Imaging in Soft Tissue Vascular Malformation

Introduction:

Vascular malformations are congenital or acquired growths composed of blood vessels involving arteries, veins, lymphatics, and capillaries alone or in combination. According to the International Society for the Study of Vascular Anomalies (ISSVA), vascular anomalies are classified into vascular neoplasm and vascular malformation. Vascular neoplasm shows increased endothelial cell turnover, whereas vascular malformation is a structural abnormality without cell turnover of arteries, lymphatics, capillaries, or veins. An MRI is an excellent imaging modality for evaluating soft tissue vascular malformation. It can show the location, the extent of the lesion, and its relation to the adjacent structures. MRI imaging is taken in three planes, including axial, sagittal, and coronal. Fat-saturated T2 weighted images and short tau inversion recovery (STIR) sequences in all three planes are helpful in the evaluation of the extent of the lesion. In these sequences, vascular malformation appears bright against a dark background.

What Are the Different Types of Vascular Anomalies?

A vascular malformation is classified into two major types: Slow flow and fast flow vascular malformations.

Slow Flow Malformation:

• Lymphatic Malformation

• Macrocytic.

• Microcytic.

• Mixed.

• Venous Malformation

• Distensible.

• Non-distensible.

• Capillary Malformation -

• Cutis marmorata telangiectasis.

• Congenital telangiectasis.

Fast Flow Vascular Malformation:

• Arterial malformation.

• Arteriovenous malformation.

• Arteriovenous fistula.

What Are the Imaging Techniques Used in the Diagnosis of Vascular Malformation?

Slow Flow Vascular Malformation:

1. Venous Malformation:

• Local and diffuse vascular malformation have different clinical presentations. A detailed clinical examination helps in differentiating slow flow from high-flow malformations.

• Ultrasound is the first choice of imaging modality used to detect vascular malformation.

• In ultrasound, slow-flow vascular malformation appears as a unilocular or multilocular heterogeneous lesion.

• Venous malformation occurs as a dysplastic vascular channel with pain, deformity, and impaired mobility.

• The venous malformation is often found in the head and neck region, trunk, and extremities. They present as compressible swelling, which increases on Valsalva.

2. Capillary Malformation:

• The capillary malformation is superficial and involves the mucosa. They occur in syndromes such as Parkes Veber, Sturge Veber, and Klippel-Trenaunay.

• The image findings of capillary malformation are similar to venous malformation, except the capillary malformation shows early enhancement compared with delayed enhancement in venous malformation.

3. Lymphatic Malformation:

• Lymphatic malformations are the second most common vascular malformation. It consists of chyle-filled cysts that are lined by endothelium.

• A lymphatic malformation is often present in early life and appears as compressible smooth masses in the axilla and posterior neck.

• In magnetic resonance imaging (MRI), lymphatic malformation appears as a hyperintense lesion on short tau inversion recovery (STIR) and T2 weighted images and iso to hypo intense on T1 weighted images due to the lack of flow void effect. On post-contrast images, it shows septal or peripheral enhancement. If the lymphatic malformation is associated with venous malformation, then the lesion shows diffuse enhancement.

• It is commonly found in the head and neck region and sometimes in the axilla, pelvis, and mesentery.

• Macrocytic lesions appear as soft masses with a cystic space of two centimetres or more. On ultrasound, macrocytic lesions are characterized by their anechoic appearance.

• Microcytic lesions appear as soft tissue with an overlying small vesicle layer.

• Macrocytic lymphatic malformation should be differentiated from venous malformation by the lack of phlebolith and the size of the chambers, whereas microcystic lymphatic malformation is easily distinguished from venous malformation, as venous malformation (VM) shows slow flow in most cases.

• In ultrasound, lymphatic malformation appears as a hypoechoic compressible lesion with no detectable blood flow on Doppler ultrasound.

• Cystic areas appear as compressible lesions in lymphatic malformations, whereas in venous malformation, they appear non-compressed.

Fast Flow Vascular Malformation:

Fast-flow malformations are pulsatile lesions often present during birth and increase in size when the child grows and show rapid growth during pregnancy, surgery, or puberty. Venous congestion leads to pain, haemorrhage, cardiac failure, and ulceration.

Arteriovenous Malformation:

• Arteriovenous (AV) malformation is a congenital lesion in which arteries are connected to veins without a capillary bed. This lesion contains a nidus in between arterial and venous beds. They are commonly found in bone, cranium, muscles, and subcutaneous fat. AV malformation is commonly found in the central nervous system and less common in the trunk and limbs.

• On ultrasound and Doppler ultrasound, arteriovenous malformation appears as blood vessels with a high systolic and diastolic flow with prominent arteriovenous shunting.

• Contrast-enhanced computed tomography (CT) scan helps in assessing the structure of the lesion.

• On MRI, arteriovenous malformation appears as a network of veins and arteries connected by a shunt. They show flow void in T1 and T2 weighted spin echo images and appear hyper-intense on T2 weighted gradient echo images that indicate rapid flow.

• In AV malformation, conventional MRI shows flow voids without enhancement or T2 hypersensitivity.

• In diagnostic angiography, AV malformation appears as dilated arteries and veins.

Arteriovenous Fistula:

• The arteriovenous fistula (AV) is commonly found in the brain and lacks a nidus between the arterial and venous beds.

• Like other vascular malformations, ultrasound is the first choice of imaging modality for the diagnosis of arteriovenous fistula.

• In ultrasound, the AV fistula appears as low resistance, high-frequency continuous flow with increased diastolic velocity.

• In MRI, arteriovenous fistula appears as high signal intensity on T1 weighted images.

• Lack of focal mass in serpiginous signal voids in T1 and T2 weighted images is an important diagnostic feature of AV fistula.

Conclusion:

Ultrasound is an effective imaging tool used to distinguish fast-flow lesions from slow-flow lesions. The use of Gadolinium in magnetic resonance imaging (MRI) can differentiate lymphatic malformation from venous malformation as they accumulate only in venous malformation. MRI provides detailed information on vascular malformation, which helps in treatment planning and detects the improvement in symptoms. Contrast-enhanced three-dimensional T1 weighted images help assess the vascularity of the lesion. A dynamic time-resolved three-dimensional fast gradient echo (GRE) sequence is helpful in the functional analysis of vascular malformation. This sequence provides images with a temporal and high spatial resolution which helps in distinguishing venous flow from arterial flow and thus reduces artifacts. T2 weighted GRE sequences help detect haemorrhage or calcification inside the lesion as high-intensity and high-flow vessels.