Kluwer - Handbook of Biomedical Image Analysis Vol

Figure 3.31: Source images of the 3D TOF MRA of the left carotid artery of a volunteer: (a and b) inferior and (c and d) at the carotid bifurcation. Images were acquired (a and c) without VTE and (b and d) with VTE (16 TE segments). The

= ˚ ˚

imaging parameters were as follows: matrix 256 A 256 A 32, slice thickness

= 1 mm, TR = 24 ms, FOV = 14 cm, and TE = 1.8/2.9 ms for VTE on/off. MT was not applied. The reduced signal indicated by arrows in a and c was much more uniform in images b and d with VTE.

bolus of Gadoteridol, and a 3D pulse sequence with a 66% sampling efficiency. This spatial resolution allowed visualization of intracranial aneurysms, carotid dissections, and atherosclerotic disease including ulcerations. Potential drawbacks of 3.0 T MRA are increased SAR and T(*)2 dephasing compared to 1.5 T.

The dependence of RF power deposition on TR for CEMRA was calculated and described.

3.4.19Magnetization Transfer MRA with RF Labeling Technique

A method for MT angiography using an RF labeling technique was suggested. The method utilized a slice-selective spin-lock pulse sequence for tagging the spins of inflowing blood [19]. The pulse sequence begins with a spatially selective 90◦ (x) RF pulses, followed by a nonselective composite locking pulse of 135◦

(y) – n[360◦ (y)] – 135◦ (y) and by a 90◦ (−x) pulse. A spoiler gradient was then applied. A rapid imaging stage, which yielded a T1 rho-weighted signal from the tagged spins, completed the sequence. Untagged spins were thoroughly dephased and consequently suppressed in the image. Thus, contrast was obtained without an injection of a contrast material or image subtraction. Furthermore, the flow of the tagged bolus could be visualized. The sequence was implemented on phantoms and on human volunteers using a 1.5 T scanner. The results indicated the feasibility of the suggested sequence.

3.4.20Oscillating Dual-Equilibrium Steady-State Angiography (ODESSA)

A novel technique of generating non-contrast angiograms was proposed [20]. This method utilized a modified steady-state free precession (SSFP) pulse sequence (see Fig. 3.32). The SSFP sequence was modified such that flowing material reaches a steady state that oscillates between two equilibrium values, while stationary material attains a single, non-oscillatory steady state. Subtraction of adjacent echoes results in large, uniform signal from all flowing spins and zero signal from stationary spins. Venous signal can be suppressed based on its reduced T2. ODESSA arterial signal was more than three times larger than that of traditional phase-contrast angiography (PCA) in the same scan time, and also compares favorably with other techniques of MR angiography (MRA). Pulse sequences are implemented in 2D, 3D, and volumetric-projection modes. Angiograms of the lower leg, generated in as few as 5 seconds, showed high arterial signal-to-noise ratio (SNR) and full suppression of other tissues.

Figure 3.32: (a) Two-dimensional and (b) 3D ODESSA pulse sequences. Two TR intervals are shown. Each axis has zero net gradient area over the TR interval. During odd TRs (at left), a bipolar flow-encoding pulse follows readout on any axis. A triphasic pulse after even readouts, though not necessary, is included to mitigate imaging system nonidealities. The numbered locations correspond to spin states.

3.4.21 Fat-Suppressed 3D MRA

Appropriate rate of fat-suppression pulses (using spec IR spectral selective inversion recovery) were determined for fat-suppressed 3D magnetic resonance angiography (MRA) with an elliptical centric view order [21]. In abdominal 3D fast spoiled gradient echo (fast SPGR) wit an elliptical centric view order, the spec IR pulse rate was changed from zero to one every 15 repetitions (in nine steps) in eight volunteers. In the equilibrium phase, abdominal contrastenhanced 3D MRA was obtained by 3D fast SPGR using an elliptical centric order without fat–suppression and with two spec IR, and by fat-suppressed 3D fast SPGR with a sequential-centric view order (3D-EFGRE). Fat and vascular signals were estimated. Although 3D fast SPGR using an elliptical centric order with two spec IR placed every 15 TR and 3D-EFGRE effectively decreased fat signals, these sequences lengthened the breath-hold by 4–6 seconds compared

with non-fat suppressed sequence. 3D fat SPGR using an elliptical centric order and two spec IR reduced the fat signal by 30%. And provided good 3D MR angiography without substantial prolongation of breath-hold. Two spec IR can be used for generation of partially fat-suppressed abdominal 3D MRA without prolongation of the breath-hold when performing 3D fast SPGR using an elliptical centric view order.

3.4.22Gadolinium Enhanced MRA with MR Cholangiography (MRC)

Simultaneously both methods were used in the preoperative evaluation of gallbladder carcinoma [22]. All MR images were analyzed in order to assess bile duct invasion, vascular invasion, hepatic invasion or metastasis, lymph node metastasis, and invasion into adjacent organs. The sensitivity and specificity of MR examination were distinctive 100% and 89% for bile duct invasion, 100% and 87% for vascular invasion, 67% and 89% for hepatic invasion, and 56% and 89% for lymph node metastasis supported by histopathologic findings. The “all-in-one” MR protocol, including MR imaging, MRC, and MRA, could be an effective diagnostic approach in the preoperative work-up for gallbladder carcinoma.

3.4.23 Ultrashort Contrast-Enhanced (CE) MRA

It was used for the morphologic evaluation of cerebral arteriovenous malformations (AVMs). The method was compared with conventional X-ray digital subtraction angiography (DSA) and time-of-flight (TOF) MRA to assess the angioarchitecture of the malformations that is essential for treatment planning and follow-up. Contrast-enhanced MRA was able to detect all AVMs seen on DSA, whereas the TOF MRA failed with a very small AVM [23]. However, there was no difference for the detection and delineation of feeding arteries and the AVM. The venous drainage patterns could always be clearly delineated in the CE MRA, whereas TOF MRA could demonstrate the exact venous drainage. Contrast-enhanced MRA was found to be superior to conventional TOF MRA in the assessment of the angioarchitecture of cerebral AVMs especially regarding the assessment of the venous drainage patterns. The superiority was

proton magnetic resonance spectroscopic imaging (MRSI) estimated individual tissue contributions to the spectroscopic voxels in multiple sclerosis (MS).

3.4.25 Coronary MRA

For assessment of patients with atherosclerotic CAD, CMRA is reported useful for detection of patency of bypass grafts. Patients with suspected coronary artery anomalies and patients with Kawasaki disease and coronary aneurysms are among those for whom CMRA has demonstrated clinical usefulness. At centers with appropriate expertise and resources, CMRA also appears to be of value for exclusion of severe proximal multivessel CAD in selected patients. Data from multicenter trials defined the clinical role of CMRA, particularly as it relates to assessment of CAD. Future developments and enhancements of CMRA promise better lumen and coronary artery wall imaging. This may become the new target in noninvasive evaluation of CAD [25].

3.4.26 4D Phase Contrast (PC) Technique

4D PC technique was demonstrated for its feasibility that permits spatial and temporal coverage of an entire 3D volume [26]. It validated quantitatively the accuracy against an established time resolved 2D PC technique to explore advantages of the approach with regard to the 4D nature of the data. Time-resolved, 3D anatomical images were generated simultaneously with registered threedirectional velocity vector fields. Improvements were compared to prior methods for gated and respiratory compensated image acquisition, interleaved flow encoding with freely selectable velocity encoding (VENC) along each spatial direction, and flexible trade-off between temporal resolution and total acquisition time. The implementation was validated against established 2D PC techniques using a well-defined phantom, and successfully applied in volunteer and patient examinations. Human studies were performed after contrast administration in order to compensate for loss of in-flow enhancement in the 4D approach. Advantages of the 4D approach included the complete spatial and temporal coverage of the cardiovascular region of interest and the ability to obtain high spatial resolution in all three dimensions with higher signal-to-noise ratio compared to 2D methods at the same resolution. In addition, the 4D nature of the data offered a variety of image processing options, such as magnitude and velocity

angiography. The extra imaging time allowed coronary artery imaging with increased spatial resolution [29].

3.4.303D Real-Time Navigator Magnetic Resonance (MR) Coronary Angiography

3D real-time navigator magnetic resonance (MR) coronary angiographic examination was reported for detection of significant coronary artery stenoses, with conventional coronary angiography as the standard of reference immediately before catheterization. It quantified coronary artery visualization, and evaluated the presence of significant narrowing or stenoses. Receiver operating characteristic (ROC) analysis signified that large portions of the coronary arteries could be visualized with MR coronary angiography. Imaging results were not consistently reliable, however, the examination was premature for routine clinical assessment of significant coronary artery stenosis owing to low sensitivity and large observer variability [30].

3.4.31Free-breathing three-dimensional (3D) coronary magnetic resonance (MR) angiography

This method was reported to determine the anatomy of anomalous coronary arteries, in particular the relationship of the vessels to the aortic root. Multiple 3D volume slabs were acquired at the level of the sinuses of Valsalva by using diaphragmatic navigators for respiratory artifact suppression. The proximal anatomy of the coronary arteries was determined. Free-breathing 3D coronary MR angiography could be used to identify the proximal anatomy of anomalous coronary arteries [31].

3.4.32BACSPIN (Breathing AutoCorrection with SPiral INterleaves) Coronary MRA Technique

Signal-to-noise ratio (SNR) of breath independent coronary magnetic resonance angiography (CMRA) was improved without increasing the number or duration of breath holds. In this BACSPIN technique, a single breath-held electrocardiogram (ECG)-gated multi-slice interleaved-spiral data set was acquired, followed

by repeated imaging of the same slices during free breathing. Each spiral interleaf from the breath-held data set was used as a standard for comparison with corresponding acquisitions at the same interleaf angle during free breathing. The most closely matched acquisitions are incorporated into a multi-slice, multi-average data set with increasing SNR over time. In-plane translations of the coronary artery could be measured and compensated for each accepted acquisition before combination with the other acquisitions. CMRA was performed with improved SNR and minimal motional blurring. BACSPIN provided a promising method for CMRA with improved SNR and limited breath-holding requirements [32].

3.4.33Motion-Adapted Gating Window in Coronary MRA

An acquisition technique was reported that used subject-specific acquisition windows in the cardiac cycle and a motion-adapted gating window for respiratory navigator gating. Cardiac acquisition windows and trigger delays were determined individually from a coronary motion scan. Motion-adapted gating used a 2-mm acceptance window for the central 35% of k-space and a 6-mm window for the outer 65% of k-space. The adaptive technique was applied in patients who underwent coronary radiographic angiography. Scanning times with the adaptive technique were reduced for the right coronary artery and left coronary artery system compared with the conventional technique, due to the use of longer subject-specific acquisition windows in patients with low heart rates. Subjective and objective measurements of image quality showed no significant differences between the two techniques. Coronary MR angiography with subject-specific acquisition windows and motion-adapted respiratory gating reduced scanning times while maintaining image quality and provided high diagnostic accuracy for the detection of coronary artery stenosis [33].

3.4.34Attenuated Coronary Blood—Myocardium In-Flow Contrast 3D Coronary Magnetic Resonance Angiography (CMRA)

The in-flow contrast between the coronary blood and the surrounding myocardium was attenuated as compared to thin-slab 2D techniques. The

application of a gadolinium (Gd)-based intravascular contrast agent provided an additional source of signal and contrast by reducing T1(blood) and supporting the visualization of more distal or branching segments of the coronary arterial tree. For imaging, an optimized free breathing, navigator-gated and -corrected 3D inversion recovery (IR) sequence was used. For comparison, state-of-the-art baseline 3D coronary MRA with T(2) preparation for non-exogenous contrast enhancement was acquired. The combination of IR 3D coronary MRA, sophisticated navigator technology, and B-22956 contrast agent allowed for an extensive visualization of the LCA system. Postcontrast showed a significant increase in both the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). Vessel sharpness of the left anterior descending (LAD) artery and the left coronary circumflex (LCx) were improved [34].

3.5 Limitations and Future Prospects

No specific MRA techniques have emerged so far that can provide sufficient sensitivity and specificity for quantification. MR angiography still remains a clinical choice of cardiovascular MR despite of cardiac and respiratory motion factors. Physical principles further highlight the intricacies and need of MRA technical improvements and modifications in coming years. From all techniques available, 2D/3D breath-hold coronary MRA(CMRA), black-blood FSE method, real-time navigator for respiratory gating with slice position correction and contrast enhanced CMRA have been evaluated clinically useful for coronary wall imaging. However, these high contrast angiography techniques suffer from limitations in temporal and spatial resolution and motion artifacts. These restrictions further limit its prediction value. Other hand, high contrast MR angiography techniques suffer from limitations in temporal and spatial resolution and motion artifacts. These advanced techniques have been described less sensitive <70% and specificity <75% while human artery risk in the wall is established >50% stenosis. These methods no doubt provide a quick way to image blood flow in a long segment of the artery for rapid burden measurements.

Other emerging MR techniques, such as water diffusion weighting, magnetization transfer weighting, steady-state free precession (SSFP) sequences, contrast enhancement methods may provide thin slices. Still measurements and plaque characterization methods are in infancy using thin slices. Some notable