On Improving Temporal and Spatial Resolution of 3D Contrast-enhanced Body MR Angiography with Parallel Imaging

MR imaging of hepatocellular carcinoma: prospective intraindividual head-to-head comparison of the contrast agents gadoxetic acid and gadoteric acid

The routine use of dynamic-contrast-enhanced MRI (DCE-MRI) of the liver using hepatocyte-specific contrast agent (HSCA) as the standard of care for the study of focal liver lesions is not widely accepted and opponents invoke the risk of a loss in near 100% specificity of extracellular contrast agents (ECA) and the need for prospective head-to-head comparative studies evaluating the diagnostic performance of both contrast agents. The Purpose of this prospective intraindividual study was to conduct a quantitative and qualitative head-to-head comparison of DCE-MRI using HSCA and ECA in patients with liver cirrhosis and HCC. Twenty-three patients with liver cirrhosis and proven HCC underwent two 3 T-MR examinations, one with ECA (gadoteric acid) and the other with HSCA (gadoxetic acid). Signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), wash-in, wash-out, image quality, artifacts, lesion conspicuity, and major imaging features of LI-RADS v2018 were evaluated. Wash-in and wash-out were significantly stronger with ECA compared to HSCA (P < 0.001 and 0.006, respectively). During the late arterial phase (LAP), CNR was significantly lower with ECA (P = 0.005), while SNR did not differ significantly (P = 0.39). In qualitative analysis, ECA produced a better overall image quality during the portal venous phase (PVP) and delayed phase (DP) compared to HSCA (P = 0.041 and 0.008), showed less artifacts in the LAP and PVP (P = 0.003 and 0.034) and a higher lesion conspicuity in the LAP and PVP (P = 0.004 and 0.037). There was no significant difference in overall image quality during the LAP (P = 1), in artifacts and lesion conspicuity during the DP (P = 0.078 and 0.073) or in the frequency of the three major LI-RADS v2018 imaging features. In conclusion, ECA provides superior contrast of HCC-especially hypervascular HCC lesions-in DCE-MR in terms of better perceptibility of early enhancement and a stronger washout.

Determining the optimal magnetic resonance imaging sequences for the efficient diagnosis of temporomandibular joint disorders

Background

To compare and analyze nine MRI sequences of the TMJ and determine the optimum sequence for the rapid diagnosis of TMDs so as to develop new clinical guidelines.

Methods

Twenty young volunteers (a total of 40 joints) aged 22-26 years were recruited. Three basic sequences, T₁-weighted imaging (T₁WI), T₂-weighted imaging (T₂WI), and proton density-weighted imaging (PDWI), together with three positions, oblique sagittal (OSag) with closed mouth, oblique coronal (OCor) with closed mouth, and OSag with opened mouth, were selected in combination for testing. In the OCor position, four regions of interest (ROIs), the condyle (C), the disc (D), the disc outside (DO), and fat (F), were analyzed. For the OSag with closed mouth position and the OSag with opened mouth position sequences, the four ROIs were the condyle (C), the disc (D), the disc ahead (DA), and the disc rear (DR). The signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and signal intensity ratio (SIR) were calculated and analyzed using independent sample t-tests and one-way analysis of variance. Two senior radiologists scored the images of the nine MRI sequences subjectively and selected three optimal sequences. Using the three selected sequences, 1479 patients with anterior disc displacement with reduction (ADDwR) or anterior disk displacement without reduction (ADDwoR) were evaluated by comparing the preoperative TMJ MRI with the outcomes of the maxillofacial arthroscopy or open surgery.

Results

The T₁WI sequence showed the highest SNR while the T₂WI group had the lowest SNR. The ROIs of the T₂WI group had the highest CNR and SIR values in the OCor and OSag sequences. In the OCor sequence, the value for the SIR F/DO group was higher than the SIR C/D and SIR C/DO values. Using subjective analysis to evaluate the quality of the scans, the highest total scores were obtained for the OSag T₂WI with opened mouth and OSag PDWI with closed mouth sequences. From the objective and subjective analysis, the three optimal sequences selected were OSag PDWI, OCor T₂WI with closed mouth, and OSag T₂WI with opened mouth. In patients with anterior disc displacement, the comparisons of the surgery and the selected MRI sequences indicated that the total diagnostic accuracy of the MRI was 96.3% (1,425/1,479 cases). For patients with ADDwoR, the diagnostic accuracy was 98.5% (1,372/1,393 cases), and for those with ADDwR it was 61.6% (53/86 cases). There were significant differences between the ADDwoR and ADDwR groups (χ²=312.92, P<0.01).

Conclusions

The three optimal MRI sequences for the rapid and efficient diagnosis of TMD were determined to be OSag PDWI, OCor T₂WI with closed mouth, and OSag T₂WI with opened mouth.

Recent advances in 3D time-resolved contrast-enhanced MR angiography

Contrast-enhanced magnetic resonance angiography (CE-MRA) was first introduced for clinical studies approximately 20 years ago. Early work provided 3-4 mm spatial resolution with acquisition times in the 30-second range. Since that time there has been continuing effort to provide improved spatial resolution with reduced acquisition time, allowing high resolution 3D time-resolved studies. The purpose of this work is to describe how this has been accomplished. Specific technical enablers have been: improved gradients allowing reduced repetition times, improved k-space sampling and reconstruction methods, parallel acquisition, particularly in two directions, and improved and higher count receiver coil arrays. These have collectively made high-resolution time-resolved studies readily available for many anatomic regions. Depending on the application, ∼1 mm isotropic resolution is now possible with frame times of several seconds. Clinical applications of time-resolved CE-MRA are briefly reviewed.

Detection of residual brain arteriovenous malformations after radiosurgery: diagnostic accuracy of contrast-enhanced four-dimensional MR angiography at 3.0 T

OBJECTIVE

To evaluate the diagnostic accuracy of four-dimensional MR angiography (4D-MRA) at 3.0 T for detecting residual arteriovenous malformations (AVMs) after Gamma Knife (Elekta Instrument AB, Stockholm, Sweden) radiosurgery (GKRS).

METHODS

We assessed 36 angiographically confirmed AVMs in 36 patients who had been treated with GKRS. 4D-MRA was performed after GKRS and the time intervals were 39.4 ± 26.0 months [mean ± standard deviation (SD)]. 4D-MRA was obtained at 3.0 T after contrast injection, with a measured voxel size of 1 × 1 × 1 mm and a temporal resolution of 1.1 s (13 patients) or a voxel size of 1 × 1 × 2 mm and a temporal resolution of 0.98 s (23 patients). X-ray angiography was performed as the standard reference within 53 ± 47 days (mean ± SD) after MRA. To determine a residual AVM, the 4D-MRA results were independently reviewed by two readers blinded to the X-ray angiography results. We evaluated diagnostic sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV) and accuracy of 4D-MRA for detection of a residual AVM.

RESULTS

A residual AVM was identified in 13 patients (13/36, 36%) on X-ray angiography. According to Readers 1 and 2, 4D-MRA had a sensitivity of 79.6% and 64.3%, a specificity of 90.9% and 100%, a PPV of 84.6% and 100% and an NPV of 90% and 81.5%, respectively, and a diagnostic accuracy of 86.1% for Readers 1 and 2, for detecting residual AVMs after GKRS.

CONCLUSION

The diagnostic accuracy of 4D-MRA at 3.0 T seems high, but there is still the possibility of further improving the spatiotemporal resolution of this technique.

Contrast-enhanced MR angiography of the abdomen with highly accelerated acquisition techniques

PURPOSE

To demonstrate that highly accelerated (net acceleration factor [R(net)] ≥ 10) acquisition techniques can be used to generate three-dimensional (3D) subsecond timing images, as well as diagnostic-quality high-spatial-resolution contrast material-enhanced (CE) renal magnetic resonance (MR) angiograms with a single split dose of contrast material.

MATERIALS AND METHODS

All studies were approved by the institutional review board and were HIPAA compliant; written consent was obtained from all participants. Twenty-two studies were performed in 10 female volunteers (average age, 47 years; range, 27-62 years) and six patients with renovascular disease (three women; average age, 48 years; range, 37-68 years; three men; average age, 60 years; range, 50-67 years; composite average age, 54 years; range, 38-68 years). The two-part protocol consisted of a low-dose (2 mL contrast material) 3D timing image with approximate 1-second frame time, followed by a high-spatial-resolution (1.0-1.6-mm isotropic voxels) breath-hold 3D renal MR angiogram (18 mL) over the full abdominal field of view. Both acquisitions used two-dimensional (2D) sensitivity encoding acceleration factor (R) of eight and 2D homodyne (HD) acceleration (R(HD)) of 1.4-1.8 for R(net) = R · R(HD) of 10 or higher. Statistical analysis included determination of mean values and standard deviations of image quality scores performed by two experienced reviewers with use of eight evaluation criteria.

RESULTS

The 2-mL 3D time-resolved image successfully portrayed progressive arterial filling in all 22 studies and provided an anatomic overview of the vasculature. Successful timing was also demonstrated in that the renal MR angiogram showed adequate or excellent portrayal of the main renal arteries in 21 of 22 studies.

CONCLUSION

Two-dimensional acceleration techniques with R(net) of 10 or higher can be used in CE MR angiography to acquire (a) a 3D image series with 1-second frame time, allowing accurate bolus timing, and (b) a high-spatial-resolution renal angiogram.

SUPPLEMENTAL MATERIAL

http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.11110242/-/DC1.

MR imaging evaluation of the hepatic vasculature

Assessment of the hepatic vasculature is essential for tumor staging, surgical planning, and understanding of liver disease. Technological advances have made contrast-enhanced magnetic resonance (MR) imaging comparable to multidetector-row computed tomography for diagnostic vascular imaging with respect to spatial resolution. Unenhanced MR angiographic sequences enable reasonable clinical assessment of vessels without contrast agents in patients with contraindications or renal insufficiency. Furthermore, MR angiography may be used to provide directional information through manipulation of the signal intensity of flowing blood. A major limitation to consistent contrast-enhanced MR angiography is the timing of MR image acquisition with arrival of the contrast bolus in the structures of interest. In this article, the authors discuss currently available techniques for imaging of the hepatic vasculature.

Abdominal magnetic resonance imaging at 3 T: oncological applications

The gain in signal-to-noise ratio at 3 T magnetic resonance (MR) imaging produces many benefits for abdominal imaging applications, including the capability to reduce acquisition times and/or improve spatial resolution for a variety of pulse sequences, the potential for broader application of parallel imaging techniques, and an increased sensitivity to gadolinium-based contrast media. These advances have the potential of improving the accuracy of MR imaging in the detection, staging, treatment planning, and follow-up of patients with abdominal tumors. At the same time, because certain high-field-strength-related drawbacks could not be compensated for, abdominal 3 T MR imaging should be clinically implemented with caution in some patients (eg, patients with massive ascites).

Direct comparison of sensitivity encoding (SENSE) accelerated and conventional 3D contrast enhanced magnetic resonance angiography (CE-MRA) of renal arteries: effect of increasing spatial resolution

PURPOSE

To assess the effect of attaining higher spatial resolution in contrast-enhanced magnetic resonance angiography (MRA) of renal arteries using parallel imaging, sensitivity encoding (SENSE), by comparing the SENSE contrast-enhanced (CE) MRA against a conventional CE-MRA protocol with identical scan times, injection protocol, and other acquisition parameters.

MATERIALS AND METHODS

Numerical simulations and a direct comparison of SENSE-accelerated versus conventional acquisitions were performed. A total of 41 patients (18 male) were imaged using both protocols for a direct comparison. Both protocols used fluoroscopic triggering, centric encoding, breath-holding, equivalent injection protocol, and lasted approximately 30 seconds.

RESULTS

Simulated point-spread functions were narrower for the SENSE protocol compared to the conventional protocol. In the patient study, although the SENSE protocol produced images with lower signal-to-noise ratio (SNR), image quality was better for all segments of the renal arteries. In addition, ringing of kidney parenchyma and renal artery blurring were significantly reduced in the SENSE protocol. Finally, reader confidence improved with the SENSE protocol.

CONCLUSION

Despite a reduction in SNR, the higher-resolution SENSE CE-MRA provided improved image quality, reduced artifacts, and increased reader confidence compared to the conventional protocol.

Increased volume of coverage for abdominal contrast-enhanced MR angiography with two-dimensional autocalibrating parallel imaging: initial experience at 3.0 Tesla

PURPOSE

To assess the feasibility and the quality of abdominal three-dimensional (3D) contrast enhanced MR angiograms acquired at 3.0 Tesla (T) using a new 2D-accelerated autocalibrating parallel reconstruction method for Cartesian sampling (2D-ARC).

MATERIALS AND METHODS

With institutional review board approval and written informed consent, a prospective trial in 6 normal healthy volunteers and 23 patients referred for evaluation of suspected renovascular disease was performed. The volunteers underwent abdominal MRA with and without 2D-ARC acceleration. Images were evaluated independently by two blinded vascular radiologists in randomized order. Vessel conspicuity was rated on a five-point scale. Evaluation for significant differences between the scores for each technique was performed using a Wilcoxon signed-rank test.

RESULTS

In the series of six volunteers, no statistical significance was found between the image quality scores for 2D-ARC accelerated and nonaccelerated exams. A high proportion of the 23 clinical 2D-ARC exams were graded as diagnostic (vessel conspicuity score >or=2; Reader 1, 96%; Reader 2, 100%) for overall image quality.

CONCLUSION

Subjective image quality of 2D-ARC accelerated MRA was equivalent to the conventional MRA method. However, the 2D-ARC accelerated sequence provided a 3.5-fold increase in imaging volume, complete abdominal coverage, and a 30% reduction in voxel volume, all within the same acquisition time.

High-resolution 3D contrast-enhanced MRA with parallel imaging techniques before endovascular interventional treatment of arterial stenosis

This study aimed to evaluate the efficacy of high-resolution 3D contrast-enhanced magnetic resonance angiography (3D CE MRA) with parallel imaging techniques for the diagnosis of various arterial stenoses and its value for planning endovascular interventional treatment. Thirty-five patients underwent 3D CE MRA before endovascular interventional treatment. Numbers of patients were as follows: clinically documented renal artery stenosis (n = 10), renal transplant artery stenosis (n = 1), carotid artery stenosis (n = 12), iliac artery stenosis (n = 11) and femoro-popliteal artery stenosis (n = 1). A total of 39 arterial segments were treated. The depiction of various arterial stenoses was evaluated. The degree and length of the stenoses were compared and analyzed between 3D CE MRA and digital subtraction angiography (DSA). The accuracy of MRA in depicting lesion characteristics (ulceration, eccentricity, post-stenotic dilatation) was reviewed. The overall value of 3D CE MRA in planning interventional treatment was determined. The quality of 3D CE MRA in the demonstration of various arterial stenoses was judged excellent or good. A strong correlation was noted between 3D CE MRA and DSA regarding severity and length of stenosis. The accuracy of 3D CE MRA in depicting lesion characteristics was good. 3D CE MRA overestimated three severe iliac artery stenoses. Except in these three segments, the value of 3D CE MRA analysis was judged high. 3D CE MRA was found to be better than DSA in revealing the distal reconstitution and occluded segment in cases of iliac artery stenosis. 3D CE MRA is accurate in demonstrating the relevant anatomy necessary to plan endovascular interventional treatment for patients with arterial stenosis.

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.

MR angiography with parallel acquisition for assessment of the visceral arteries: comparison with conventional MR angiography and 64-detector-row computed tomography

The purpose of the study was to retrospectively compare three-dimensional gadolinium-enhanced magnetic resonance angiography (conventional MRA) with MRA accelerated by a parallel acquisition technique (fast MRA) for the assessment of visceral arteries, using 64-detector-row computed tomography angiography (MDCTA) as the reference standard. Eighteen patients underwent fast MRA (imaging time 17 s), conventional MRA (29 s) and MDCTA of the abdomen and pelvis. Two independent readers assessed subjective image quality and the presence of arterial stenosis. Data were analysed on per-patient and per-segment bases. Fast MRA yielded better subjective image quality in all segments compared with conventional MRA (P = 0.012 for reader 1, P = 0.055 for reader 2) because of fewer motion-induced artefacts. Sensitivity and specificity of fast MRA for the detection of arterial stenosis were 100% for both readers. Sensitivity of conventional MRA was 89% for both readers, and specificity was 100% (reader 1) and 99% (reader 2). Differences in sensitivity between the two types of MRA were not significant for either reader. Interobserver agreement for the detection of arterial stenosis was excellent for fast (kappa = 1.00) and good for conventional MRA (kappa = 0.76). Thus, subjective image quality of visceral arteries remains good on fast MRA compared with conventional MRA, and the two techniques do not differ substantially in the grading of arterial stenosis, despite the markedly reduced acquisition time of fast MRA.

Assessment of arteriovenous malformations with 3-Tesla time-resolved, contrast-enhanced, three-dimensional magnetic resonance angiography

OBJECT

Although conventional catheter angiography is commonly used in the evaluation of intracranial arteriovenous malformations (AVMs), less invasive tools are more suitable for screening or follow-up. Older MR angiography techniques cannot provide high enough temporal and spatial resolution for assessing AVMs. Threetesla time-resolved imaging of contrast kinetics (TRICKS)-a time-resolved, contrast-enhanced 3D MR angiography technique-achieves subsecond time resolution without sacrificing spatial resolution. The purpose of this study was to assess the accuracy of TRICKS at 3 T in the evaluation of AVMs.

METHODS

Between November 2006 and November 2007, 31 patients who were known to have AVMs underwent evaluation in a 3-T unit with the TRICKS technique. The TRICKS images were then evaluated independently by 2 radiologists for nidus detection, early venous filling detection, and Spetzler-Martin classification, and these results were compared with the results of catheter angiography.

RESULTS

Time-resolved imaging of contrast kinetics achieved 96% sensitivity and 100% specificity both in nidus detection and early venous filling detection. The Spetzler-Martin grades also showed excellent correlation with catheter angiography findings (kappa= 0.89).

CONCLUSIONS

Although this is a preliminary study, the authors' results indicate that time-resolved contrast-enhanced 3D MR angiography at 3 T is a good tool to assess AVMs, and has the potential to replace catheter angiography in screening or follow-up examinations of patients with AVMs.

Three-dimensional dynamic time-resolved contrast-enhanced MRA using parallel imaging and a variable rate k-space sampling strategy in intracranial arteriovenous malformations

PURPOSE

To evaluate the effectiveness of three-dimensional (3D) dynamic time-resolved contrast-enhanced MRA (TR-CE-MRA) using a combination of a parallel imaging technique (ASSET: array spatial sensitivity encoding technique) and a time-resolved method (TRICKS: time-resolved imaging of contrast kinetics) and to compare it with 3D dynamic TR-CE-MRA using ASSET alone in the assessment of intracranial arteriovenous malformations (AVMs).

MATERIALS AND METHODS

Twenty consecutive patients with angiographically confirmed AVMs were investigated using both 3D dynamic TR-CE-MRA techniques. Examinations were compared with respect to image quality, spatial resolution, number and type of feeders and drainers, nidus size, presence of early venous filling and temporal resolution. Digital subtraction angiography was used as standard of reference.

RESULTS

The higher temporal and spatial resolution of 3D dynamic TR-CE-MRA TRICKS ASSET allowed a better assessment of intracranial vascular malformations, namely better depiction of feeders, drainers and better detection of early venous drainage. There was no significant difference between them in terms of nidus size.

CONCLUSION

3D dynamic TR-CE-MRA combining parallel imaging and a time-resolved method with subsecond and submillimeter resolution could become the first-line investigation technique in both diagnosis and follow-up of intracranial AVMs.

Measurement of signal-to-noise ratios in MR images: influence of multichannel coils, parallel imaging, and reconstruction filters

PURPOSE

To evaluate the validity of different approaches to determine the signal-to-noise ratio (SNR) in MRI experiments with multi-element surface coils, parallel imaging, and different reconstruction filters.

MATERIALS AND METHODS

Four different approaches of SNR calculation were compared in phantom measurements and in vivo based on: 1) the pixel-by-pixel standard deviation (SD) in multiple repeated acquisitions; 2) the signal statistics in a difference image; and 3) and 4) the statistics in two separate regions of a single image employing either the mean value or the SD of background noise. Different receiver coil systems (with one and eight channels), acquisitions with and without parallel imaging, and five different reconstruction filters were compared.

RESULTS

Averaged over all phantom measurements, the deviations from the reference value provided by the multiple-acquisitions method are 2.7% (SD 1.6%) for the difference method, 37.7% (25.9%) for the evaluation of the mean value of background noise, and 34.0% (38.1%) for the evaluation of the SD of background noise.

CONCLUSION

The conventionally determined SNR based on separate signal and noise regions in a single image will in general not agree with the true SNR measured in images after the application of certain reconstruction filters, multichannel reconstruction, or parallel imaging.

Intraindividual Comparison of High-Spatial-Resolution Abdominal MR Angiography at 1.5 T and 3.0 T: Initial Experience

PURPOSE

To prospectively compare three-dimensional (3D) contrast material-enhanced abdominal magnetic resonance (MR) angiography at 1.5 and 3.0 T intraindividually in healthy volunteers.

MATERIALS AND METHODS

After institutional review board approval and informed consent were obtained, 15 healthy male volunteers (age range, 24-41 years) underwent one abdominal 3D contrast-enhanced MR angiographic examination each at 1.5 and 3.0 T in random order. Fast 3D gradient-echo sequence with parallel imaging acceleration factor of three was used for MR angiography; acquired spatial resolutions were 1x0.8x1 mm3 (imaging time, 19 seconds) at 1.5 T and 0.9x0.8x0.9 mm3 (imaging time, 18 seconds) at 3.0 T. With the latter, volume of the 3D slab was 8% larger. At 1.5 T, 20-mL bolus of gadobenate dimeglumine was delivered at 2 mL/sec; at 3.0 T, 15-mL bolus was delivered at 2.5 mL/sec. Two blinded radiologists rated image quality of aorta and proximal renal arteries in consensus with five-point scale (4=very good, 0=nondiagnostic) according to sequence and in direct intraindividual comparison. Visibility of proximal and segmental renal arteries was rated with three-point scale (3=completely visible, 1=nonvisible). Signal-to-noise ratio (SNR) was determined with phantoms. For statistical analysis of the SNRs, t tests were used.

RESULTS

All MR angiographic measurements were diagnostic. Median score for image quality at both field strengths was 4. Depiction of proximal renal arteries was rated 3 at both field strengths. The visibility of the distal renal arteries was better at 3.0 T (median score, 3) than at 1.5 T (median score, 2). With direct comparison, 3.0-T MR angiography was better in 14 of 15 cases; no field strength was preferred in the other case. Mean SNR was significantly (P<.001) higher at 3.0 T (17.8+/-0.09 [standard deviation]) than at 1.5 T (11.9+/-0.10).

CONCLUSION

MR angiography at 3.0 T provided better vessel visibility and SNR than did that at 1.5 T, although voxel size and imaging time were reduced.

Assessment of aortoiliac and renal arteries: MR angiography with parallel acquisition versus conventional MR angiography and digital subtraction angiography

PURPOSE

To prospectively compare the image quality, sensitivity, and specificity of three-dimensional gadolinium-enhanced magnetic resonance (MR) angiography accelerated by parallel acquisition (ie, fast MR angiography) with MR angiography not accelerated by parallel acquisition (ie, conventional MR angiography) for assessment of aortoiliac and renal arteries, with digital subtraction angiography (DSA) as the reference standard.

MATERIALS AND METHODS

The study was approved by the institutional review board; informed consent was obtained from all patients. Forty consecutive patients (33 men, seven women; mean age, 63 years) suspected of having aortoiliac and renal arterial stenoses and thus examined with DSA underwent both fast (mean imaging time, 17 seconds) and conventional (mean imaging time, 29 seconds) MR angiography. The arterial tree was divided into segments for image analysis. Two readers independently evaluated all MR angiograms for image quality, presence of arterial stenosis, and renal arterial variants. Image quality, sensitivity, and specificity were analyzed on per-patient and per-segment bases for multiple comparisons (with Bonferroni correction) and for dependencies between segments (with patient as the primary sample unit). Interobserver agreement was evaluated by using kappa statistics.

RESULTS

Overall, the image quality with fast MR angiography was significantly better (P=.001) than that with conventional MR angiography. At per-segment analysis, the image quality of fast MR angiograms of the distal renal artery tended to be better than that of conventional MR angiograms of these vessels. Differences in sensitivity for the detection of arterial stenosis between the two MR angiography techniques were not significant for either reader. Interobserver agreement in the detection of variant renal artery anatomy was excellent with both conventional and fast MR angiography (kappa=1.00).

CONCLUSION

Fast MR angiography and conventional MR angiography do not differ significantly in terms of arterial stenosis grading or renal arterial variant detection.

Contrast-enhanced hepatic magnetic resonance angiography at 3 T: does parallel imaging improve image quality?

PURPOSE

To evaluate whether hepatic magnetic resonance angiography (MRA) performed at 3 T and acquired with a parallel imaging acceleration factor of 2 provides satisfactory image quality and adequate contrast-to-noise, compared with an acquisition without parallel imaging.

MATERIALS AND METHODS

Ten volunteers underwent both a standard (A) and an experimental (B) MRA protocol. Both protocols used a gadoteridol dose of 20 mL injected at 2 mL/s in a 3-T magnetic resonance (MR) system. Protocol B was identical to protocol A, except protocol B was performed with a parallel imaging acceleration factor of 2. Contrast-to-noise ratios (CNRs) were calculated in the suprarenal abdominal aorta, the common hepatic artery, and the right and left hepatic arteries. The same 4 vessels were rated for quality of arterial visualization using a 5-point scale (1 = poor to 5 = excellent). The paired t test and Wilcoxon rank sum test were used for statistical analysis.

RESULTS

The CNRs and qualitative scores were higher in all 4 vessels using protocol B. Mean CNRs in the suprarenal abdominal aorta, common hepatic artery, and the right and left hepatic arteries were 31, 33, 28, and 22 for protocol A and 36, 40, 36, and 25 for protocol B, respectively (P < 0.05 except the left hepatic arteries [P = 0.35]). Mean qualitative scores of the same vessels were 4.3, 3.7, 3.1, and 2.9 using protocol A and 4.4, 3.8, 3.5, and 3.2 for protocol B, respectively (P > 0.44).

CONCLUSIONS

Parallel imaging performed in a 3-T MR system improves hepatic MRA both quantitatively and qualitatively.

Intraarterial contrast-enhanced MR aortography with and without parallel acquisition technique in patients with peripheral arterial occlusive disease

OBJECTIVE

Repeated intraarterial gadolinium injections are necessary in endovascular MRI-guided interventions; therefore a low-dose protocol with a short acquisition time is preferable. The purpose of this study was to conduct a quantitative comparison of intraarterial MR aortograms obtained with and without high-speed parallel acquisition technique.

SUBJECTS AND METHODS

Intraarterial MR aortography was performed at 1.5 T on nine patients with peripheral arterial occlusive disease and in an aortic phantom with pulsatile flow. A 3D fast low-angle shot MRI sequence was used for standard technique (acquisition time, 20 seconds) and for parallel acquisition technique (acquisition time, 14 seconds). In all patients, a pigtail catheter was left in the suprarenal position after digital subtraction angiography. Contrast-enhanced intraarterial MR aortography was performed after automated injection of 50 mmol/L gadoterate dimeglumine at an injection rate of 4 mL/s. Contrast-to-noise ratio (CNR) and image quality were evaluated in both imaging series at different locations. In an aortic phantom with pulsatile flow, CNR was determined 1, 30, and 60 cm distal to the catheter tip with standard and parallel acquisition techniques.

RESULTS

In all patients, intraarterial MR aortography was feasible with both acquisition techniques. No significant difference in CNR or image quality was observed in the patient study. Similar results were calculated for the pulsatile aortic flow phantom at all locations.

CONCLUSION

Intraarterial MR aortography is feasible with parallel acquisition technique without a significant loss of CNR. This technique reduces contrast agent consumption approximately 30% owing to an approximately 30% reduction in acquisition time.

Efficacy and safety of gadodiamide (Gd-DTPA-BMA) in renal 3D-magnetic resonance angiography (MRA): a phase II study

PURPOSE

To determine the most efficacious dose of gadodiamide for three-dimensional (3D) contrast-enhanced (CE) magnetic resonance angiography (MRA) of the renal arteries on a patient level based on the sensitivity in detecting the main hemodynamically relevant (> or =50% or occlusion) renal artery stenosis (RAS) using intra-arterial digital subtraction angiography (IA DSA) as the gold standard.

MATERIALS AND METHODS

This prospective, randomized, double-blind, parallel-group, multicenter study included 273 patients referred to IA DSA for suspected RAS. Patients underwent 3D CE MRA after injection of 0.01, 0.05, 0.1, or 0.2mmol/kg of body weight gadodiamide (0.5mmol/ml). The images were assessed for location and degree of RAS by independent blinded readers (MRA: three readers, IA DSA: one reader). Hypothesis testing for a significant trend in sensitivity across dose groups was based on the one-sided Cochran-Armitage style trend test for each independent MRA reader.

RESULTS

The lowest dose group (0.01mmol/kg) proved non-efficacious in detecting hemodynamically relevant (i.e., > or =50% or occlusion) RAS. A statistically significant dose trend (p<0.001) was shown for each of the three independent readers. Depending on reader, the sensitivity obtained with 0.05, 0.1, and 0.2mmol/kg was 63.9-86.1%, 75.8-91.4% and 80.6-90.6%, the specificity was 66.7-73.9%, 59.3-75.0%, and 59.3-75.0% and accuracy was 67.8-78.9%, 75.4-77.4%, and 76.3-81.0%, for the three dose groups, respectively. There were eight non-severe adverse events (AEs). Three serious AEs occurring in one patient were judged not related to gadodiamide by the on-site investigator.

CONCLUSION

A significant dose trend between the four doses examined was observed. The lowest dose (0.01mmol/kg) differed significantly from those of the other three doses. Based on the analysis of the primary and secondary endpoints, 0.1mmol/kg gadodiamide appears to be the most suitable dose in diagnosing hemodynamically relevant RAS. The present study also demonstrated gadodiamide to be safe and well tolerated.

Nouvelles techniques en imagerie vasculaire cervico-encéphalique et médullaire

Recent technical progress of MRI and CT made it possible to widen the field of exploration of the noninvasive vascular imaging in the study of supra-aortic, encephalic and medullary vessels. MRI of the carotid plaques, CT angiography in the detection of the intracranial aneurysms, intracranial time-resolved MRA and MRA of the spinal cord took their place in the field of the noninvasive vascular imaging.

Whole-body three-dimensional contrast-enhanced magnetic resonance (MR) angiography with parallel imaging techniques on a multichannel MR system for the detection of various systemic arterial diseases

Using a 1.5-T magnetic resonance (MR) imager equipped with 32 receiving channels and integrated parallel acquisition techniques, 37 patients underwent whole-body three-dimensional (3D) contrast-enhanced MR angiography (WB 3D CE MRA). The patients included had clinically documented or suspected peripheral arterial occlusive disease (PAOD, n = 19), Takayasu arteritis (n = 8), polyarteritis nodosa (n = 1), type-B dissection (n = 4), thoracic and/or abdominal aneurysm (n = 5). Sixty-eight surface coils were employed to encompass the whole body. Four 3D CE MRA stations were acquired successively through automatic table moving. The spatial resolution was 1.6 x 1.0 mm and slice thickness was 1.5 mm for all stations. A total scan range of 188 cm was acquired. Overall image quality of each arterial segment and venous overlay were assessed. The depiction of various systemic arterial diseases was evaluated and compared, in 20 patients, with other imaging modalities. This WB 3D CE MRA yielded a detailed display of the arterial system with an average MR room time of 17.4 min. The image quality was considered diagnostic in 99.3% of the arterial segments. In 7 of 19 patients with PAOD, WB MRA showed additional vascular narrowing apart from peripheral arterial disease. In nine patients with vasculitis, WB MRA depicted luminal irregularity, narrowing or occlusion, aneurysm, and collateral circulation involving multiple vascular segments. WB MRA also clearly revealed the severity and extent of dissection and aortic aneurysm. In 20 cases where additional imaging investigations have been carried out, the vascular pathologies demonstrated by WB MRA agree with these additional imaging investigations.

Coronary magnetic resonance angiography using magnetization-prepared contrast-enhanced breath-hold volume-targeted imaging (MPCE-VCATS)

OBJECTIVES

Coronary artery x-ray angiography (XRA) is currently the gold standard for the assessment of coronary artery disease. A substantial minority of patients referred for coronary angiography have no significant coronary artery disease. The purpose of this study is to evaluate magnetization-prepared contrast-enhanced breath-hold volume-targeted imaging (MPCE-VCATS), a new 3-dimensional breath-hold coronary magnetic resonance angiography (MRA) technique, in detecting hemodynamically significant coronary artery stenoses in a patient population, with XRA correlation.

MATERIALS AND METHODS

A total of 19 subjects who were referred for conventional coronary angiography were enrolled in the study. ECG-triggered MPCE-VCATS coronary artery scans were acquired for the left main coronary artery (LCA), left anterior descending (LAD), and right coronary artery (RCA). Coronary MRA and XRA results were compared.

RESULTS

The overall sensitivity, accuracy, and negative predictive value for diagnosing any hemodynamically significant coronary artery disease (> or =50% diameter reduction) was 91%, 80%, and 90%, respectively. The sensitivity of the technique in the LCA, LAD, and RCA was 100%, 100% and 78%, respectively. The negative predictive value of the technique was 100%, 100%, and 71%, respectively.

DISCUSSION

MPCE-VCATS is a promising technique for coronary artery imaging. It has a relatively high sensitivity as well as a high NPV. The results of the study may indicate a future role for the technique in obviating the need for some patients to undergo XRA.

Utilizing SENSE to reduce scan duration in high-resolution contrast-enhanced renal MR angiography

PURPOSE

To evaluate the use of sensitivity encoding (SENSE) to reduce scan time and decrease detrimental artifacts arising from motion and bolus profile effects during contrast-enhanced MR angiography (CE-MRA) of the renal arteries (RAs).

MATERIALS AND METHODS

A direct comparison of conventional and SENSE (acceleration factor 2) CE-MRA protocols was performed on 20 patients. Each patient underwent both scans. Both protocols achieved the same resolution, but the SENSE protocol was 50% faster and utilized a faster injection than the conventional scan. Three radiologists graded the images for image quality, artifact levels, and reader confidence.

RESULTS

While the signal-to-noise ratio (SNR) decreased (26+/-5 vs. 30+/-10; P=0.04) with the SENSE protocol, the image-quality scores for four identified segments of the RAs increased or were unchanged. The largest improvements in image quality occurred in the more distal segments of the RAs. Parenchymal ringing (P=0.005) and RA blurring (P=0.006) were significantly reduced, and there was a trend toward improvement of RA ringing despite the increased injection rate.

CONCLUSION

The faster SENSE scan maintained nearly the same SNR (due to faster injection of Gd-chelate), reduced artifact levels, and improved image quality ratings for the distal renal vessels.

Assessment of the abdominal aorta and its visceral branches by contrast-enhanced dynamic volumetric hepatic parallel magnetic resonance imaging: feasibility, reliability and accuracy

The purpose of this study was to evaluate a new three-dimensional gradient-echo (GRE) MR sequence performed with a parallel acquisition technique to shorten breath-hold times (parallel GRE MRI) in the detection of arterial variants and stenosis of the abdominal aorta and its visceral branches. A total of 102 patients underwent dynamic parallel GRE MRI, timed to the arterial phase by a test bolus (mean breath-hold time, 17 s). For both quantitative and qualitative analysis, the abdominal aorta and its visceral branches were divided into 13 arterial segments. In a subanalysis of 55/102 patients, the accuracy of parallel GRE MRI compared to MDCT in the detection arterial variants and stenosis was calculated for two independent readers. Mean SNRs and CNRs were 47.2 and 35.6, respectively. Image quality was rated good or excellent in 1,234/1,326 segments (93%). Hepatic and renal arterial variants were identified with an accuracy of 93 and 95%, respectively (reader 1) and 98 and 100%, respectively (reader 2). Both readers detected arterial stenosis with an accuracy of 98%. Interobserver agreement was good to excellent for the detection of hepatic (kappa=0.69) and renal (kappa=0.92) variants and for the diagnosis of stenosis (kappa=0.96). Dynamic three-dimensional parallel GRE MRI is feasible and allows a reliable and accurate diagnosis of arterial variants and stenosis of the abdominal aorta and its visceral branches in a short breath-hold-time.

MRA of abdominal vessels: technical advances

Magnetic resonance angiography (MRA) in general and MRA of the abdominal vessels in particular have undergone substantial improvements in the past 5 years triggered by the introduction and application of parallel imaging (PI), new sequence techniques such as centric k-space trajectories and undersampling, dedicated contrast agents and clinical high-field scanners. All of these techniques have the potential to improve image quality and resolution or decrease the image acquisition time. However, each of them has its own specific advantages and drawbacks. This review describes the main technical innovations and focuses on the impact these developments may have on abdominal MRA. Special consideration is given to the interaction of these various technical advances. The clinical value of advanced MRA techniques is discussed and illustrated by characteristic cases.

High-resolution renal MRA: Comparison of image quality and vessel depiction with different parallel imaging acceleration factors

PURPOSE

To investigate the image quality and vessel depiction of renal MRA with integrated parallel imaging techniques (iPAT) using acceleration factors of 2 and 3.

MATERIALS AND METHODS

In this prospective study renal MRA was performed on 14 and 12 patients with acceleration factors of 3 and 2, respectively. For the MRA a 3D-GRE sequence with an acquired spatial resolution of 0.9 x 0.8 x 1.0 mm(3) was applied (TR/TE = 3.79 msec/1.39 msec, FOV = 400 mm x 320 mm, acquired matrix = 512 x 384, flip angle = 25 degrees) on a 32-channel 1.5T MR scanner. The acquisition time was 26 seconds with iPAT 2, and 19 seconds with iPAT3. All parameters other than acquisition time and acceleration factor were kept constant. To assess the signal-to-noise ratio (SNR) we performed repetitive phantom measurements using iPAT 2 and 3. The images were rated by two radiologists in terms of noise, artifacts, and the quality of vessel depiction for the proximal, segmental, and subsegmental renal artery. A Mann-Whitney U-test and kappa-test were used for statistical analysis.

RESULTS

SNR decreased significantly with iPAT 3 in the phantom measurements. The two readers found no difference in noise, but significantly fewer artifacts with iPAT 3. The depiction of segmental vessels was significantly better for both readers with iPAT 3, and the subsegmental vessels were rated significantly better by one reader. iPAT 3 also resulted in a better interreader agreement.

CONCLUSION

The use of iPAT 3 for renal MRA enables a better depiction of the distal parts of the renal artery. The decrease in SNR is not diagnostically impairing.

Three-dimensional dynamic magnetic resonance angiography for the evaluation of radiosurgically treated cerebral arteriovenous malformations

We assessed the value of three-dimensional (3D) dynamic magnetic resonance angiography (MRA) for the follow-up of patients with radiosurgically treated cerebral arteriovenous malformations (AVMs). Fifty-four patients with cerebral AVMs treated by radiosurgery (RS) were monitored using conventional catheter angiography (CCA) and 3D dynamic MRA with sensitivity encoding based on the parallel imaging. Cerebral AVM was qualitatively classified by two radiologists into one of five categories in terms of residual nidus size and persistence of early draining vein (I, >6 cm; II, 3-6 cm; III, <3 cm; IV, isolated early draining vein; V, complete obliteration). 3D MRA findings showed a good agreement with CCA in 40 cases (kappa=0.62). Of 23 nidus detected on CCA, 3D dynamic MRA showed 14 residual nidus. Of 28 occluded nidus on 3D dynamic MRA, 22 nidus were occluded on CCA. The sensitivity and specificity of 3D dynamic MRA for the detection of residual AVM were 81% and 100%. 3D dynamic MRA after RS may therefore be useful in association with MRI and can be repeated as long as opacification of the nidus or early venous drainage persists, one CCA remaining indispensable to affirm the complete occlusion at the end of follow-up.

A review of technical advances in interventional magnetic resonance imaging

Initial research in the development of interventional magnetic resonance (MR) imaging in the late 1980s and early to mid-1990s focused on pulse sequences, devices, and clinical applications. This focus was largely a result of the limited number of areas in which the academic research community leading the development could provide innovation on the MR systems of the time. However, during the past decade, computational power, higher bandwidth graphical displays, faster computer networks, improved pulse sequence architectures, and improved technical specifications have accelerated the pace of development on modern MR systems. Today, it is the combination of multiple system factors that are enabling the future of interventional MR. These developments, their impact on the field, and newly emerging applications are described.

Applications de l’arm dynamique dans la pathologie vasculaire du système nerveux central

Conventional catheter angiography (CCA) remains the gold standard for the evaluation of most intracranial vascular malformations. MRA techniques such as Time of Flight, Phase Contrast or 3D contrast-enhanced MRA, provide anatomic evaluation but without hemodynamic information. Recently developed, dynamic MRA is based on dynamic acquisition of images and image subtraction; these two principal characteristics produce images comparable to those obtained by CCA. The purpose of this review is to explain the principles, advantages and drawbacks of this technique in the evaluation of arteriovenous malformations, arteriovenous fistulas, aneurysms and venous thrombosis.