Imaging of Peripheral Vascular Fistulas and Arteriovenous Malformations

Narasimham Dasika

Arterial and venous malformations, also referred to as vascular anomalies, are a result of anomalous development of the vascular system during embryogenesis and are present from birth. Imaging studies and arteriography support the classification adopted by the International Society for the Study of Vascular Anomalies (ISSVA), modified from an earlier classification proposed by Mulliken and Glowacki. This classification differentiates vascular anomalies into slow-flow malformations, high-flow malformations, and vascular tumors. Invasive studies such as arteriography and closed-system venography are not indicated for diagnosis.

Imaging Modalities

Multiplanar imaging with magnetic resonance imaging (MRI), ultrasound (US), and sometimes computed tomography (CT) is essential to eliminate misclassification and to show the exact anatomic extent of the lesions and associated hemodynamic changes. Angiographic imaging sequences, three-dimensional (3-D) reconstructions with these imaging modalities, are extremely helpful for proper treatment planning and follow-up. Invasive angiography is recommended only during endovascular treatment. Although MRI is the imaging modality of choice in the evaluation of vascular anomalies, US and CT also have a role in the pediatric population and in specific clinical situations.

Magnetic Resonance Imaging And Magnetic Resonance Angiography

MRI and magnetic resonance angiography (MRA) are noninvasive modalities of choice in the imaging of vascular anomalies. MRA can be performed with or without intravenous contrast agents. Imaging with specific pulse sequences such as time-of-flight (TOF), phase contrast (PC), and steady-state free precession (SSFP) can be obtained without contrast to produce images with bright blood in vascular structures from flow-related enhancement. This imaging is generally referred to as noncontrast MRA (NC-MRA). A second method of visualizing vascular structures, by altering T1 relaxation characteristics with addition of gadolinium-based contrast agents, is called contrast-enhanced MRA (CE-MRA).

MRI and MRA are complementary to each other in the diagnosis and comprehensive evaluation of these lesions. MRI has the ability to depict anatomic extent within the soft tissues and bone, whereas MRA images vascular structures and flow dynamics within these tissues. The high-contrast resolution of MRI and MRA with specific sequences, large field of view, and 3-D reformation shows focal, multifocal, and diffuse lesions; details of contiguous extent of the lesion from subcutaneous tissue into muscles, tendons, and bone cortex; and extension into the bone marrow. The specific signal properties differentiating blood flowing with various velocities and flow patterns, depiction of feeding arteries and draining veins, pressure effects on adjacent organs, relation to neural structures, and 3-D imaging are very useful in endovascular or percutaneous access planning. Placement of external markers over the symptomatic component of large lesions is helpful for percutaneous treatment. Because vascular anomalies are common in children, imaging without ionizing radiation with MRI is a significant advantage for diagnosis and follow-up.

Imaging of vascular anomalies with MRI is a very complex process because it involves multiple sets of images obtained with specific protocols to enhance contrast between various tissues and also to differentiate normal from abnormal. A simplified summary of pulse sequences frequently used in evaluating vascular anomalies and findings is shown in Table 1.

TABLE 1

Frequently Used Pulse Sequences and Common Findings in the Evaluation of Vascular Anomalies

Sequence	Capillary Malformations	Venous Malformations	Arteriovenous Malformations	Lymphatic Malformations	Arteriovenous Fistulas	Hemangiomas
Spin echo T1-weighted for anatomic detail	Low signal Only skin or subcutaneous tissue No flow voids	Low to intermediate signal intensity Darker No flow voids	Large multiple tubular loops of flow voids in vague mass (arteries and veins with fast flow)	Low to intermediate signal intensity Cystic mass Dark septa	No mass Large tubular flow voids Artery and adjacent vein	Intermediate Flow voids around the lesion
Fast spin echo (FSE) T2-weighted or short T1 inversion recovery (STIR) (anatomic extent)	Bright signal No flow voids	High signal intensity Bright septated lesions	Same as above or rarely enhancement in veins	Increased signal to bright cystic lesions of varying sizes	No mass Large tubular flow voids Artery and adjacent vein	Bright mass surrounded by flow voids
3D TWIST, dynamic 3+ phases for flow dynamics	Normal artery enhancement Early filling of normal veins	No arterial or venous phase, but only slow delayed enhancement	Bright enlarged tufts of arteries and veins Nidus itself is the mass Early venous filling	No enhancement in any phase	Simultaneous arterial and adjacent venous enhancement	Veins appear marginally early
Pre and post CE MRA with 3-D GRE for enhancement	Enhancement is seen	Very delayed heterogeneous enhancement, dysplastic draining veins	Simultaneous filling of large arteries, nidus, and veins	No enhancement or only peripheral enhancement	Potentially shows exact area of fistula and often some venous stenosis	Intense early enhancement
Other findings	MR is not needed unless it is a feature of a complex syndrome	Multifocal or diffuse lesions Foci of low signal (phleboliths)	Focal or multifocal Recognition of single draining vein is helpful for treatment planning	Large cysts are seen in macrocystic LM	No mass, only vascularity Usually focal	Gradual reduction in size with age and fat replacement

Note: Noncontrast MRA sequences such as time of flight (TOF), phase contrast (PC), ECG gated 3-D half Fourier FSE (also called fresh blood imaging [FBI]), and balance steady state free procession (SSFP or bSSFP) sequences can also be used if gadolinium cannot be administered in patients with renal insufficiency.

CE, Contrast enhanced; ECG, electrocardiograph; GRE, gradient echo; LM, lymphatic malformation; MR, magnetic resonance; MRA, magnetic resonance angiography; 3-D, three-dimensional.

UltrasonOgraphy

High-resolution gray-scale US allows excellent visualization of most lesions and masses. The enlarged dysplastic veins and blood-filled cavities of slow-flow malformations are seen as compressible anechoic tubular tortuous channels and hypoechoic cystic lesions. Compression with the US probe shows emptying and filling of the spaces.

Doppler US allows hemodynamic assessment of a lesion and differentiates slow-flow from pulsatile high-flow malformations. Change in size with position, manual compression, and Valsalva maneuvers can be studied in real time. Phleboliths and calcifications are seen as hyperechoic foci. US examination of high-flow malformations shows enlarged pulsatile feeding arteries and large draining veins with pulsatile flow. Color Doppler ultrasound and duplex ultrasound show numerous vascular structures with turbulent high-velocity flow spectrum. Low-resistance flow in the large feeding arteries and high-velocity arterialized waveforms in the draining veins are seen. Tumors are seen as heterogeneous hypervascular infiltrative lesions in the soft tissues.

US imaging is an excellent, cost-effective imaging modality for diagnosis and classification of vascular anomalies in the clinic, but it is limited by small field of view, restricted penetration, and operator experience. It remains an excellent imaging method for screening, diagnosis, and follow-up of hemangiomas in children without any additional imaging studies or ionizing radiation.

Computed Tomography

CT is not the imaging modality of choice for evaluating vascular anomalies. The relatively low contrast resolutions and the high ionizing radiation from multiphase imaging following bolus injection are the limitations in other locations. However, CT and CTA are the choice for evaluating lesions in the lung and gastrointestinal tract. CT and CTA are also considered in patients who have contraindications for MRI such as pacemakers and intracranial vascular clips, intravascular coils, metal hardware, and claustrophobia. Other advantages include shorter scan times and detection of calcifications and phleboliths.

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