Low-Dose, Time-Resolved, Contrast-Enhanced 3D MR Angiography in the Assessment of the Abdominal Aorta and Its Major Branches at 3 Tesla

Rationale and Objectives

The aims of this study were to evaluate the effectiveness of low-dose, contrast-enhanced (CE), time-resolved, three-dimensional magnetic resonance angiography (MRA) in the assessment of the abdominal aorta and its major branches at 3 T and to compare the results with those of high–spatial resolution CE MRA.

Materials and Methods

Twenty-two consecutive patients (eight men, 14 women; mean age, 43.9 ± 17.9 years) underwent CE time-resolved three-dimensional MRA and high–spatial resolution three-dimensional MRA. Studies were performed using a 3-T magnetic resonance system; gadolinium-based contrast medium was administered at a dose of 3 to 5 mL for time-resolved MRA, followed by 0.1 mmol/kg gadopentetate dimeglumine for single-phase CE MRA. For analysis purposes, the abdominal arterial system was divided into 11 arterial segments, and image quality as well as the presence and degree of vascular pathology were evaluated by two independent magnetic resonance radiologists.

Results

A total of 242 arterial segments were visualized with good image quality. Time-resolved MRA was able to visualize the majority of arterial segments with good definition in the diagnostic range. Vascular pathologies (stenosis, occlusion) or abnormal vascular anatomy was detected in 19 arterial segments, with good interobserver agreement (κ = 0.78). All image findings were detected with time-resolved CE MRA by both observers and were confirmed by correlative imaging.

Conclusion

Low-dose, time-resolved MRA at 3 T yields rapid and important anatomic and functional information in the evaluation of the abdominal vasculature. Because of its limited spatial resolution, time-resolved MRA is inferior to CE MRA in demonstrating fine vascular details.

Three-dimensional (3D) contrast-enhanced (CE) magnetic resonance (MR) angiography (MRA) has made substantial technical advances in the past decade and is now widely used in many applications . Furthermore, CE MRA has been accepted in clinical routine as a substitute for the standard of reference, digital subtraction angiography (DSA), for evaluating the abdominal vasculature and its major branches .

Today most conventional CE MR angiographic methods aim to capture a single imaging volume at peak arterial contrast enhancement with minimal venous overlay. Therefore, these methods require that the acquisition and breath-hold coincide with the presence of the contrast bolus in the vessels of interest for optimal visualization of the arterial tree. But the timing of contrast material arrival may be crucial, because sufficient T1 shortening of blood is achieved only for few seconds during peak vascular concentration. As a result, delayed acquisition will lead to a substantial decrease in diagnostic accuracy. To guarantee accurate bolus timing, different strategies have been developed, including the test-bolus technique , automated detection of the contrast material , and real-time depiction of bolus arrival .

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Materials and methods

Patient Population

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MR Imaging

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Time-resolved CE MRA

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Single-phase CE MRA

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Table 1

Sequence Parameters of Time-resolved CE MRA and High–spatial resolution Single-phase CE MRA

Parameter Time-resolved 4D CE MRA Single-phase 3D CE MRA Orientation Coronal Coronal Repetition time (ms) 2.54 3.0 Echo time (ms) 1.05 1.14 Flip angle (°) 12–19 ∗ 19–23 ∗ Slice thickness (mm) 3.0 1.0 FOV read (mm) 420 420 FOV phase (%) 75 75 Base resolution 512 512 Phase resolution (%) 75 80 Slices per slab 32–36 ∗ 88–96 ∗ Matrix size 512 × 288 512 × 308 True voxel size (mm) 1.1 × 0.8 × 3.8 1.0 × 0.8 × 1.6 Interpolated voxel size (mm) 0.8 × 0.8 × 3.0 0.8 × 0.8 × 1.0 PAT mode GRAPPA GRAPPA Acceleration factor 3 3 Reference lines 24 24 Bandwidth (Hz/pixel) 750 650 Number of measurements 14–16 ∗ 1 Total scan time (s) 26 19

CE, contrast-enhanced; 4D, four-dimensional; FOV, field of view; GRAPPA, generalized autocalibrating partially parallel acquisitions; MRA, magnetic resonance angiography; PAT, parallel acquisition technique; 3D, three-dimensional.

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Table 2

Demographic Data of Patient Population

Patient Age (y) Height (m) Weight (kg) BMI (kg/m 2 ) Reason for MRA Referral 1 56 1.52 73 31.60 RAS 2 46 1.75 66 21.55 Abdominal angina 3 59 1.78 90 28.41 RAS 4 40 1.71 83 28.38 RAS 5 19 1.65 60 22.04 RAS 6 63 1.64 61 22.68 RAS 7 37 1.90 77 21.33 RAS 8 51 1.72 83 28.06 KD 9 61 1.67 84 30.12 RAS 10 59 1.69 53 18.56 AAA 11 32 1.86 112 32.37 RAS 12 50 1.63 70 26.35 Abdominal angina 13 66 1.67 79 28.33 KD 14 65 1.80 75 23.15 Abdominal angina 15 33 1.73 70 23.39 RAS 16 58 1.65 63 23.14 RAS 17 63 1.64 62 23.05 RAS 18 52 1.66 86 31.21 RAS 19 26 1.78 83 26.20 KD 20 45 1.77 78 24.90 KD 21 54 1.70 69 23.88 KD 22 27 1.73 55 18.38 FMD

AAA, abdominal aortic aneurysm; BMI, body mass index; FMD, fibromuscular dysplasia; KD, kidney donation; MRA, magnetic resonance angiography; RAS, renal artery stenosis.

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Image Analysis

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Semiquantitative Analysis

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Correlation with DSA

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Statistical Analysis

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Results

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Vascular Pathology

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Vessel Depiction and Visibility Score

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Table 3

Results for Total Vessel Visibility Score on a per Patient Basis Calculated for Both Sequence Types

Time-resolved 4D CE MRA (TWIST) Single-phase 3D CE MRA Patient Reader 1 Reader 2 Mean Reader 1 Reader 2 Mean 1 34 35 34.5 40 39 39.5 2 30 30 30 34 35 34.5 3 26 28 27 40 40 40 4 24 24 24 39 37 38 5 24 23 23.5 33 35 34 6 28 29 28.5 36 36 36 7 21 22 21.5 40 39 39.5 8 25 24 24.5 40 38 39 9 26 26 26 40 40 40 10 25 25 25 44 42 43 11 20 21 20.5 40 40 40 12 35 36 35.5 42 40 41 13 37 36 36.5 41 41 41 14 35 37 36 44 44 44 15 24 25 24.5 31 32 31.5 16 31 32 31.5 42 43 42.5 17 32 31 31.5 39 39 39 18 22 23 22.5 36 38 37 19 19 20 19.5 35 34 34.5 20 28 31 29.5 40 39 39.5 21 29 31 30 40 38 39 22 28 32 30 40 40 40 Maximum 37 37 37 44 44 44 Minimum 19 20 19.5 31 32 31.5 Mean 27.4 28.2 27.8 38.9 38.6 38.8 SD 4.96 5.11 5.03 3.30 2.84 3.07

CE, contrast-enhanced; 4D, four-dimensional; MRA, magnetic resonance angiography; 3D, three-dimensional; SD, standard deviation; TWIST, time-resolved angiography with stochastic trajectories.

The mean visibility score for conventional single-phase 3D CE MRA was significantly higher than for time-resolved multiphase 4D CE MRA.

Table 4

Results for Total Vessel Visibility Score on a per Segment Basis Calculated for Both Sequence Types

Segment 1 2 3 4 5 6 7 8 9 10 11 4D 3D 4D 3D 4D 3D 4D 3D 4D 3D 4D 3D 4D 3D 4D 3D 4D 3D 4D 3D 4D 3D Sum 176 176 175 176 61 167 59 128 125 167 168 174 34 46 63 166 13 153 176 176 174 176 Visibility score 0 0 0 0 0 16 0 14 5 3 0 0 0 34 32 18 0 35 4 0 0 0 0 1 0 0 0 0 10 0 13 3 5 0 0 0 0 0 8 0 5 0 0 0 0 0 2 0 0 0 0 7 2 9 5 8 1 1 0 2 0 4 2 4 2 0 0 0 0 3 0 0 1 0 7 5 4 9 8 7 6 2 2 2 9 6 0 3 0 0 2 0 4 44 44 43 44 4 37 4 22 20 36 37 42 6 10 5 36 0 35 44 44 42 44

4D, four-dimensional; 3D, three-dimensional.

Vessel segments with smaller diameters, such as the hepatic artery (segment 5), splenic artery (segment 6), superior mesenteric artery (segment 8), and the inferior mesenteric artery (segment 9), were significantly better shown in detail on single-phase 3D contrast-enhanced magnetic resonance angiography.

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Image Artifacts

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Discussion

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Technical Considerations

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Advantages of Higher Field Strength

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Potential Clinical Applications

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Image Postprocessing and Multiplanar Reformatting

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Limitations

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Conclusion

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References

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Low-Dose, Time-Resolved, Contrast-Enhanced 3D MR Angiography in the Assessment of the Abdominal Aorta and Its Major Branches at 3 Tesla

Rationale and Objectives

Materials and Methods

Results

Conclusion

Materials and methods

Patient Population

MR Imaging

Time-resolved CE MRA

Single-phase CE MRA

Image Analysis

Semiquantitative Analysis

Correlation with DSA

Statistical Analysis

Results

Vascular Pathology

Vessel Depiction and Visibility Score

Image Artifacts

Discussion

Technical Considerations

Advantages of Higher Field Strength

Potential Clinical Applications

Image Postprocessing and Multiplanar Reformatting

Limitations

Conclusion

References

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