrast-enhanced intra-arterial 3D-MR-angiography (IA-MRA) of the infra-popliteal arteries in an open-bore magnet. Number, severity of arterial lesions, and artefacts were compared to routinely performed intra-arterial digital-subtraction angiography (IA-DSA) in patients suffering from symptomatic peripheral arterial occlusive disease (PAOD).

MATERIAL AND METHODS

Fifteen patients admitted for PAOD underwent percutaneous transluminal angioplasty (PTA) by IA-DSA. After PTA, IA-MRA of the infra-popliteal station was performed on an open-bore 1.5T MR-scanner applying a low dose intra-arterial contrast-enhanced 3D-gradient-echo-MRA with gadopentate dimeglumine. The reading was performed by three blinded readers distinguishing moderate (< or =50%), significant stenoses (51-99%) and vessel occlusions. Imaging artefacts were recorded and binary classified as not disturbing or compromising the observation of the arterial tree.

RESULTS

Overall IA-DSA revealed 36 moderate stenoses (< or =50%), 38 significant stenoses (51-99%), and 10 vessel occlusions. For the detection of significant stenoses and occlusions, the overall sensitivity, specificity, positive predictive value, negative predictive value and accuracy of IA-MRA were 96%, 83%, 88%, 94% and 90%. The only observed artefact was venous overlay in four stations. The readout was not hampered in any case.

CONCLUSION

Intra-arterial contrast-enhanced 3D-gradient-echo-MRA on an open-bore MR-scanner offers an acceptable diagnostic accuracy in diagnosing peripheral arterial occlusive disease in the infra-popliteal region and correlates well with DSA.

[Screening in cardiovascular diseases]

Cardiovascular disease still ranks number one in the mortality statistics in the industrialized world. In Germany the five most common causes of death are all associated with arteriosclerotic changes of the arterial vasculature. As the treatment often extends over long periods and it can be impossible for patients to work, peripheral arterial occlusive disease (PAOD) constitutes a not inconsiderable economic factor. Thus, screening for arteriosclerotic disease seems to be reasonable, because the potential for influencing arteriosclerotic changes is known to be higher in an early stage of the disease even before symptoms become apparent. Not every case can be cured, but progression can frequently be slowed down. The need for invasive procedures, some of them associated with ionizing radiation, limited the use of imaging of the arterial vasculature for a long time. Noninvasive clinical examinations such as the "ankle brachial index" (ABI) can indicate the presence of PAOD, though exact localization of the pathologic changes is not possible except with imaging methods. In contrast to these, MRI is a noninvasive imaging modality that does not involve ionizing radiation but offers high spatial resolution arterial imaging.

Magnetic resonance evaluation of peripheral arterial disease

This article provides the reader with a clinically relevant practical approach to obtaining high-quality magnetic resonance angiograms of the lower extremities. The physics underlying this imaging are discussed as they relate to practical matters such as contrast timing schemes. Model protocols for various scanners are presented, and the rationale for the choices. In addition to the standard core protocol, alternative protocols are presented, so that the reader will have a sense of the full palette of options available, and be readily able to appreciate the relative advantages and disadvantages of each. Applications of these protocols in imaging various disorders then are discussed, with clinical exams presented. Pearls and pitfalls regarding imaging techniques and various tricks of the trade also are discussed.

High-resolution magnetic resonance angiography of the lower extremities with a dedicated 36-element matrix coil at 3 Tesla

INTRODUCTION

Recent developments in hard- and software help to significantly increase image quality of magnetic resonance angiography (MRA). Parallel acquisition techniques (PAT) help to increase spatial resolution and to decrease acquisition time but also suffer from a decrease in signal-to-noise ratio (SNR). The movement to higher field strength and the use of dedicated angiography coils can further increase spatial resolution while decreasing acquisition times at the same SNR as it is known from contemporary exams. The goal of our study was to compare the image quality of MRA datasets acquired with a standard matrix coil in comparison to MRA datasets acquired with a dedicated peripheral angio matrix coil and higher factors of parallel imaging.

MATERIALS AND METHODS

Before the first volunteer examination, unaccelerated phantom measurements were performed with the different coils. After institutional review board approval, 15 healthy volunteers underwent MRA of the lower extremity on a 32 channel 3.0 Tesla MR System. In 5 of them MRA of the calves was performed with a PAT acceleration factor of 2 and a standard body-matrix surface coil placed at the legs. Ten volunteers underwent MRA of the calves with a dedicated 36-element angiography matrix coil: 5 with a PAT acceleration of 3 and 5 with a PAT acceleration factor of 4, respectively. The acquired volume and acquisition time was approximately the same in all examinations, only the spatial resolution was increased with the acceleration factor. The acquisition time per voxel was calculated. Image quality was rated independently by 2 readers in terms of vessel conspicuity, venous overlay, and occurrence of artifacts. The inter-reader agreement was calculated by the kappa-statistics. SNR and contrast-to-noise ratios from the different examinations were evaluated.

RESULTS

All 15 volunteers completed the examination, no adverse events occurred. None of the examinations showed venous overlay; 70% of the examinations showed an excellent vessel conspicuity, whereas in 50% of the examinations artifacts occurred. All of these artifacts were judged as none disturbing. Inter-reader agreement was good with kappa values ranging between 0.65 and 0.74. SNR and contrast-to-noise ratios did not show significant differences.

CONCLUSION

Implementation of a dedicated coil for peripheral MRA at 3.0 Tesla helps to increase spatial resolution and to decrease acquisition time while the image quality could be kept equal. Venous overlay can be effectively avoided despite the use of high-resolution scans.

Value of single-dose contrast-enhanced magnetic resonance angiography versus intraarterial digital subtraction angiography in therapy indications in abdominal and iliac arteries

The objective of the study was to prove the value of single-dose contrast-enhanced magnetic resonance angiography [three-dimensional (3D) ceMRA] in abdominal and iliac arteries versus the reference standard intra-arterial digital subtraction angiography (i.a.DSA) when indicating a therapy. Patients suspected of having abdominal or iliac artery stenosis were included in this study. A positive vote of the local Ethics Committe was given. After written informed consent was obtained, 37 patients were enrolled, of which 34 were available for image evaluation. Both 3D ceMRA and i.a. DSA were performed for each patient. The dosage for 3D ceMRA was 0.1 mmol/kg body weight in a 1.5-T scanner with a phased-array coil. The parameters of the 3D-FLASH sequence were as follows: TR/TE 4.6/1.8 ms, effective thickness 3.5 mm, matrix 512 x 200, flip angle 30 degrees , field of view 420 mm, TA 23 s, coronal scan orientation. Totally, 476 vessel segments were evaluated for stenosis degree by two radiologists in a consensus fashion in a blinded read. For each patient, a therapy was proposed, if clinically indicated. Sensitivity, specificity, positive and negative predictive values, and accuracy for stenoses > or = 50% were 68%, 92%, 44%, 97%, and 90%, respectively. In 13/34 patients, a discrepancy was found concerning therapy decisions based on MRA findings versus therapy decisions based on the reference standard DSA. The results showed that the used MRA imaging technique of abdominal and iliac arteries is not competitive to i.a. DSA, with a high rate of misinterpretation of the MRAs resulting in incorrect therapies.

A multicenter, site-independent, blinded study to compare the diagnostic accuracy of contrast-enhanced magnetic resonance angiography using 1.0M gadobutrol (Gadovist™) to intraarterial digital subtraction angiography in body arteries

PURPOSE

Prospective evaluation of diagnostic accuracy of single field-of-view contrast-enhanced MR Angiography (ceMRA) with 1.0M gadobutrol compared to intraarterial DSA in body arteries.

MATERIALS AND METHODS

In an European multicenter study 179 patients underwent ceMRA and DSA. For each indication five prospectively defined vessel segments were evaluated by local investigators onsite and by three site-independent blinded readers (BR) independently.

RESULTS

The agreement between ceMRA and DSA diagnosis was statistically significant in the onsite (96.6%) and blinded reader (86.6-90.2%) evaluation. Sensitivity, specificity, accuracy, positive (PPV) and negative predictive values (NPV) for detection of relevant stenosis (>50%) were calculated for the right and left internal carotid arteries, and common and external iliac arteries: Sensitivity was 95-98% (onsite) and 76-96% (BR), specificity 94-96% (onsite) and 86-94% (BR), accuracy 96% (onsite) and 87-93% (BR), NPV 98-99% (onsite) and 84-98% (BR), and PPV 79-93% (onsite) and 44-91% (BR), respectively.

CONCLUSION

CeMRA of body arteries using 1.0M gadobutrol provides diagnostic information comparable to intraarterial DSA.

Magnetic Resonance Evaluation of Peripheral Arterial Disease

This article provides the reader with a clinically relevant practical approach to obtaining high-quality magnetic resonance angiograms of the lower extremities. The physics underlying this imaging are discussed as they relate to practical matters such as contrast timing schemes. Model protocols for various scanners are presented, and the rationale for the choices. In addition to the standard core protocol, alternative protocols are presented, so that the reader will have a sense of the full palette of options available, and be readily able to appreciate the relative advantages and disadvantages of each. Applications of these protocols in imaging various disorders then are discussed, with clinical exams presented. Pearls and pitfalls regarding imaging techniques and various tricks of the trade also are discussed.

Optimization of spatial resolution for peripheral magnetic resonance angiography

RATIONALE AND OBJECTIVES

To determine optimum spatial resolution when imaging peripheral arteries with magnetic resonance angiography (MRA).

MATERIALS AND METHODS

Eight vessel diameters ranging from 1.0 to 8.0 mm were simulated in a vascular phantom. A total of 40 three-dimensional flash MRA sequences were acquired with incremental variations of fields of view, matrix size, and slice thickness. The accurately known eight diameters were combined pairwise to generate 22 "exact" degrees of stenosis ranging from 42% to 87%. Then, the diameters were measured in the MRA images by three independent observers and with quantitative angiography (QA) software and used to compute the degrees of stenosis corresponding to the 22 "exact" ones. The accuracy and reproducibility of vessel diameter measurements and stenosis calculations were assessed for vessel size ranging from 6 to 8 mm (iliac artery), 4 to 5 mm (femoro-popliteal arteries), and 1 to 3 mm (infrapopliteal arteries). Maximum pixel dimension and slice thickness to obtain a mean error in stenosis evaluation of less than 10% were determined by linear regression analysis.

RESULTS

Mean errors on stenosis quantification were 8.8% +/- 6.3% for 6- to 8-mm vessels, 15.5% +/- 8.2% for 4- to 5-mm vessels, and 18.9% +/- 7.5% for 1- to 3-mm vessels. Mean errors on stenosis calculation were 12.3% +/- 8.2% for observers and 11.4% +/- 15.1% for QA software (P = .0342). To evaluate stenosis with a mean error of less than 10%, maximum pixel surface, the pixel size in the phase direction, and the slice thickness should be less than 1.56 mm2, 1.34 mm, 1.70 mm, respectively (voxel size 2.65 mm3) for 6- to 8-mm vessels; 1.31 mm2, 1.10 mm, 1.34 mm (voxel size 1.76 mm3), for 4- to 5-mm vessels; and 1.17 mm2, 0.90 mm, 0.9 mm (voxel size 1.05 mm3) for 1- to 3-mm vessels.

CONCLUSION

Higher spatial resolution than currently used should be selected for imaging peripheral vessels.

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.

Superficial femoral artery occlusive disease severity correlates with MR cine phase-contrast flow measurements

PURPOSE

To evaluate how cine phase-contrast (PC) flow data correlate with the severity of peripheral vascular disease (PVD).

MATERIALS AND METHODS

Flow waveforms were obtained in 48 patients proximal and distal to superficial femoral artery (SFA) disease using the 2D cine PC technique with velocity encoding (venc) = 100 cm/second. Flow data were correlated with SFA disease severity and compared with data from nine healthy volunteers.

RESULTS

Of 96 arterial segments in 48 patients, 26 were patent or only mildly stenotic, 35 had moderate-to-severe stenosis, and 35 were occluded. The flow patterns tended to become low-resistant below severe stenoses or occlusion. The mean peak flow velocity above/below SFA lesions was significantly higher in patients with severe disease (1.9 +/- 1.0, P = 0.01) or occlusion (2.0 +/- 1.0, P = 0.003) compared to normal volunteers (1.4 +/- 0.6). The delay in peak velocity below the lesions showed a significant positive correlation with lesion severity (r = 0.65, P < 0.001). The mean flow volume ratio above/below SFA lesions was greater in patients with occluded vessels compared to normal volunteers (3.9 and 2.3 respectively; P = 0.04).

CONCLUSION

Cine PC flow waveform changes across atherosclerotic lesions correlate with disease severity. This may help determine which lesions are hemodynamically significant.

Comprehensive MR angiography of the lower limbs: a hybrid dual-bolus approach including the pedal arteries

The purpose of this study was to include the pedal vasculature into the coverage of peripheral multistation magnetic resonance angiography (3DceMRA). A total of 216 patients suffering from peripheral vascular disease were examined with a modified hybrid dual-bolus technique. The cruropedal arteries were acquired first with two sagittal slabs and time-resolved 3D sequences. Then the aortofemoral vessels were visualized using the bolus-chase technique and a second contrast injection. Interventional procedures were performed in 104 patients, and in 69 of those, the cruropedal vessels were also examined with digital subtraction angiography (iaDSA). Using 3DceMRA, the cruropedal arteries were displayed with both excellent and good quality in 95% (205/216 cases), and without any venous overlay in 94% (203/216 cases). The aortofemoral vessels were not jeopardized by the first contrast injection. With iaDSA as the standard of reference, observed sensitivity of 3DceMRA was found in ranges from 80% (29%, 99%) to 100% (86%, 100%) for assessing significant stenoses, and observed specificity ranged between 93% [80%, 98%] and 100% (82%, 100%). In conclusion, hybrid dual-bolus 3DceMRA significantly reduces the limitations of standard single-bolus 3DceMRA in anatomic coverage and temporal resolution of the cruropedal arteries, thus providing high-quality images of the entire peripheral vasculature.

Diagnostic Accuracy of Time-Resolved 2D Projection MR Angiography for Symptomatic Infrapopliteal Arterial Occlusive Disease

OBJECTIVE

Our objective was to evaluate the diagnostic accuracy of time-resolved 2D projection MR angiography in detecting calf and pedal artery occlusive disease.

MATERIALS AND METHODS

Time-resolved MR angiography of calf and pedal arteries was performed on 59 symptomatic legs of 52 patients using the head coil and bolus injections of 6 mL of gadolinium contrast medium. Selective X-ray digital subtraction angiography was performed within 30 days after MR angiography. Calf and pedal arteries were divided into 10 segments. X-ray digital subtraction angiography and MR angiography images were retrospectively interpreted by three expert observers, who graded segments as having no significant stenosis, significant stenosis (> 50%), or occlusion. The accuracy of MR angiography interpretations was compared with the accuracy of consensus X-ray digital subtraction angiography interpretations as the standard of reference. Arterial segments with discrepant grading on X-ray digital subtraction angiography and MR angiography were reviewed again to determine the reasons for disagreement.

RESULTS

Arterial phase MR angiography images free of venous contamination were obtained in every case. The agreement between MR angiography and X-ray digital subtraction angiography in depicting infrapopliteal arterial disease was fair to good (kappa = 0.44-0.92). Overall sensitivity and specificity were 83% and 87%, respectively, for detecting significant stenosis of calf and pedal arteries and 86% and 93%, respectively, for detecting occlusions. Accuracy was higher in the larger vessels-for example, calf (84%) compared with foot (71%). In 21% (22/105) of the segments graded differently on MR angiography than on X-ray digital subtraction angiography, it was believed that MR angiography was more likely to be correct than X-ray digital subtraction angiography because of visualization of late-filling arteries on MR angiography that did not opacify on X-ray digital subtraction angiography.

CONCLUSION

Time-resolved 2D projection MR angiography accurately evaluates calf and pedal arteries without degradation from venous contamination.

Magnetic resonance angiography

MRA and MRI have become increasingly important diagnostic modalities in vascular surgery. The ability to obtain cross-sectional and angiographic images by these noninvasive and non-nephrotoxic modalities represents one of the most significant advances in vascular surgery over the past decade. We review the current status of MRI and MRA in vascular surgical practice.

Quand, comment et pourquoi réaliser une imagerie des artères des membres inférieurs ?

Noninvasive screening of lower limb arterial disease has long been performed using color Doppler ultrasonography, whereas surgical or endovascular treatment planning relied on conventional angiography. With continued improvements of noninvasive imaging modalities, it is now possible to image the entire lower limb vasculature without arterial catheterization. Multidetector row helical CT angiography has the advantage of visualizing the arterial lumen and arterial wall calcifications, and nephrotoxicity is reduced by decreasing amounts of contrast medium. Three-dimensional MR angiography is a safe procedure, with high contrast sensitivity, and has recently benefited from step table technology that allows a single injection of contrast medium. The literature shows that both helical CT and MR angiography have high levels of accuracy, but outcome studies of their respective role in the setting of acute and chronic lower limb ischemia are lacking. These noninvasive techniques should play a major role in two main situations: first, in critical ischemia where therapeutic planning should ideally be achieved non invasively; second, in the follow-up of treated patients presenting with recurrent ischemia.