Magnetic Resonance Angiography of the Upper Extremity




The magnetic resonance angiography (MRA) toolbox includes a wide array of versatile methods for diagnosis and therapy planning in patients with a variety of upper extremity vascular pathologies. MRA can provide excellent image quality with high spatial and high temporal resolution without the disadvantages of ionizing radiation, iodinated contrast, and operator dependency. Contrast-enhanced techniques are preferred for their robustness, image quality, and shorter scan times. This article provides an overview of the available MRA techniques and a description of the clinical entities that are well suited for evaluation with contrast-enhanced MRA.


Key points








  • Magnetic resonance angiography (MRA) can provide excellent image quality with high spatial resolution of the vessels of the upper extremity.



  • Time-resolved MRA can be used to depict the flow dynamics of pathology to the upper extremity in the same way that digital subtraction angiography (DSA) provides functional information about vascular pathology.



  • Practicing radiologists should be familiar with MRA imaging techniques used for individual cases to establish a correct diagnosis.



  • MRA techniques must to be tailored for each individual to achieve optimal image quality and maximal diagnostic yield.



  • Contrast-enhanced MRA (CE-MRA) techniques are currently preferred to noncontrast MRA techniques for imaging the upper extremity.






Introduction


MRA is a noninvasive imaging modality with high spatial resolution that can be used for diagnosis and treatment planning of vascular abnormalities of the upper extremity. Unlike CT angiography (CTA) or DSA, it avoids the need for ionizing radiation and exposure to iodinated contrast agents. Targeted MRA of anatomic regions, such as the hand, can achieve high spatial resolution and in some instances may surpass the performance of CTA. In addition, the high attenuation of bone necessitates the need for complex postprocessing algorithms that segment bone from CTAs, because bone can obscure the visualization of small arteries of the distal extremities. DSA has the highest spatial and temporal resolution and remains the standard of reference for imaging the upper extremity. Recent developments in MRA, however, including time-resolved imaging, have made dynamic imaging of the vasculature feasible in the clinical setting to assess the anatomic and hemodynamic abnormalities seen in vascular disease and to bring MRA closer to the DSA reference standard.


MRA of the upper extremity can be performed with or without the use of contrast agents. Although CE-MRA is generally preferred, non–contrast-enhanced techniques are increasingly available and may be a good option for patients with impaired renal function or who have allergies to gadolinium-based contrast agents (GBCAs). Non–contrast-enhanced techniques often overestimate stenoses in the setting of complex vessel anatomy or abnormal blood flow and can be time consuming. CE-MRA is robust and less time consuming. A major challenge for all CE-MRA techniques is to achieve optimal timing of the contrast bolus relative to the sampling of the center of k -space, which is crucial for optimal imaging. The specific MRA protocol chosen should be tailored to the patient to provide the best possible image quality.


A broad spectrum of vascular disorders of the upper extremity, ranging from the thoracic outlet syndrome to distal disease, such as thromboangiitis obliterans and hypothenar hammer syndrome, can be assessed accurately using MRA of the upper extremity. This review discusses MRA techniques and MRA findings of common disease entities of the upper extremity.




Introduction


MRA is a noninvasive imaging modality with high spatial resolution that can be used for diagnosis and treatment planning of vascular abnormalities of the upper extremity. Unlike CT angiography (CTA) or DSA, it avoids the need for ionizing radiation and exposure to iodinated contrast agents. Targeted MRA of anatomic regions, such as the hand, can achieve high spatial resolution and in some instances may surpass the performance of CTA. In addition, the high attenuation of bone necessitates the need for complex postprocessing algorithms that segment bone from CTAs, because bone can obscure the visualization of small arteries of the distal extremities. DSA has the highest spatial and temporal resolution and remains the standard of reference for imaging the upper extremity. Recent developments in MRA, however, including time-resolved imaging, have made dynamic imaging of the vasculature feasible in the clinical setting to assess the anatomic and hemodynamic abnormalities seen in vascular disease and to bring MRA closer to the DSA reference standard.


MRA of the upper extremity can be performed with or without the use of contrast agents. Although CE-MRA is generally preferred, non–contrast-enhanced techniques are increasingly available and may be a good option for patients with impaired renal function or who have allergies to gadolinium-based contrast agents (GBCAs). Non–contrast-enhanced techniques often overestimate stenoses in the setting of complex vessel anatomy or abnormal blood flow and can be time consuming. CE-MRA is robust and less time consuming. A major challenge for all CE-MRA techniques is to achieve optimal timing of the contrast bolus relative to the sampling of the center of k -space, which is crucial for optimal imaging. The specific MRA protocol chosen should be tailored to the patient to provide the best possible image quality.


A broad spectrum of vascular disorders of the upper extremity, ranging from the thoracic outlet syndrome to distal disease, such as thromboangiitis obliterans and hypothenar hammer syndrome, can be assessed accurately using MRA of the upper extremity. This review discusses MRA techniques and MRA findings of common disease entities of the upper extremity.




MRA techniques


The main requisite for diagnostic MRA is to achieve sufficient spatial resolution and sufficient vessel contrast. A high-field magnetic resonance (MR) scanner (1.5T or 3T) with modern phased-array receive coils is necessary to obtain images with high spatial resolution and high signal-to-noise ratio, while still covering the region of interest. Dedicated phased-array extremity or wrist coils should be used whenever possible, depending on the region of interest and required coverage.


Contrast-Enhanced MRA Techniques


Contrast-enhanced MRA


Of all available methods, CE-MRA has evolved as the preferred technique for MR imaging of the arterial vasculature. CE-MRA relies on the T1 shortening effect of paramagnetic GBCAs. This results in a significant difference in signal intensity between blood and adjacent tissue when using heavily T1-weighted arterial-phase imaging. The image acquisition must be timed with the contrast bolus peak during the sampling of the center of k -space to achieve maximum vessel contrast. Before the contrast material is injected, a nonenhanced acquisition with the same sequence settings as the contrast-enhanced scans can be acquired to allow for subtraction with the subsequent arterial-phase images. Subtracted images can be further manipulated with a maximum intensity projection (MIP) visualization to produce 3-D representations of the arterial anatomy. Recent approaches in the lower extremity using Dixon-based methods are an alternative approach that can be used to suppress background fat signal potentially obviating subtraction. This approach is promising for lower extremity MRA and also may be helpful for MRA of the upper extremity.


The advantages of CE-MRA include short scan times and high spatial resolution with minimal flow-related artifacts. CE-MRA findings are highly reproducible, and image quality is comparable to that of DSA. The main disadvantage of CE-MRA is the need for injection of GBCA, which has been associated with nephrogenic systemic fibrosis (NSF) in patients with renal insufficiency. Overall, however, the safety profile of GBCAs is excellent and generally exceeds that of iodinated agents.


Time-resolved MRA


Temporal resolution can add clinically valuable information to an examination of the upper extremity, including collateral flow pathways associated with stenoses and visualization of arterial to venous shunting. Another major advantage of time-resolved imaging is that it obviates a timing bolus or real-time fluorotriggering techniques. In this way, time-resolved imaging provides a point-and-shoot approach that is highly advantageous for imaging challenging anatomy where bolus timing/triggering methods are impractical.


Most time-resolved MRA techniques use view-sharing methods to achieve high temporal resolution while maintaining high spatial resolution. Such approaches use frequent sampling of low spatial frequencies (center of k -space) with less frequent sampling of higher spatial frequencies (periphery of k -space) that are subsequently shared between the final reconstructed 3-D data sets. These methods are commonly used in combination with subtraction, obtained by acquiring a precontrast mask, ultimately providing a set of time-resolved 3-D data images showing progressive enhancement of vessels akin to DSA. A major advantage is that not only morphologic but also dynamic information is obtained, which may allow for evaluation of the hemodynamic relevance of a stenosis. From the rapidly acquired multiple images, the best arterial or other relevant phase can be chosen. Time-resolved MRA can be performed in the same imaging session together with high-resolution standard MRA for more detailed depiction of small vessels.


Noncontrast MRA


Time of flight (TOF) and phase contrast (PC) are currently the most commonly used noncontrast MRA techniques. Newer techniques are ECG-gated balanced steady-state free precession, flow-sensitive dephasing, and arterial spin labeling techniques. Appropriate use of these techniques can allow diagnosis of vascular diseases in patients with chronic kidney disease without using contrast materials.


Time of flight MRA


TOF MRA is based on the principle that the protons in blood flowing into a presaturated imaging section of interest are unsaturated and thereby provide fresh magnetization when excited with the radiofrequency pulse. A major advantage of TOF MRA is that no contrast agents are required. Furthermore, by placing a saturation band above or below the section of interest, the direction of blood flow can be assessed. Disadvantages of this method include small imaging volumes, long acquisition times, and artifacts related to either slow or turbulent flow. These artifacts can impede assessment of vascular disease, potentially causing false-positive overdetection of vascular stenoses.


Phase-contrast MRA


PC MRA requires the acquisition of 2 or more (typically 4) data sets or flow-compensated pulses to generate flow-sensitive phase images. Phase data can be used either to reconstruct velocity-encoded flow-quantification images or MRA images. This technique is commonly used for flow imaging and measurement of flow but only in rare cases is it used for pure angiographic imaging of the upper extremity. The direction of flow can also be depicted with PC imaging and may be helpful verifying the presence of functionally significant vascular pathology.




Imaging protocol


Contrast Agents


CE-MRA can be performed with any extracellular contrast agents, such as gadopentetate dimeglumine (Magnevist, Bayer HealthCare, Whippany, New Jersey). Furthermore, there has been great interest in the use of high relaxivity agents, such as gadobenate dimeglumine (MultiHance, Bracco Diagnostics, Princeton, New Jersey) or gadobutrol (Bayer HealthCare, Wayne, New Jersey). Extracellular contrast agents have a short blood pool half-life and are best suited for arterial-phase or early delayed-phase imaging. This makes the need for accurate bolus timing important.


The limitations of imaging with extracellular agents led to the development of intravascular contrast agents with a prolonged intravascular half-life, referred to as blood pool contrast agents. Gadofosveset trisodium (Lantheus Medical Imaging, North Billerica, Massachusetts) binds reversibly to human serum albumin, which effectively prolongs the serum half-life to several hours. As with other extracellular contrast agents, first-pass MRA can be performed with gadofosveset. The main advantage of gadofosveset is the ability to perform steady state imaging that is helpful for facilitating delayed imaging using provocative maneuvers, such as arm position, to evaluate for thoracic outlet syndrome.


Injection Protocol


The contrast agent should be injected at a minimum rate of 1.5 to 2 mL/s using a power injector followed by a 30- to 50-mL saline flush at the same rate. Injecting the contrast agent in the contralateral extremity (from that with suspected pathology) avoids susceptibility artifacts due to high concentrations of gadolinium during first pass of the injection in the symptomatic extremity ( Fig. 1 ). In some circumstances, it may be necessary to inject from a foot vein or femoral central venous access. Furthermore, dilution of contrast agents in saline to large bolus volumes (approximately 30 mL) is an off-label practice that the authors find helpful for prolonging the contrast bolus and providing better matching of a uniform bolus during the k -space acquisition. Dilution of contrast may also be helpful when small volumes of contrast (eg, with an infant or small child) are needed and are difficult to handle using power injectors. Dilution also mitigate susceptibility related artifacts from highly concentrated contrast.




Fig. 1


Susceptibility artifact. The high concentration of gadolinium during first pass of the injection leads to a susceptibility artifact with signal drop in the right subclavian vein as well as in the adjacent subclavian artery ( arrowhead ). Injecting the contrast agent in the contralateral extremity (from that with suspected pathology) avoids susceptibility artifacts in the symptomatic extremity. Dilution also mitigates susceptibility related artifacts from highly concentrated contrast.


Positioning of the Patient


Patient comfort is of paramount importance to ensure a successful examination with minimal motion artifact. Feet-first imaging may be recommended for claustrophobic patients, but head-first imaging allows the injection site to be monitored and is preferable. For upper arm imaging, patients should be positioned supine with the arm next to the body. The arm and site of interest should be moved as close as possible to the center of the magnet. For forearm and hand imaging, patients should be positioned in a decubitus prone position, with the arm of interest extended above the head (superman position). The arm and region of interest should be positioned within the extremity coil as close to the center of the magnet as possible. In cases of clinically suspected vasospasm, wrapping the hand in a warm towel may be helpful in depicting the peripheral digital vessels. When the hand is imaged, the palm should lie flat within the coil with the fingers slightly spread to include the entire arterial tree in the imaging volume.


MRA Protocols


Proximal upper extremity


Imaging of the proximal upper extremity should be performed with phased-array receive coils. The authors use an 8-element cardiac coil that is placed over a patient’s chest and arm of interest. In general, a single-phase MRA can be performed for imaging of the proximal upper extremity; however, time-resolved MRA can also be performed to assess the flow dynamics. The single PC-enhanced MRA can be performed coronal or coronal-oblique. The authors use a 3-D spoiled gradient-echo sequence with a repetition time (TR) of 3.0 to 3.5 ms and an echo time (TE) of 1.0 to 1.2 ms, 25° to 30° flip angle, field of view (FOV) approximately 30 × 38 cm, approximately 192 × 256 matrix, and 1.6- to 2.0-mm slice thickness. Zero-filling in the in-plane and through plane direction is recommended to avoid stair-step artifacts when performing multiplanar reformatting (MPR).


Forearm


Imaging of the forearm should be performed with dedicated phased-array extremity coils. The authors use either an 8-element cardiac coil or 16-element extremity wrap coils. In general, a time-resolved MRA should be performed for imaging of the forearm. The authors recommend performing the excitation in sagittal orientation to avoid foldover (wrapping) artifacts from the body. The authors use 3-D time-resolved imaging of contrast kinetics (TRICKS) with a TR of 3.5 to 4.0 ms and a TE of 1.0 to 1.4 ms, 25° to 30° flip angle, FOV approximately 13 × 42 cm, approximately 320 × 320 matrix, and 1.6- to 2.0-mm slice thickness. Zero-filling is performed to facilitate interpolation, necessary to performing MPRs.


Hand


Imaging of the hand should be performed with dedicated phased-array extremity coils. The authors use a 16-element extremity wrap coil or a quadrature knee coil. In general, a time-resolved MRA should always be performed for imaging of the hand. We use 3-D TRICKS with a TR of 2.8 to 3.0 ms and a TE of 1.0–1.2 ms, 25° to 30° flip angle, FOV approximately 14.4 × 24 cm, approximately 224 × 256 matrix, and 1.2- to 1.8-mm slice thickness. Zero-filling is performed to facilitate interpolation, necessary to perform MPRs.




Clinical applications


Subclavian Steal Syndrome


The subclavian steal syndrome is caused by a stenosis or occlusion of the proximal subclavian artery or the brachiocephalic trunk. The stenosis/occlusion leads to retrograde blood flow in the vertebral artery to supply the affected arm with blood at the expense of the vertebrobasilar circulation, hence the name subclavian steal. This steal may cause symptoms, such as vertigo, presyncope, syncope, and even stroke, and is often exacerbated by exercise of the affected upper extremity. Atherosclerosis is the most common cause of subclavian stenosis, although any pathology, such as vasculitis, that affects the arch vessels can lead to this syndrome. The prevalence is less than 2% in the general population and a majority of cases are asymptomatic.


MRA not only can assess the severity of the stenosis/occlusion but also can demonstrate the presence of reversed flow in the vertebral artery. CE-MRA should be used to localize and quantify the degree of the stenosis of the proximal subclavian artery ( Fig. 2 ). TOF MRA with appropriate saturation bands, time-resolved CE-MRA, and PC MRA all can demonstrate retrograde flow in the vertebral artery (see Fig. 2 ). In the authors’ experience, PC imaging in a single axial plane with flow encoding in the superior/inferior direction is a rapid and unambiguous method to demonstrate flow reversal in a vertebral artery.




Fig. 2


Subclavian steal syndrome. ( A ) Coronal MIP image of a contrast-enhanced 3-D time-resolved MRA demonstrates short segment occlusion ( arrowhead ) at the origin of the right proximal subclavian artery. The occlusion is located just distal to the origin of the right common carotid artery but proximal of the origin of the right vertebral artery. Note the delayed enhancement of the right subclavian artery compared with the fully enhanced left subclavian artery. Contrast agent was injected from the left arm to avoid first-pass venous overlay of the area of interest on the affected right side. ( B ) Axial PC image demonstrates reversal (caudad) of flow in the right vertebral artery (RVA) ( white , arrow ) compared with cephalad flow in the left vertebral artery (LVA) ( black ) and common carotid arteries, right carotid artery (RCA) and left carotid artery (LCA) ( black ). The dotted line ( A ) indicates the level of the axial PC section ( B ).


Takayasu Arteritis


Takayasu arteritis is an uncommon form of large-vessel granulomatous vasculitis. The inflammatory process results in stenosis, occlusion, dilatation, or aneurysm formation in the arterial wall. It affects mainly the aorta and its branches but may also affect other vessels, such as the pulmonary arteries. The left subclavian artery may be involved in up to 50% of patients. Patients develop systemic symptoms during an initial inflammatory phase, such as malaise, fever, night sweats, weight loss, arthralgia, and fatigue. Patients may also develop anemia and elevated C-reactive protein, although these are not reliable for diagnosis. The initial phase is often followed by a secondary pulseless phase, characterized by vascular stenoses and occlusions from intimal narrowing. The disease is more frequently in the Asian population and in young women (<50 years of age).


MRA aids in diagnosis and allows for assessment of the extent of disease. CE-MRA allows detection of typical findings, such as luminal narrowing from arterial stenosis, occlusion, or dilatation ( Fig. 3 ). Fat-suppressed T2-weighted imaging is helpful to assess vessel wall edema and perivascular inflammation, particularly in the early, active stages. Postcontrast T1-weighted fat-saturated imaging shows thickening and enhancement of the vessel walls in patients with active disease.




Fig. 3


Takaysu arteritis in a 34-year-old woman. Coronal MIP of contrast-enhanced 3-D time-resolved MRA demonstrates a long-segment complete occlusion ( arrowhead ) of the left subclavian artery with supply of the distal artery via a collateral vessel. Also the descending aorta has a tapered narrowing ( arrow ). The other vessels are of normal caliber without occlusion or stenosis. Although this is an example of complete occlusion, long-segment stenosis is also common in Takayasu arteritis. Contrast was injected from the nonaffected right arm to avoid first-pass venous overlay of the symptomatic side.


Giant Cell Arteritis


Giant cell arteritis (GCA), also known as temporal arteritis or Horton disease, is the most common chronic vasculitis of medium- and large-sized arteries in persons ages 50 and older. The characteristic histopathologic features of GCA include granulomatous inflammation of the vessel wall with multinucleated giant cells. The involvement of epicranial arteries facilitates histopathologic confirmation. The superficial temporal artery, however, may not be involved, and establishing a diagnosis of GCA may be challenging if other vessels, such as the subclavian arteries, are affected.


MRA is helpful for diagnosis and assessment of disease extent in GCA. MRA can identify the localization and inflammatory changes of the arteries and guides clinicians to alternative biopsy sites when initial results are negative. MRA findings include segmental smooth arterial stenoses alternating with normal-caliber or dilated segments. The stenoses are smooth and tapered ( Fig. 4 ). Extraluminal findings may include wall thickening, hyperintensity in the vessel wall on fat-suppressed T2-weighted images, and wall enhancement on delayed-phase imaging. In general, the findings of GCA are often indistinguishable from Takayasu arteritis. Patient age, presentation, and other clinical information, including biopsy and/or laboratory values, may be needed to distinguish the 2 entities. Distinguishing these 2 types of arteritis based on clinical information is usually straightforward.


Sep 18, 2017 | Posted by in MAGNETIC RESONANCE IMAGING | Comments Off on Magnetic Resonance Angiography of the Upper Extremity
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