Introduction
Deep venous thrombosis (DVT) and its primary sequela, pulmonary embolism (PE) is associated with significant morbidity and mortality. The long- term sequelae of DVT may cause venous insufficiency.

Purpose
The diagnosis of DVT based solely on clinical signs and symptoms is difficult and unreliable. Duplex imaging is the examination of choice of the lower extremity veins, and is performed to assess the deep and superficial venous system to detect or exclude the presence of venous thrombosis.

Indications for Duplex
· Symptomatic lower limb/s
· Asymptomatic lower limbs with symptoms of PE
· Asymptomatic lower limbs with risk factors
· Surveillance of high risk patients
· Surveillance of post thrombotic episode

Duplex and Diagnostic Criteria
Evaluation of the lower limb venous system utilises a combination of B Mode, spectral waveform analysis and colour.

Characteristics of normal veins
· Compressibility
· Spontaneous flow
· Phasic flow
· Augmentable flow
· Competence

Characteristics of venous thrombus
· Partial or total incompressibility
· Venous distension (acute)
· Visualisation of thrombus
· Abnormal spectral waveforms
· Poor/absent colour filling

Conclusion
Improvement in diagnosis and treatment of DVT depends on early, accurate diagnosis, recognition of risk factors and appropriate treatment.
It is important for sonographers to have a good working knowledge of lower extremity venous anatomy and anatomical variations. Knowledge of risk factors, clinical implications, differential diagnosis, limitations and pitfalls of the study are also essential.