Understanding the Radiological Diagnosis of Deep Vein Thrombosis

Venous thromboembolism (VTE) comprises two related conditions: pulmonary embolism (PE) and deep vein thrombosis (DVT). Deep vein thrombosis remains a significant healthcare problem because it is associated with complications that contribute to high morbidity and mortality, including acute and chronic pulmonary embolism, pulmonary hypertension, and post-thrombotic syndrome. DVT usually begins in the calf veins but can extend proximally and lead to life-threatening pulmonary embolism. In clinical practice, prevention and treatment of this disease are essential; therefore, early and accurate diagnosis, including precise assessment of the location and extent of DVT, is particularly important.

Significance of Venography

For a long time, the "gold standard" for diagnosing DVT was venography (phlebography), the only radiological method that allowed precise localization of a suspected thrombus in the deep veins of the legs or pelvis and in the inferior vena cava. During the examination, an iodine-containing contrast agent is injected into a vein on the dorsum of the foot, and a series of X-ray images are obtained as the contrast flows through the venous system. At the site of a thrombus, interruption of contrast flow within the vein can be observed as a filling defect, or the entire vein may fail to fill with contrast and therefore remain invisible during the examination.

The procedure is painful for the patient, involves considerable radiation exposure, requires a large amount of contrast material, and can only be performed by specialists experienced in the technique at major medical centers. Nowadays, venography is rarely performed in routine clinical practice. It is mainly used when complex venous anatomy cannot be adequately assessed by other methods or when an anti-embolic filter needs to be inserted into the inferior vena cava to protect a patient with leg DVT from the life-threatening complication of pulmonary embolism. Filters can be implanted through the femoral vein of the thrombosed leg and positioned below the level of the renal veins.

Because of the limitations of diagnostic methods for assessing the deep veins and diagnosing thrombosis, other radiological techniques have also been investigated.

Ultrasound Diagnostics

The possibility of examining blood vessels using ultrasound (US) emerged approximately 50 years ago, around 1964, with the development of the first Doppler ultrasound devices. The examination is non-invasive, does not involve ionizing radiation, does not require contrast material, is easy to perform, including at the bedside when necessary, and can be repeated multiple times.

Several years after the introduction of continuous-wave Doppler devices, pulsed-wave Doppler systems were developed, allowing blood flow velocity in the examined vessel to be detected and measured. Approximately 10 years later, around 1980, equipment was developed that enabled a pulsed Doppler signal and a two-dimensional image of the examined organ to be displayed simultaneously on the ultrasound screen. This type of ultrasound equipment is known as a duplex scanning system.

Further development of these systems, around 1990, led to the emergence of three-dimensional ultrasound systems, in which a three-dimensional reconstruction is generated from the acquired planes.

Ultrasound assessment of blood flow in organs is based on measurements of blood flow direction because ultrasound frequency changes proportionally to blood flow velocity and direction: the more intense the blood flow, the higher the ultrasound frequency. Usually, both continuous-wave and pulsed-wave Doppler modes are used when examining blood flow with ultrasound:

during continuous-wave scanning, overall high blood flow velocity can be measured, but blood flow at a specific, diagnostically selected location cannot be assessed;

during pulsed-wave scanning, blood flow can be recorded within a locally selected area of defined depth and volume, but very high blood flow velocities cannot be measured.

Therefore, both Doppler ultrasound techniques are used in blood flow assessment. Power Doppler reflects the strength of the Doppler signal, which depends on the number of moving red blood cells. It is highly sensitive to slow blood flow but cannot determine its direction.

With color-coded blood flow imaging, the direction of blood flow toward the ultrasound probe is displayed in red, while flow away from the probe is displayed in blue on the two-dimensional ultrasound image. When power Doppler signals are used, blood flow, regardless of its velocity or direction, is displayed in a yellow color spectrum. When both color and spectral Doppler measurements are performed during the same ultrasound examination, the technique is referred to as triple real-time scanning.

Venous ultrasound examination includes several methods:

• compression ultrasonography (images are obtained in simple B mode while the probe is used to compress the area of the visible veins) is most suitable for evaluating the proximal veins of the leg: v. femoralis and v. poplitea;
• duplex ultrasonography combines a B-mode image with a color Doppler signal;
• Doppler imaging alone.

In current clinical practice, the diagnosis of venous thrombosis still largely relies on the compression method. DVT is considered absent when the vein collapses completely under compression, whereas DVT is suspected when the vein does not collapse completely.

Ultrasonography has certain limitations. It is less accurate for evaluating thrombosis in some femoral vein segments, while intestinal gas makes assessment of the deep pelvic veins difficult. The method is also less suitable in obese patients and those with leg edema.

Computed Tomography

In some cases, to avoid angiographic examination and more accurately assess the structure of the tissues surrounding the leg blood vessels, computed tomography angiography may be performed. Computed tomography (CT) is an X-ray examination method based on the uneven absorption of X-ray radiation in tissues and organs. During a CT scan, the X-ray tube emits a narrow beam of X-ray radiation and rotates around the object being examined.

When examining the limbs and their anatomical structures, larger-volume CT scans are important – helical multidetector CT is performed. This examination provides continuous spiral-shaped volumetric information, from which, using special software, a spatial reconstruction of the organs being examined is created. Various reconstruction techniques can be used to visualize blood vessels, such as multiplanar reconstructions, maximum intensity pixel reconstructions, surface-shaded reconstructions, and volume rendering. When applying the latter, other structures that interfere with image evaluation, such as bones and soft tissues, are removed from the final CT image. All these image reconstruction algorithms can also be applied during other sectional examinations, such as MRI.

By performing helical multidetector CT with contrast injected into a vein, angiography is performed, and three-dimensional contrast-enhanced images of blood vessels can be created. During CT venography, about 80% less contrast material is used than during venography. Iodine-based contrast material is injected into the cubital vein, and after a certain period, the legs are scanned from the knees (or ankles) upward, including the pelvis (acetabular wings). Thrombosis is indicated by the absence of contrast material in a certain segment of the vein – a filling defect (Figures 3, 4, 5). CT examination of the deep veins of the legs can be performed together with pulmonary angiography to assess pulmonary embolism.

The advantage of CT venography, like MRI, is the ability to obtain cross-sectional images. They can also reveal non-vascular pathologies, such as causes of external vein compression (adenopathy, masses in the pelvis) that can cause DVT. The latest CT machines emit a lower radiation dose during the examination than older ones, allow various reconstructions, and provide three-dimensional images, facilitating diagnosis.

It is also important to consider the contraindications for CT venography: the examination cannot be performed in patients allergic to iodine or with impaired kidney function; pregnant women; patients experiencing claustrophobia or unable to remain still during the examination; children; patients with metal implants that cause artifacts interfering with image evaluation; and severely obese patients. Atherosclerotic changes in the vessel walls also interfere with the assessment of changes in the vein lumen – they can be difficult to distinguish from true DVT.

Magnetic Resonance Imaging

When DVT is suspected in patients who are allergic to iodine or cannot be exposed to ionizing radiation, magnetic resonance (MR) imaging can be performed. MR venography can be performed in pregnant women when deep vein thrombosis in the pelvis needs to be diagnosed.

The time-of-flight (TOF) method is used to examine peripheral leg blood vessels (veins). When performing MR angiography using this method and acquiring images in three-plane projections, fast blood flow and small blood vessels are evaluated, while two-plane images provide more information when examining a larger area and assessing slower blood flow.

This method has certain limitations: possible circulatory disorders can weaken the MR signal, long segments of blood vessels may be inadequately covered, and some tissues, such as fat surrounding blood vessels, may obscure them.

During MR angiography using intravenous paramagnetic contrast agents and obtaining high-quality spatial reconstructions, blood vessels are visualized more distinctly against the darker background of surrounding tissues, enabling rapid examination of a large anatomical area and providing higher-quality images. It is also important that vessel diameters can be accurately assessed regardless of blood flow direction. However, when evaluating retrograde or collateral circulation, as well as in the presence of stents, imaging inaccuracies or artifacts may occur.

Thermographic Circulatory Studies

In some countries around the world, including Lithuania, thermographic measurements of human body thermal radiation can be used in specific cases to assess the condition of peripheral veins. One of these methods is infrared thermography, which helps determine the intensity of blood circulation.

It can be applied to evaluate deep vein thrombosis or other disorders of limb circulation, necrosis, or areas with impaired blood perfusion where hypo- or hyperthermal zones are formed. Unfortunately, this thermographic technique is not yet routinely used in Lithuania, and examinations are performed only in individual private diagnostic centers.

Source: "Lithuanian Doctor's Journal"