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MR Angiography of the Lower Extremities

¹ Cardiovascular Imaging Section, Department of Radiology, Brigham and Women's Hospital and Harvard Medical School, 75 Francis St., Boston, MA 02115.

Received March 11, 2007; accepted after revision December 21, 2007.

 
Address correspondence to H. Ersoy (hersoy{at}partners.org).

CME This article is available for CME credit. See www.arrs.org for more information.

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This article is available for CME credit. See www.arrs.org for more information.

Abstract

Top Abstract Introduction System Requirements Peripheral MRA Protocols Tailored MRA Examinations for... Three-Dimensional Contrast... Gadolinium Dose and Infusion... Bolus Timing for Single... Data Postprocessing Alternative Contrast Agents for... Unenhanced MRA Techniques Summary References

 
OBJECTIVE. Current MRI technology and postprocessing tools have enabled 3D contrast-enhanced MR angiography (MRA) to evolve into a first-line noninvasive diagnostic tool to evaluate vascular disorders.

CONCLUSION. In this article, 3D MRA techniques, bolus timing issues, new IV contrast agents allowing a steady-state acquisition, principals of postprocessing, and unenhanced MRA techniques are reviewed and how to effectively use 3D gadolinium-enhanced MRA for peripheral arterial imaging is described.

Keywords: gadolinium • MR angiography • MR angiography techniques • peripheral artery • popliteal artery • unenhanced MR angiography

Introduction

Top Abstract Introduction System Requirements Peripheral MRA Protocols Tailored MRA Examinations for... Three-Dimensional Contrast... Gadolinium Dose and Infusion... Bolus Timing for Single... Data Postprocessing Alternative Contrast Agents for... Unenhanced MRA Techniques Summary References

 
Three-dimensional gadolinium-enhanced MR angiography (MRA) noninvasively facilitates the accurate and detailed assessment of the peripheral arteries without sedation, catheterization, ionizing radiation, or potentially nephrotoxic iodinated contrast agents. Koelemay et al. [1] published a meta-analysis of 34 studies (1,090 patients) between January 1985 and May 2000, reporting high accuracy for the assessment of the lower extremity arteries using MRA. Furthermore, 3D gadolinium-enhanced MRA improved diagnostic performance compared with 2D MRA; the estimated points of equal sensitivity and specificity were 94% and 90% for 3D gadolinium-enhanced MRA and 2D MRA, respectively [1]. More recent studies focused on the diagnostic performance of lower extremity 3D gadolinium-enhanced MRA (Figs. 1A and 1B) compared with digital subtraction angiography are provided in Table 1 [2-21]. The accuracy of 3D MRA for evaluating bypass grafts and recurrent disease in the graft lumen is equal to that in native arteries [22, 23]. The sensitivity and specificity of foot and calf MRA are more than 80% and 90%, respectively [14, 24]. Unlike digital subtraction angiography, gadolinium-enhanced MRA provides a 3D data set that can then be reformatted to reproduce multilane digital subtraction angiography-like displays of the vessels that highlight information most relevant to prognosis and treatment planning (e.g., arterial wall inflammation, plaque composition, and mural and intramural thrombus formation).

View larger version (29K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A —80-year-old woman with increasing pain in left foot. Coronal three-station 3D gadolinium-enhanced MR angiography (MRA) of left lower extremity arteries after IV administration of 45 mL of gadolinium-based contrast agent is performed on 1.5-T MR system. Iliac arteries are widely patent. Left superficial femoral artery is occluded at its origin. Extensive collaterals from femoral artery attempt to reconstitute run-off vessels. Note segmental reconstitution of posterior tibial artery in distal calf (arrow). After 1 week, patient developed rest pain in left foot and underwent emergency digital subtraction angiography.

View larger version (20K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B —80-year-old woman with increasing pain in left foot. Digital subtraction angiography findings are identical to MRA findings except that digital subtraction angiography does not show reconstituted segment of posterior tibial artery at ankle despite significant contrast volume and delayed imaging.

System Requirements

Top Abstract Introduction System Requirements Peripheral MRA Protocols Tailored MRA Examinations for... Three-Dimensional Contrast... Gadolinium Dose and Infusion... Bolus Timing for Single... Data Postprocessing Alternative Contrast Agents for... Unenhanced MRA Techniques Summary References

 
Three-dimensional gadolinium-enhanced MRA requires state-of-the-art scanners equipped with high-performance gradients that are essential to providing ultrashort TEs and TRs for dynamic acquisitions. Current 1.5-T systems are highly optimized for MRA studies. Three-Tesla MR scanners offer a higher signal-to-noise ratio (SNR), but, in general, optimization is still required to resolve technical issues and image artifacts. These challenges include field distortions due to strong attenuation of the radiofrequency field that results in signal loss in the deep tissues, dielectric resonances, and specific absorption rate limits.

The complete examination includes vascular segments from the juxtarenal abdominal aorta to the ankles. For a single injection, automated table translation between the abdominopelvic, thigh, and calf stations is required. Although the entire peripheral vasculature can be studied by combining a body coil with a surface coil, homogeneous signal reception is optimized with a dedicated lower extremity reception coil. A full-length peripheral vascular reception coil with an optimized protocol may substantially improve the usefulness of single-bolus peripheral 3D gadolinium-enhanced MRA [25]. Such a coil can provide a higher SNR than the body coil, and the improved signal can in turn be used for higher resolution or shorter scanning times with parallel imaging, without sacrificing vessel-to-background contrast. The improved scanning time can be used to increase the anatomic coverage in the anteroposterior direction.

Some problems are associated with imaging at multiple fields of view. Gradient nonlinearity leads to distortions at the field-of-view edges. Moreover, extensive postprocessing is required to "stitch" multiple fields of view together. Recent work toward incremental field-of-view imaging methods includes integrating continuous acquisition with continuous table motion [26-29]. Although the multiple challenges include motion correction artifacts, gradient wrap effects, and the requirement of table velocity adjustment according to the contrast travel time, Vogt et al. [21] reported a sensitivity, specificity, and accuracy of 92.8%, 100%, and 89.2%, respectively for detection of significant peripheral arterial occlusive disease of the lower extremities [21].

Peripheral MRA Protocols

Top Abstract Introduction System Requirements Peripheral MRA Protocols Tailored MRA Examinations for... Three-Dimensional Contrast... Gadolinium Dose and Infusion... Bolus Timing for Single... Data Postprocessing Alternative Contrast Agents for... Unenhanced MRA Techniques Summary References

 
No standard MRA protocol exists that can be applied to all scanners. Multiple techniques successfully cover long peripheral vascular territories. Separate contrast injections for each station are not used because of the large contrast volume needed and the unacceptable background soft-tissue and venous enhancement, particularly in distal stations. Although single-injection multistation bolus-chase MRA improves speed, eliminates motion artifacts due to patient repositioning, and simplifies bolus timing, venous contamination in the calves is often unacceptable. Moreover, the narrow artery-only imaging window in the distal station results in inadequate isotropic resolution.

One approach to address these issues has been introduced by Maki et al. [30]: "Waki-Trak" (wide aperture kinematic table imaging with isotropic resolution). In this technique, parallel imaging, such as simultaneous acquisition of spatial harmonics (SMASH) [31], sensitivity encoding (SENSE) [32], or generalized autocalibrating partially parallel acquisition (GRAPPA) [33], is implemented in the proximal station to provide a relatively long artery-only imaging window, thus achieving submillimeter isotropic resolution in the calf and foot stations. The rationale is that smaller calf and foot vessels require higher spatial resolution. Another approach to reduce venous contamination is to apply subsystolic midfemoral venous compression to slow venous return in the calves [34, 35]. Pressure cuffs may induce graft thrombosis and should not be used in patients with bypass graft repair of the lower extremity arteries.

Hybrid MRA protocols [36] use two stages. The first stage is high-spatial-resolution MRA of the calf and foot. The second stage (i.e., after the second injection) is aortoiliac and femoral bolus-chase MRA. Hybrid techniques are more accurate for evaluating the trifurcation and foot vessels than single-injection multistation MRA [12, 14, 18, 36-38]. Alternatively, temporally resolved MRA techniques, described in detail in the following text, eliminate the need for bolus timing. With these methods, selective arterial phase images can be obtained in most cases, regardless of the rate of venous enhancement [12]. For hybrid methods, the calf and foot station should be acquired first. This minimizes venous contamination for smaller arteries closely associated with veins and the associated lower accuracy in infragenicular interpretation [12]. A single-injection multistation acquisition of the aortoiliac and femoral stations follows the calf and foot station.

Optimal prescription of the 3D slabs requires projection images from a low-resolution axial 2D time-of-flight MRA locator through all stations (Fig. 2). The prescribed slabs overlap at the common femoral artery bifurcation and the popliteal artery trifurcation to visualize both regions on two stations at different phases. Image acquisition is performed both before (mask) and after contrast administration. Mask images ensure proper placement of the 3D slabs and are subsequently subtracted from the contrast-enhanced acquisition.

View larger version (29K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2 —50-year-old man with claudication. Sagittal projections of axial 2D time-of-flight localizer images are used for determining most anterior and most posterior extensions of arterial territories. Rectangles represent oblique coronal MR angiography slabs that are prescribed from these sagittal projections. Note overlaps at common femoral and popliteal arteries. Also note that when blood pressure cuffs are inflated, thigh may raise anteriorly a few centimeters. Thus, slab prescribed for thigh station should have sufficient coverage anterior to artery so that image field of view fully covers artery.

View larger version (89K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 3 —78-year-old man with claudication. Three-dimensional time-resolved gadolinium-enhanced MR angiography of left foot after administration of 10 mL of gadolinium-based contrast agent shows widely patent dorsalis pedis artery and plantar arch. Posterior tibial artery is not visualized at ankle. Peroneal artery attempts to reconstitute plantar artery (arrows).

Tailored MRA Examinations for Specific Conditions and Disorders

Top Abstract Introduction System Requirements Peripheral MRA Protocols Tailored MRA Examinations for... Three-Dimensional Contrast... Gadolinium Dose and Infusion... Bolus Timing for Single... Data Postprocessing Alternative Contrast Agents for... Unenhanced MRA Techniques Summary References

 
Because of their complexity in patients with peripheral vascular disease, MRA examinations are often patient-specific. The imaging workup always begins with an interventional and surgical history, such as bypass grafts, stents, and prostheses. Metallic materials cause T2^* susceptibility artifacts, and if unrecognized, may lead to misinterpretations as focal stenosis or occlusion (Figs. 4A and 4B). The type and the location of the grafts, particularly extraanatomic bypass grafts (i.e., axillofemoral bypass grafts) must be known before imaging because such superficial grafts can otherwise be inadvertently excluded from the imaging volume.

View larger version (37K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4A —61-year-old man with rest pain in both feet that is worse in right foot. Three-dimensional gadolinium-enhanced MR angiography after IV administration of 45 mL of gadolinium contrast material at 1.5 T. Coronal maximum-intensity-projection image shows focal areas of stenosis (arrows) of Dacron (DuPont) graft lumen and its distal anastomosis.

View larger version (63K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 4B —61-year-old man with rest pain in both feet that is worse in right foot. Digital subtraction angiography shows patent right common femoral artery to above-knee popliteal artery bypass graft. Note that focal areas of decreased lumen diameter on MR angiogram correspond to levels of metallic surgical clips (arrows) located around graft and at distal anastomosis site. Metallic susceptibility artifact can be recognized by characteristic signal buildup at edge of signal void area due to intravoxel phase distortions.

Popliteal Artery Imaging
There are also two conditions of the popliteal artery for which specialized protocols are used to answer specific questions. Adventitial cystic medial necrosis most commonly occurs in the popliteal artery (90%), followed by the femoral and external iliac arteries (9%), and rarely in arteries in the upper extremity. Enlarged cysts in the artery wall result in progressive claudication, paresthesia, and burning in the lower extremity. MRA shows segmental lumen encroachment. In patients with suspected adventitial cystic disease, T2-weighted fat-suppressed spin-echo and or proton density-weighted imaging through the stenotic segment should be acquired to determine the actual size and location of the cysts [48].

The other special examination is imaging of popliteal artery entrapment syndrome, in which the medial head of the gastrocnemius muscle entraps the popliteal artery and vein against the medial condyle of the femur, resulting in transient tingling or coldness in the foot and intermittent claudication. Irreversible arterial damage, occlusion, or distal embolization may occur as a result of repeated arterial compression. Therefore, early recognition and surgical correction is important. Our current protocol is managed using either conventional or time-resolved 3D gadolinium-enhanced MRA techniques and involves two contrast injections with the patient at a rest position and hyperextension of the knee (or dorsiflexion of the foot) (Figs. 5A, 5B and 5C).

View larger version (37K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5A —35-year-old man after placement of popliteal artery-to-anterior tibial artery vein graft for treatment of claudication resulting from popliteal artery stenosis. Follow-up MR angiography (MRA) was performed at 1.5 T using 30 mL of gadolinium contrast agent. Volume-rendered image of coronal 3D gadolinium-enhanced MRA at neutral position shows mild stenosis of popliteal artery at knee level (arrow) and patent bypass graft.

View larger version (40K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5B —35-year-old man after placement of popliteal artery-to-anterior tibial artery vein graft for treatment of claudication resulting from popliteal artery stenosis. Follow-up MR angiography (MRA) was performed at 1.5 T using 30 mL of gadolinium contrast agent. Volume-rendered image of coronal 3D gadolinium-enhanced MRA at dorsiflexion of feet shows greater stenosis at same level (open arrow).

View larger version (50K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 5C —35-year-old man after placement of popliteal artery-to-anterior tibial artery vein graft for treatment of claudication resulting from popliteal artery stenosis. Follow-up MR angiography (MRA) was performed at 1.5 T using 30 mL of gadolinium contrast agent. Contrast-enhanced axial fat-suppressed T1-weighted image confirms evidence of type III popliteal artery entrapment on left. During dorsiflexion of foot, popliteal artery is compressed by slip of medial head of gastrocnemius muscle (curved arrow) originating more laterally than normal.

Intraarterial MRA
Although this method requires an arterial puncture, substantial gadolinium dose reduction can be achieved without loss of image quality, allowing repetitive contrast administration. The overall values for sensitivity, specificity, and accuracy of intraarterial MRA for the characterization of significant stenosis or occlusion in the peripheral arteries are comparable with those in previously published reports [49, 50].

Three-Dimensional Contrast-Enhanced MRA Pulse Sequence and Acquisition Parameters

Top Abstract Introduction System Requirements Peripheral MRA Protocols Tailored MRA Examinations for... Three-Dimensional Contrast... Gadolinium Dose and Infusion... Bolus Timing for Single... Data Postprocessing Alternative Contrast Agents for... Unenhanced MRA Techniques Summary References

 
Short TR and TE for fast acquisition are accomplished with 3D spoiled gradient-echo pulse sequences. Spoiling increases the contrast-to-noise ratio (CNR) by suppressing residual background signal. As in other MR applications, the acquisition time is determined by the TR, the number of phase-encoding steps, the number of slices, the fraction of k-space sampled, and the acceleration factor (when parallel imaging is used). The gradient strength governs the shortest possible TR (< 5 milliseconds) and TE (< 3 milliseconds), although parameters such as wider bandwidth, smaller flip angles, and fractional echo can shorten the TR and TE. A flip angle of 15-45° is typically used. Additional modifications for shortening the acquisition include partial-Fourier and partial-phase field-of-view techniques.

The trade-off from any adjustment to shorten scanning time will result in either lower SNR or lower spatial resolution. For example, choosing a shorter TR will result in a signal reduction proportional to the square root of the TR. The vessel lumen signal is significantly enhanced with gadolinium agents that transiently shorten the blood T1 below the background tissues.

Although 3D gadolinium-enhanced MRA does not suffer from signal loss due to intravoxel spin dephasing as much as flow-dependent unenhanced MRA techniques, a short TE (< 3 milliseconds) is still necessary to minimize flow artifacts, such as signal loss caused by spin dephasing as a result of turbulent flow, and to minimize T2^* metallic susceptibility artifacts [51]. At 3 T, specific absorption rate limitations will determine the flip angle, which should be chosen as high as possible without increasing the TR.

Further reduction in scanning time can be achieved by applying zero filling and incorporating parallel imaging. Although zero filling does not add information content to the raw data, it can effectively increase spatial resolution by providing overlapping voxels, thereby reducing partial volume artifacts [52]. The disadvantage of parallel imaging is the reduction in SNR by a factor of approximately the square root of the acceleration factor times a geometry factor [53].

Thicker slices decrease scanning time at the expense of spatial resolution. Note that stenoses are measured by dividing the minimal luminal diameter in the stenotic segment by the maximal observed luminal diameter. Accurate stenosis assessment requires a spatial resolution in all planes no less than approximately one third of the vessel diameter. Adequate spatial resolution is particularly essential for evaluating the small vessels in the calves and the foot. On the other hand, achieving isometric spatial resolution is difficult because in-plane resolution is often higher than in the slice-select direction. Using thicker slices will emphasize the nonisometric voxel problem via poor spatial resolution for vessels coursing in the slice-select direction.

Stent imaging requires specific adjustments in pulse sequence parameters. The severity of the artifact is determined by the type and mass of the metal used in the stent [54-56]. An ultrashort TE (< 1 millisecond) can be used to minimize the T2^* effect of metal [51]. This is typically achieved with a wider receiver bandwidth at the expense of SNR (Figs. 6A and 6B). The Faraday cage effect of the stent mesh can be partially overcome by using a higher (e.g., 75°) flip angle. Nevertheless, despite pulse sequence adjustments, distinguishing stent restenosis from susceptibility artifact is not consistently possible with current techniques.

View larger version (86K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6A —57-year-old man with left renal artery stent referred for imaging of renal and lower extremity arteries. Three-dimensional gadolinium-enhanced MR angiography was performed at 1.5 T. Imaging parameters are bandwidth, 31.25 MHz/s; TR/TE, 8.8/2.9; slice thickness, 1.3 mm (A) and bandwidth, 62.5 MHz/s; 5.9/1.4; slice thickness, 0.8 mm (B). Metallic susceptibility artifact from stent (arrow) is less when using wider bandwidth with shorter TE in comparison with narrower bandwidth and longer TE.

View larger version (103K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 6B —57-year-old man with left renal artery stent referred for imaging of renal and lower extremity arteries. Three-dimensional gadolinium-enhanced MR angiography was performed at 1.5 T. Imaging parameters are bandwidth, 31.25 MHz/s; TR/TE, 8.8/2.9; slice thickness, 1.3 mm (A) and bandwidth, 62.5 MHz/s; 5.9/1.4; slice thickness, 0.8 mm (B). Metallic susceptibility artifact from stent (arrow) is less when using wider bandwidth with shorter TE in comparison with narrower bandwidth and longer TE.

Gadolinium Dose and Infusion Rate for 3D Contrast-Enhanced MRA

Top Abstract Introduction System Requirements Peripheral MRA Protocols Tailored MRA Examinations for... Three-Dimensional Contrast... Gadolinium Dose and Infusion... Bolus Timing for Single... Data Postprocessing Alternative Contrast Agents for... Unenhanced MRA Techniques Summary References

 
The MRA signal is provided by the blood T1 shortening effect of the gadolinium agents. During the first-pass, T1 shortening is mainly determined by the peak gadolinium concentration. That, in turn, is determined by the injection rate and the cardiac output [57]:

Thus, good bolus timing requires an injection rate adjusted according to the variables affecting the contrast travel time, including cardiac output and intervening vascular pathology.

For three-station bolus-chase MRA, the injection rate should be fast enough to obtain sufficient arterial enhancement at successive stations. However, a rate that is too fast will not allow complete vascular filling in subsequent stations. We typically use 0.2 mmol/kg of an extracellular gadolinium agent injected at a rate of 1.5-2 mL/s. Higher injection rates do not provide significant improvement in signal for individuals with normal cardiac output [58]. The peak contrast concentration in the vessel of interest is synchronized with the k-space center. Slower injection rates are preferred for prolonged and uniform T1 shortening [59, 60]. This is particularly important for showing the collateral circulation secondary to severe stenosis or occlusion of the arteries. The preferred injection duration is approximately 50-60% of the overall acquisition time. Contrast material should be flushed with 20 mL of saline to advance the entire bolus centrally. A split dose delivery of 2 mL/s for 10 seconds followed by 0.5-1.0 mL/s for the remaining contrast agent can be used for long scanning times when proper sharing of the bolus is problematic. This approach can also be used to highlight venous anatomy. The typical cumulative dose of the gadolinium agent for multistation 3D MRA should be 0.3 mmol/kg.

View larger version (79K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 7 —33-year-old man with ischemic left foot. Coronal time-resolved images acquired using 3D time-resolved imaging of contrast kinetics (TR/TE, 5.2/1.3; slice thickness, 1.8 mm, interpolated to 0.9-mm; matrix, 320 x 224; number of excitations, 1) performed at 1.5 T using 12 mL of gadolinium contrast agent followed by 20 mL of saline, both at rate of 2.5 mL/s. Temporal resolution of time-resolved images is one image per 9 seconds. Scanning delay is 10 seconds. Contrast arrives in both calf arteries 28 seconds after antecubital IV injection. Venous enhancement begins at 46 seconds, allowing 180-second window for artery-only imaging. Left popliteal artery is occluded, with segmental reconstitution of left anterior tibial and posterior tibial arteries. Left peroneal artery is not visualized. Right posterior tibial artery is occluded at origin, but right peroneal and right anterior tibial arteries are widely patent. Blood flow arrival time is same on both sides.

Top Abstract Introduction System Requirements Peripheral MRA Protocols Tailored MRA Examinations for... Three-Dimensional Contrast... Gadolinium Dose and Infusion... Bolus Timing for Single... Data Postprocessing Alternative Contrast Agents for... Unenhanced MRA Techniques Summary References

A contrast bolus injected in an antecubital vein arrives in the common femoral artery in 24 ± 6 seconds, and in the popliteal artery after an additional 5 ± 2 seconds [47]. In the calf, peak arterial and venous enhancement can be determined from time-resolved images that also show asymmetric arrival times between the legs (Fig. 7). In case of asymmetric flow between the legs, the scanning delay time should be adjusted according to the symptomatic leg, usually the side with slower flow.

Data Postprocessing

Top Abstract Introduction System Requirements Peripheral MRA Protocols Tailored MRA Examinations for... Three-Dimensional Contrast... Gadolinium Dose and Infusion... Bolus Timing for Single... Data Postprocessing Alternative Contrast Agents for... Unenhanced MRA Techniques Summary References

 
As in all complex 3D imaging acquisitions, postprocessing is an important step in the work flow. High-quality postprocessed images are also useful for the referring physicians, many of whom prefer images that resemble those of digital subtraction angiography. The first postprocessing step is subtraction of the mask data from its corresponding contrast-enhanced data to eliminate background signal. This subtraction dramatically improves the CNR (Figs. 8A and 8B). Details of the subtraction in the k-space and image space can be found in the literature [61]. The clinically relevant pitfall is motion between the mask and the contrast-enhanced images. Unequal patient respiration can cause misregistration and result in inferior image quality and significant artifacts when compared with non-subtracted images. Subtle artifacts are more problematic because they may lead to under- or overestimation of the stenoses in small vessels. Subtraction of the arterial phase from a delayed phase may improve visualization of the veins. Fat suppression can also be helpful because an improved SNR can be more prominent with fat suppression than with a subtraction technique, at the expense of a slightly lengthened acquisition time.

View larger version (84K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8A —72-year-old woman with claudication that is worse on right side. Coronal 3D gadolinium-enhanced MR angiography images before (A) and after (B) mask subtraction. Note that MR angiogram after mask subtraction (B) allows visualization of more vessel segments and smaller branches than nonsubtracted image.

View larger version (105K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 8B —72-year-old woman with claudication that is worse on right side. Coronal 3D gadolinium-enhanced MR angiography images before (A) and after (B) mask subtraction. Note that MR angiogram after mask subtraction (B) allows visualization of more vessel segments and smaller branches than nonsubtracted image.

View larger version (70K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9A —22-year-old man after kidney transplantation presented with extensive varicose veins in left calf. Three-dimensional gadolinium-enhanced MR angiography was performed at 1.5 T after administration of 45 mL of gadolinium contrast material. Coronal MR angiogram shows aneurysm of infrarenal abdominal aorta and left common and internal iliac arteries. Note left persistent sciatic artery (solid arrow) identified as ectatic and dominant inflow vessel to popliteal region. Left common femoral and external iliac arteries are hypoplastic (open arrow), and left superficial femoral artery is atretic. Left thigh is supplied by widely patent profunda femoral artery branches. Transplanted kidney is seen in left iliac fossa (asterisk).

View larger version (113K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 9B —22-year-old man after kidney transplantation presented with extensive varicose veins in left calf. Three-dimensional gadolinium-enhanced MR angiography was performed at 1.5 T after administration of 45 mL of gadolinium contrast material. Sagittal oblique maximum-intensity-projection image shows widely patent transplanted renal artery anastomosed to left external iliac artery (curved arrow).

Finally, automated and semiautomated "vascular segmentation" techniques allow vessel tracking based on lumen signal intensity. Such algorithms can also remove the vessel from the 3D data set. Because such algorithms rely heavily on the ability to determine the center line and lumen borders mathematically, errors in segmentation can occur. Thus, it is essential to review reformatted and, when necessary, the source images, as opposed to basing an interpretation on automated systems.

Alternative Contrast Agents for Contrast-Enhanced MRA Applications

Top Abstract Introduction System Requirements Peripheral MRA Protocols Tailored MRA Examinations for... Three-Dimensional Contrast... Gadolinium Dose and Infusion... Bolus Timing for Single... Data Postprocessing Alternative Contrast Agents for... Unenhanced MRA Techniques Summary References

 
Extracellular contrast agents show rapid extracellular distribution, resulting in decreased SNR and CNR at the steady state. Recently, MR contrast agents confined to the intravascular space, also known as blood pool agents, have become available for research and clinical use. Blood pool agents provide a much longer time window for data acquisition—that is, data can be repeatedly acquired over minutes to hours with little loss in intravascular signal intensity. Moreover, true blood pool agents produce only minimal soft-tissue enhancement. These features allow extensive signal averaging to improve the intravascular SNR and thereby the steady-state high-resolution imaging of small vessels and vessels with slow or complex flow.

View larger version (67K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 10 —74-year-old woman with occluded left superficial femoral artery imaged with unenhanced MR angiography. Pulse sequence parameters are described in text. Imaging parameters were as follows: Aortoiliac station: TR/TE, 3 R-R ECG intervals/80; 256 x 256 matrix interpolated to 512 x 512; 2-mm slice thickness (interpolated to 1 mm); parallel imaging factor, 2; field of view, 400 x 380 mm; total acquisition time, 4 minutes. Thigh station: 3RR/80; 256 x 256 matrix; 3-mm slices (interpolated to 1.5 mm); parallel imaging factor, 1.5; field of view 400 x 380 mm; total acquisition time, 3.5 minutes. (Courtesy of Dr. Masaaki Akahane, University of Tokyo, Japan)

Other blood pool agents proposed for first-pass and steady-state MRA are ultra small superparamagnetic iron oxide particles (USPIO): ferumoxtran-10, ferumoxytol, and SHU-555C. Iron oxide particles significantly reduce both T1 and T2 relaxation times. They act as positive enhancers if the T2 and T2^* shortening effect is minimized and T1 relaxivity is enhanced by using short-TE gradient-echo pulse sequences. Sequences with short TEs are required to minimize confounding susceptibility effects.

Although blood pool agents have high potential, there are challenges such as venous contamination and SNR loss as a result of less pronounced T1 shortening and high-resolution imaging. As is the case for all agents, lower extremity arteries pose a challenge because of their small caliber and their close anatomic relationship to the veins. Longer acquisition times can compensate for SNR loss from increased spatial resolution. Multiple strategies and their associated trade-offs can be found in the literature [62, 70, 71].

Unenhanced MRA Techniques

Top Abstract Introduction System Requirements Peripheral MRA Protocols Tailored MRA Examinations for... Three-Dimensional Contrast... Gadolinium Dose and Infusion... Bolus Timing for Single... Data Postprocessing Alternative Contrast Agents for... Unenhanced MRA Techniques Summary References

 
In patients with moderate and severe renal failure, high doses of gadolinium-based contrast agents must be used with extreme prudence, or not at all, because of the risk of nephrogenic systemic fibrosis (NSF) [72]. Details regarding the pathophysiology of NSF are beyond the scope of this review, but the topic has been recently reviewed [73-75]. It is important to follow U.S. Food and Drug Administration (FDA) guidelines. The recognition of the relationship between NSF and gadolinium and the considerable attention it has received have renewed interest in robust unenhanced MRA sequences.

In comparison with contrast-enhanced MRA, conventional unenhanced bright-blood MRA techniques (e.g., phase contrast and time-of-flight) and black-blood vessel wall imaging technique (double inversion recovery) have relatively long acquisition times, directional dependency, and sensitivity to a predetermined vascular flow speed. For these reasons, 3D gadolinium-enhanced MRA has supplanted unenhanced imaging for clinical decision making. However, more robust unenhanced MRA techniques such as fresh blood imaging are currently available commercially. Fresh blood imaging is an ECG-gated 3D fast spin-echo strategy that acquires images over one acquisition by triggering in both systole and diastole, thus allowing separation of arteries from veins. Diastolic triggering is applied to visualize fresh blood that enters the veins and arteries, where the flow is relatively slow in comparison with systole and during which only venous flow is typically depicted. In large vessels such as the aorta, the difference in flow rates between systole and diastole is large enough to perform subtraction to separate arteries from veins and to depict both vascular systems.

In the slower-flow peripheral vessels, additional flow-spoiling pulses are required to yield bright blood during diastole and black blood during systole [76]. MIP of the subtracted (diastole - systole) images eliminates venous and background signals for interpretation (Fig. 10). Although this and future methods hold great promise, clinical trials are necessary to determine whether the technique is suitable to routinely separate slow flow in a population of patients with peripheral arterial disease.

Summary

Top Abstract Introduction System Requirements Peripheral MRA Protocols Tailored MRA Examinations for... Three-Dimensional Contrast... Gadolinium Dose and Infusion... Bolus Timing for Single... Data Postprocessing Alternative Contrast Agents for... Unenhanced MRA Techniques Summary References

 
MRA is an excellent noninvasive imaging method that routinely guides clinical management decisions between catheter-based and surgical lower extremity interventions. Continuing advancements include multichannel systems with whole-body multidetector arrays in combination with parallel acquisition techniques, continuous table motion, whole-body MRA techniques, high-performance gradients, new contrast agents with high relaxivity, and strategies to reduce or eliminate the dose of gadolinium contrast material in patients with impaired renal function.

References

Top Abstract Introduction System Requirements Peripheral MRA Protocols Tailored MRA Examinations for... Three-Dimensional Contrast... Gadolinium Dose and Infusion... Bolus Timing for Single... Data Postprocessing Alternative Contrast Agents for... Unenhanced MRA Techniques Summary References

 

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