Thermal Imaging in the Investigation of Deep Venous Thrombosis

Harding, J. Richard. St. Woolos Hosp., Newport (U.K.)

Preliminary assessment of clinically suspected deep venous thrombosis (DVT) of the lower limb by

thermography avoids the need for over one third of venograms or duplex Doppler ultrasound scans.

Clinical diagnosis of DVT is notoriously unreliable - hence the need for an accurate means of clini-

cal investigation. Untreated DVT is dangerous as it can progress to pulmonary embolism (PE)

which is frequently fatal or life-threatening. Treatment of DVT by anticoagulation poses risks of its

own however, and should not be undertaken without a confirmed diagnosis. Thermal imaging is

quick, simple, non- invasive, risk-free, cost-effective and highly sensitive in the initial investigation

of suspected DVT; a negative thermogram excludes DVT and avoids the necessity for further inves-

tigation. Thermal imaging is, however, non-specific; a positive thermogram has a number of possi-

ble causes and is an indication for further assessment by venography or Doppler ultrasound to con-

firm or exclude DVT. Thermography should be considered the initial investigation of choice in

clinically suspected DVT, proceeding to venography or Doppler ultrasound only when thermogra-

phy is positive.

INTRODUCTION

Deep venous thrombosis (DVT) poses an important diagnostic challenge in medical practice. Along with

secondary pulmonary embolism (PE), DVT is a significant cause of emergency hospital admission, and

also occurs in patients already undergoing hospital in-patient investigation or treatment; post-operative pa-

tients and those suffering from malignant disease (especially abdominal and pelvic malignancy) are par-

ticularly at risk. The major complication of DVT is of fragments of blood clot (thrombi) becoming de-

tached (emboli) and passing through the veins of the lower limb, the inferior vena cava, and the right

atrium and right ventricle of the heart into the pulmonary arterial circulation of the lungs where they be-

come lodged in, and occlude branches of the pulmonary arterial tree as pulmonary emboli causing pulmo-

nary infarction of the distal perfused pulmonary tissues. This is a serious and life-threatening event imme-

diately fatal in 30% of cases [1]. Fatal pulmonary embolism causes over 20,000 deaths annually in Great

Britain and accounts for 10% of all deaths in hospital [2].

DVT is treated by anticoagulation; this treatment has risks, in particular haemorrhage, and is expensive and

time-consuming and should not be undertaken without a confirmed diagnosis of DVT or PE. Clinical diag-

nosis of DVT is notoriously unreliable [2], hence the need for reliable investigations to confirm or exclude

DVT.

The traditional method of investigation of DVT, still regarded as the "gold standard", is lower limb veno-

graphy (phlebography)[3], which outlines the deep venous system of the lower limb with radiographic con-

trast medium giving excellent detail of anatomy and pathology. This suffers the disadvantages of being in-

vasive, at best uncomfortable for the patient if not painful, with risks of allergy to intra-venous injection of

contrast medium (at worst fatal), the potential of dislodging emboli from lower limb venous thrombi due to

the forced volume injection of contrast, irritation of the venous endothelium by the contrast medium actu-

ally causing DVT when not already present, precipitation of cardiac failure in susceptible subjects due to

the fluid load of hypertonic contrast medium drawing fluid into the vascular space, and exposes the patient

to ionising radiation with its attendant risks. Venography is also expensive for consumable materials

(mostly radiographic contrast medium and film).

Various other methods of investigation of DVT have been utilised including isotope scanning for lower

limb thrombosis, light reflection rheography, impedance plethysmography and measurement of C reactive

protein or fibrin degradation products, but none have gained widespread acceptance due to various draw-

backs.

In recent years, duplex Doppler ultrasound has gained popularity [4], replacing venography in many insti-

tutions, but it has the disadvantages of being time-consuming, operator dependent, interpretation being to

some extent subjective, and exclusion of DVT confined to the calf veins can be difficult or impossible [5].

Thermography was first described in the investigation of DVT in 1972 [6]. It has been shown that thermo-

graphic investigation of DVT can avoid the need for further investigation by venography or duplex Doppler

ultrasound [7]. Whilst thermography is non-specific, it has the advantage of being highly sensitive

(approaching 100%) [8], and is non-invasive, risk-free, does not expose the patient to ionising radiation, is

not operator dependent or subjective in interpretation, and it is simple, quick and cheap to perform.

In acute DVT, the presence of propagating thrombus in the deep veins causes increased skin temperature in

the affected lower limb due to the action of vaso-active amines on the capillary skin circulation [9]; thermal

imaging detects this increased skin temperature along with loss of the normal 3oc temperature gradient

down the affected lower limb [10].

The aim of this study was to prospectively assess clinically suspected DVT in order to determine what pro-

portion of patients could avoid further investigation by virtue of normal thermography.

METHODS

1,000 patients with clinically suspected DVT were investigated by thermography by a single operator over

a period of 6 years 3 months. Thermal imaging of the lower limbs was initially performed by liquid crystal

thermography using the NovaTherm apparatus, followed by imaging with an Aga Thermovision 782 infra-

red camera. The lower limbs are first equilibrated with normal room temperature for 10 minutes by posi-

tioning the patient supine with both legs fully exposed and slightly elevated, with the heels supported, al-

lowing free circulation of air over the skin, away from localised sources of heat or cold such as radiators,

direct sunlight or draughts. Images are then obtained of the anterior aspect of both thighs, the antero-medial

aspects of the calves, and the posterior calves.

RESULTS

There were 362 male (36.2% of total) and 638 female patients (63.8% of total). The males had an age range

of 22-91 years and the females an age range of 15-96 years with a mean age of 65.0 years for males and

63.5 years for females.

Thermal imaging was not possible in two patients due to lack of patient co-operation. There was no differ-

ence in result in patients examined by both liquid crystal thermography and I-R thermography. Thermogra-

phy was normal in 374 patients out of 1,000 (37.4%) i.e. no increased skin temperature or loss of the nor-

mal temperature gradient was demonstrated in the lower limb in which DVT was suspected; these examina-

tions were thus negative for DVT and no further investigation was undertaken. There were no cases of

clinically suspected or proven PE on follow-up of these patients.

Of the 624 patients with increased temperature in the lower limb in which DVT was suspected, i.e. positive

thermography, DVT was confirmed by venography and/or duplex Doppler ultrasound in 297 (47.6%), ex-

cluded in 211 (33.8%) and no further investigation was possible in 116 (18.6%), due to lack of accessible

veins, patient refusal to undergo venography, unavailability of Doppler ultrasound in the earlier years of

this study, or request for no further investigation to be undertaken by the referring clinician.

DISCUSSION

Thermal imaging avoided the need for further investigation by venography or duplex Doppler ultrasound in

37.4% of patients. Thermal imaging offers considerable financial savings over the more expensive conven-

tional investigations which it has been demonstrated can be avoided in over one third of cases.

By comparison with venography, thermography results in the most significant revenue savings, assessed at

US $26,350 (Canadian $37,230) per 1,000 patients. This potential saving is of great importance to health

providers with cash constraints. Compared with duplex Doppler ultrasound there is less financial saving, as

thermography costs about half as much as Doppler for consumables, but there is considerable saving of

skilled ultrasonographer and equipment time.

Thermography has the advantage of being totally non-invasive and risk-free and quick and easy to perform.

It is suggested that the ideal investigative pathway in clinically suspected DVT should be initial thermogra-

phy, proceeding to Doppler ultrasound only when thermography is positive, and finally proceeding to

venography only after negative or equivocal duplex Doppler ultrasound.

ACKNOWLEDGMENTS

Financial support has kindly been provided by The U.S. Department of Defense Agencies. I am indebted to

Professor E F J Ring, Royal National Hospital for Rheumatic Diseases, Bath, UK, for the provision of an

Aga Thermovision 782 I-R thermal imaging camera.

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