Steven C. Rose, MD #{149}William J. Zwiebel, MD #{149}Brent D. Nelson, MD #{149}Derek L. Priest, RVT .

Rhonda A. Knighton, RVT #{149}Joyce W. Brown, LPN-VT #{149}Peter F. Lawrence, MDBarry M. Stults, MD #{149}James C. Reading, PhD #{149}Franklin J. Miller, MD

Symptomatic Lower Extremity Deep VenousThrombosis: Accuracy, Limitations, and Roleof Color Duplex Flow Imaging in Diagnosis’

From the Departments of Radiology (S.C.R., W.J.Z., B.D.N., D.L.P., R.K., J.W.B., F.J.M.), Surgery

(P.F.L.), Internal Medicine (B.M.S.), and Family and Preventive Medicine (J.C.R.), University ofUtah Medical Center, 50 N Medical Dr. Salt Lake City, UT 84132; and the Veterans AdministrationMedical Center, Salt Lake City. From the 1989 RSNA annual meeting. Received November 28,1989; revision requested January 19, 1990; revision received February 2; accepted February 20. Ad-dress reprint requests to S.C.R.

C. RSNA 1990

639

Cardiovascular Radiology

Color duplex flow imaging (CDFI)permits pain- and risk-free directimaging of the deep venous systemof the lower extremities. To pro-spectively ascertain the accuracyand limitations of this technique,CDFI was performed in 75 lowerlimbs of 69 consecutive patients re-ferred for venographic evaluation ofclinically suspected lower extremitydeep venous thrombosis (DVT). TheCDFI study was obtained within 24hours of the contrast venogram.Both studies were interpreted with-out knowledge of the patient’s clini-cal findings or the results of theother test. Contrast venography wasregarded as the standard for diagno-sis of DVT. Accuracy was 99% fordetection of DVT above the kneeand 81% below the knee. Sono-graphic evaluation of the calf veinswas technically adequate in 60% oflimbs; accuracy was 98% in thisgroup. In the 40% of limbs withtechnically limited CDFI studies ofthe calf, accuracy decreased to 57%.Although small nonocclusivethrombi occurred infrequently inthis series of symptomatic patients,CDFI missed three of four suchthrombi. It is concluded that CDFI,when not technically compromised,is sufficiently accurate to defini-tively diagnose symptomatic lowerextremity DVT.

Index terms: Extremities, thrombosis, 93.751 #{149}

Extremities, US studies, 44.12984, 45.12984

Thrombosis, US studies, 93.12984 #{149}Thrombo-

sis, venous, 93.751 #{149}Ultrasound (US), Doppler

studies #{149}Veins, stenosis or obstruction, 93.751 #{149}

Veins, US studies, 93.12984

Radiology 1990; 175:639-644

D EEP venous thrombosis (DVT) of

the lower extremities is a com-

mon and serious disorder that re-

quires diagnosis and treatment be-

fore life-threatening complications

(eg, pulmonary embolization) occur.

Unfortunately, the primary methods

for diagnosis of DVT have been sub-

stantialiy underutilized because of

associated pain and complications

(contrast venography), suboptimal

diagnostic accuracy (most noninva-

sive techniques), or logistic encum-

brances (serial impedance plethys-

mography or radiofibrmnogen uptake

tests) (1). Postvenography phlebitis is

reported to occur in 8%-9% of pa-

tients, approximately one-third of

the cases involving the deep venous

system (2,3).

Color duplex flow imaging (CDFI)

displays blood flow within vascular

structures in colors. The color and

hue depend on the velocity and di-

rection of blood flow relative to the

transducer. Veins are readily distin-

guished from arteries by flow direc-

tion and Doppler flow characteris-

tics. Thrombus is identified as a flow

void within the imaged lumen (Fig

1). Because CDFI can directly demon-

strate most of the deep venous sys-

tern in the lower extremities, we hy-

pothesized that this technique could

enable detection of symptomatic

DVT with an accuracy close to that of

contrast venography and substantial-

ly better than the reported accuracy

of other noninvasive techniques. Us-ing contrast venography as the refer-

ence test, we evaluated the accuracy

and technical limitations of CDFI in

69 consecutive patients with clinical-

ly suspected lower extremity DVT.

MATERIALS AND METHODS

All patients with symptoms or signs

(pain, tenderness, or swelling) suggesting

lower extremity DVT who underwentsuccessful contrast venography, regard-

less of result, were offered CDFI at no ad-

ditional cost, provided the latter study

could be performed within 24 hours of

venography. Nineteen patients were ex-

cluded from the study because elective

CDFI was not performed within 24 hours.All patients offered free CDFI consented

to be evaluated. This study included 69

consecutive patients who underwent

both venography and CDFI. Bilateral

studies were performed in six patients

with bilateral symptoms; therefore, a total

of 75 lower extremities were evaluated. In

no case was the same extremity studied

more than once. The pelvic portion ofone extremity and the calf portion of a

second limb were excluded from the

study because those segments of the yen-

ograms were technically inadequate.

The venographic technique was a mod-

ification of that of Rabinov and Paulin

(4). Specifically, the patient was tilted onthe fluoroscopic table to at least 30#{176}re-

verse Trendelenburg (head elevated). In

all patients, both tight-tourniquet (ankle

and knee) and tourniquet-free views of

the calf were obtained. Venous opacifica-

tion was fluoroscopically monitored and

documented with overhead radiographs.On average, 120-150 mL of contrast mate-

rial (Conray 43%; Mallinckrodt, St. Louis)

was used in each lower extremity. Thestudy was concluded when the entire

deep venous system was demonstratedfrom the ankle to the iliocaval junction.

After all imaging was completed, the ye-

nous system was irrigated with 250 mL of

heparinized saline (4 U/mL).The sonographic technique used was a

modification of that published by Talbot

(5). A high-resolution CDFI ultrasound

(US) unit (Quantum Angiodynograph I;

Quantum Medical Systems, Issaquah,Wash) was used with a 3.0-, 5.0-, or 7.5-

MHz transducer coupled to a wedge-shaped water path. Transducer selection

depended on patient habitus and the

Abbreviations: CDFI color duplex flowimaging. CI = 95% confidence interval; DVT

deep venous thrombosis.

2. 3.

Figures 2, 3. (2) Contrast venography demonstrates a nonocclusive 2-cm-long thrombus(arrowheads) within a branch of the profunda femoris vein in a paraplegic patient with legswelling and fever. CDFI, which missed this thrombus, was generally not used in our series

to evaluate the deep portions of the profunda femoris vein. (3) Venogram shows nonocclu-

sive soleal sinus thrombus (arrowheads) missed on a technically adequate CDFI study. Al-

though the soleal sinuses are not routinely imaged on venous sonograms, new-onset focal

calf pain and swelling 4 days after knee arthroplasty should have directed attention to thispossibility.

640 #{149}Radiology

Figure 1. CDFI display of blood flow. Col-or-encoded flow allows unequivocal differ-entiation of vessels (colored) from surround-

ing tissues (gray), arterial flow (red) from

venous flow (blue), and patent venous lu-mens (blue) from thrombosed venous lu-

mens (black; ie, anechoic thrombus).

thickness of overlying soft tissues. Instru-ment controls (software mediated) wereset to detect low (minimum, 0.3 cm/sec)or medium (minimum, 0.7 cm/sec) flow

rates, adjusted to the venous flow velocityof each patient. The patient lay supine onan examination table tilted to 20#{176}-30#{176}re-verse Trendelenburg (head elevated). Theinvolved lower extremity was rotated ex-ternally 30#{176}-45#{176}with 20#{176}-40#{176}of flexionat the hip and knee. Typical examinationtime was 20-30 minutes per extremity.

Direct CDFI of the iliac veins was per-formed with the 3.0- or 5.0-MHz trans-ducer. Because of the depth of the iliacveins and their obscuration by overlyingbowel gas, only 3-5 cm of external iliacvein could be imaged in most patients. Il-iac vein patency was further assessed bymeans of CDFI study of flow signals inthe common femoral vein. Normal flowin the common femoral vein characteristi-

cally produces spontaneous centripetalDoppler signals, with respiration-in-duced modulation of flow velocity, cessa-tion of flow during a vigorous Valsalvamaneuver, and augmentation of flowwith compression of the calf.

Below the inguinal ligament, the 5.0- or7.5-MHz transducer was used. Longitudi-nal image planes over the anteromedial

thigh were used to evaluate common fem-

oral, superficial femoral, deep femoral,and greater saphenous vein anatomy, pa-tency, and blood flow characteristics. Thetransverse transducer orientation was usedto confirm anatomic relationships and to

assess vein compressibility (coaptation of

vein walls with minimal pressure).

The distal superficial femoral and pop-liteal veins were evaluated with the

transducer placed posteriorly in the pop-liteal fossa. The posterior tibial and pero-

neal veins were examined with the trans-

ducer positioned posteromedially along

the calf. Longitudinal image planes were

used in the popliteal region and calf toevaluate venous anatomy, patency, andaugmentation of venous flow with foot or

distal calf compression. Transverse trans-

ducer orientation in the popliteal area

was used to confirm anatomic relation-ships and to assess vein compressibility.

The anterior tibial veins were examined

only if anterolateral calf symptoms were

present. The anterior tibial veins were ex-amined in longitudinal sections, with the

transducer positioned anterolateral to the

tibia. In addition to examination of the

deep venous system, regions of pain, ten-derness, or local swelling were evaluatedfor evidence of superficial venous throm-

bosis or nonvenous disease.

CDFI of the veins was performed byone of three experienced vascular tech-

nologists. The studies were recorded on

videotape for blinded interpretation at alater time.

Contrast venograms were independent-ly double-read by two experienced angio-graphers (5CR., F.J.M.) who were blind-ed to both the clinical symptoms and in-

terpretation of the CDFI studies. Discor-

dant interpretation occurred infrequently

and was resolved by consensus. Similarly,

the CDFI studies were interpreted by one

of three experienced reviewers (W.J.Z.,P.F.L., F.J.M.) who were blinded to bothclinical information and venographic in-

terpretation.

Thrombosis was diagnosed venogra-

phically if there was a consistently

present intraluminal filling defect or seg-

mental venous occlusion that was not at-

tributable to venographic technique (eg,

tourniquet or needle placement). CDFI

diagnosis of thrombosis was based pri-

manly on the presence of a focal void

within the color-encoded blood flow im-

age or the absence of visible flow withina segment of a vessel. Usually the flowvoids were accompanied by intraluminalechogenicity, vein distention, and ab-

sence of normal vein compressibility. For

the purposes of this study, no attempt

was made to estimate thrombus age.Patient charts were retrospectively re-

viewed to confirm presenting symptomsand to determine which patients had a

history of DVT. Forty-seven of the pa-

tients (68%) were men, and 22 (32%) were

women. Mean patient age was 54 years(range, 21-92 years). Presenting symp-

toms included swelling (69 limbs), pain

(55 limbs), tenderness (28 limbs), erythe-

ma (eight limbs), and palpable cord (two

June 1990

a. b. c.

Volume 175 #{149}Number 3 Radiology #{149}641

Figure 4. Variable image adequacy of CDFI studies of the calf. (a) Technically adequate image of the posterior tibial and peroneal pairedbranch veins. (b) Increased sound attenuation caused by excessive accumulation of soft-tissue edema (hypoechoic subcutaneous striations)

impairs imaging of deep structures. (c) Multiple small collateral veins (red and blue dots) in a patient with previous DVT cause confusion

when an attempt is made to distinguish and follow the main conduit veins of the calf. In addition, the multiple acoustic interfaces contribute

to sound attenuation.

limbs). The mean duration of symptoms

was 14 days (range, i day to 6 months).Thirty patients (43%) were inpatients, and

39 (57%) were initially evaluated as out-

patients. DVT was found with venogra-

phy in 32 of the 75 (43%) lower extrem-

ities examined. For purposes of this

study, each lower extremity examination

was tabulated as if it were an indepen-dent event.

Ninety-five percent confidence inter-

vals (CIs) were calculated with the tech-nique described by Fleiss (6). Standard

two-by-two x2 tests were performed to

compare accuracies.

Overall

RESULTS

Correct identification of DVT with

CDFI (true-positive cases)-regard-

less of thrombus size, location, or ex-

amination quality-occurred in 27

limbs. Thirty-six limbs were correctly

identified as not having DVT (true-

negative cases). Five CDFI studies

were false-positive relative to venog-

raphy, and seven were false-nega-

tive. Overall sensitivity was 79%,

specificity 88%, positive predictive

value 84%, negative predictive value

84%, and accuracy 84% (CI 73.3%-

91.1%).

Anatomic Location

CDFI results for detection of DVT

above the knee (both iliac and femo-

ropopliteal venous segments) were as

follows: 25 true-positive, 49 true-neg-

ative, no false-positive, and one false-

negative result (sensitivity, 96%;

specificity, 100%; positive predictive

value, 100%; negative predictive val-

ue, 98%; and accuracy, 99% [CI

91 .8%-99.9%}).

For detection of DVT proximal to

the inguinal ligament (common and

external iliac veins), CDFI results

were true-positive in three limbs and

true-negative in 62 limbs, with no

false-positive or false-negative re-

sults. All three cases of identified iii-

ac vein DVT also had noncontiguous

DVT of the femoropopliteal segment.

In nine limbs, common femoral vein

thrombosis precluded assessment of

iliac vein patency with Doppler sig-

nal. The pelvic portion of one pa-

tient’s study was excluded because

the venogram was nondiagnostic

(underexposed film). Isolated iliac

vein thrombosis did not occur in this

series of patients. For the iliac veins,sensitivity, specificity, positive and

negative predictive values, and accu-

racy were 100%.

For detection of DVT between the

ingumnal ligament and knee (corn-

rnon femoral, superficial femoral,

deep femoral, and popliteal veins),

CDFI results were true-positive in 23

limbs, true-negative in 50, false-posi-

tive in none, and false-negative in

two. In one of the false-negative

cases the thrornbus was of small cali-

ber and less than 3 cm in length (Fig

2). The CDFI study in the other false-

negative case was of limited quality

because of the presence of soft-tissue

edema and extensive collateral vein

formation that obscured a segment of

partially recanalized old thrombus in

the distal superficial femoral vein.

For the region between the inguinal

ligament and the knee, sensitivitywas 92%, specificity 100%, positive

predictive value 100%, negative pre-

dictive value 96%, and accuracy 97%.

For detection of thrombus in the

infrapopliteal deep veins (tibioper-

oneal trunk and trunks and branches

of the posterior tibial and peroneal

veins), CDFI results were true-posi-

tive in 22 limbs, true-negative in 38,

false-positive in six, and false-nega-

tive in eight. The calf portion of one

patient’s study was excluded because

numerous overlapped veins rendered

the venogram nondiagnostic for calfDVT. In this region, sensitivity was

73%, specificity 86%, positive predic-tive value 79%, negative predictive

value 83%, and accuracy 81% (CI

70.0%-88.9%).

Eleven patients had venographic

DVT of the anterior tibial veins. In

no case, however, was DVT isolated

to the anterior tibial veins; each of

these patients had concomitant femo-

ropopliteal DVT (extensive in 10 pa-

tients). Supplementary CDFI studies

of the anterior tibial veins were per-

formed in only two patients with an-

terolateral calf symptoms. One study

642 #{149}Radiology June 1990

was technically adequate and had

true-positive results. The other was

technically limited and had false-

positive results.

Calf Studies

The calf was the most difficult re-

gion to visualize with CDFI. In 45 ex-

tremities (60%), CDFI of the calf was

technically adequate (ie, the tibioper-

oneal trunk and all portions of the

posterior tibial and peroneal veins

were imaged, or definite thrombus

was identified). In the group with

technically adequate examinations,

the CDFI study was true-positive in

19 calves, true-negative in 24, false-

positive in none, and false-negative

in one. One calf was excluded be-

cause of a nondiagnostic venogram.

In the false-negative case the nonoc-

clusive thrombus was within a soleal

sinus and was small (less than 3 cm

long) (Fig 3). For this subgroup of ad-

equate calf studies, sensitivity was95%, specificity 100%, positive pre-

dictive value 100%, negative predic-

tive value 96%, and accuracy 98%

(CI = 87.0%-99.9%).

DVT was absent above the knee in

50 limbs (those with potential isolat-

ed calf DVT). Twenty-nine of these

50 extremities (58%) had technically

adequate CDFI studies. Four calves

had true-positive results for DVT, 23

true-negative, none false-positive,

and one false-negative. One calf was

excluded because of a nondiagnostic

calf venogram. For technically ade-

quate CDFI studies of the calf in the

setting of possible isolated calf DVT,

sensitivity was 80%, specificity 100%,

positive predictive value 100%, nega-

tive predictive value 96%, and accura-

cy96%.

In 30 extremities (40%), technical

factors limited the ability to visualize

the deep venous system of the calf

(Fig 4). In these cases, the recorded

causes of compromised visibility

were prominent calf swelling (18limbs), obscuration by numerous ad-

jacent collateral veins (seven limbs),

obesity (three limbs), local pain (one

limb), small-caliber nondistended

vessels in a paraplegic patient (one

limb), an open calf wound (one

limb), overlying wound adhesive

strips (one limb), a combative patient

(one limb), and technical oversight

(one limb). In two extremities, the

reason for poor visualization was not

recorded. Eight calves had more than

one cause for a limited study. Results

in calves with technically limited

studies were true-positive in three

cases, true-negative in 14, false-posi-

tive in six, and false-negative in 5ev-

en. Sensitivity was 30%, specificity

70%, positive predictive value 33%,

negative predictive value 67%, and

accuracy 57% (CI 37.7%-74.0%). A

x2 test comparing the accuracy of ad-equate CDFI studies of the calf (98%)

with the accuracy of limited calf

studies (57%) was statistically signifi-

cant (P < .001).

Twenty-one of the 50 extremities

(42%) without above-knee thrombus

(those with potential isolated calf

DVT) had technically limited studies.

In this group, CDFI results were true-

positive in none, true-negative in 12,

false-positive in five, and false-nega-

tive in four. Sensitivity was 0%,

specificity 71%, positive predictive

value 0%, negative predictive value

75%, and accuracy 57% (CI 34.4%-

77.4%).

Small Thrombi

Four extremities had small-caliber

nonocclusive thrombi less than 3 cm

long on venograms. Two were isolat-

ed thrombi: one in a deep femoral

vein and the other in a large soleal si-

nus. CDFI missed both thrombi. Two

additional cases involved small

thrombi in calf veins of extremities

with concurrent occlusive iliofe-

moral DVT. CDFI showed one of

these small thrombi but missed the

other.

History of DVT

Nineteen patients (28%) (19 lower

extremity studies) had a history of at

least one episode of DVT in the ex-

tremity examined. When thrombus

size, location, and imaging quality

were disregarded, CDFI results were

true-positive for DVT in 1 1 cases,

true-negative in six, false-positive in

none, and false-negative in two. Sen-

sitivity was 85%, specificity 100%,

positive predictive value 100%, nega-

tive predictive value 75%, and accura-

cy 89%. Eleven of the 13 patients

with venographically demonstrated

DVT had above-knee thrombus; all

1 1 thrombi were detected with CDFI.

The quality of the venous study of

the calf in patients with a history of

DVT was deemed adequate in eight

patients (42%). CDFI results were

true-positive in five cases and true-

negative in three, and there were no

false-positive or false-negative re-

sults. CDFI of the calf veins was tech-

nically inadequate in 1 1 patients

(58%), mainly because of calf swell-

ing and the presence of numerous

collateral venous channels. Among

these inadequate calf studies, CDFI

results were true-positive in one case,

true-negative in four, false-positive

in one, and false-negative in five.

DISCUSSION

The clinical diagnosis of DVT is

generally inaccurate, with a sensitiv-

ity ranging from 14% to 78% and a

specificity ranging from 4% to 21%,

depending on the clinical sign being

considered (7-1 1). Contrast venogra-

phy has been accepted as the defini-

tive modality for diagnosis of DVT.

Clinical follow-up has confirmed the

validity of withholding anticoagu-

lant therapy in patients with a nor-

mal lower extremity venogram (10).

Unfortunately, venography is associ-

ated with significant patient discom-

fort and a low level of risk for con-

trast material-induced nephropathy

and phlebitis, contrast material reac-

tions, and skin slough due to extrava-

sation of contrast material at the in-

jection site (2,3,12,13). For these

reasons, venography has been

underutilized and many patients

with lower extremity DVT remain

undiagnosed (1).

In an effort to improve patient

safety and tolerance, multiple nonin-

vasive techniques have been devel-

oped for the detection of DVT. Im-

pedance plethysmography measures

venous capacitance and the rate of

venous outflow from the lower ex-

tremities (14,15). In expert hands, im-

pedance plethysmography has a sen-sitivity and specificity ranging from

87% to 100% and 92% to 100%, respec-

tively, for detection of DVT above

the knee (16). However, these results

have not been consistently reproduc-

ible in other centers (17). Further-

more, the sensitivity of impedance

plethysmography for detection of

isolated calf DVT is only 17%-33%

(18). Multiple radionuclide tech-

niques have been developed but

have not been sufficiently accurate to

replace contrast venography (19-21).

Gray-scale US, usually supple-

mented with Doppler flow assess-

ment, has proved to be accurate for

detection of femoropopliteal DVT

(sensitivity, 88%-100%; specificity,

92%-100%) and occlusive thrombosis

of the common and external iliac

veins (1 1,22-32). One significant lim-

itation of standard duplex sonogra-

phy has been the difficulty many in-

vestigators have experienced in iden-

tifying and therefore assessing the

deep veins of the calf (11,22-32). A

second limitation has been the in-

ability to directly image the common

Volume 175 #{149}Number 3 Radiology #{149}643

and external iliac veins because of

their depth from the skin and the

presence of overlying bowel gas

(31,33). In most studies the patency

of the iliac veins has been assessed

indirectly by Doppler analysis of

blood flow in the common femoral

vein. False-positive results may be

caused by venous compression by

pelvic masses (24,27,33-35). Sources

of false-negative results from gray-

scale duplex studies include nonoc-

clusive thrombi, the development of

sizable collateral venous channels,

and the presence of anechoic throm-

bus (which occurs in as many as 56%

of patients with acute thrombus)

mimicking a patent lumen (36). Lack

of venous compressibility when gen-

tie pressure is applied accurately in-

dicates anechoic thrombus in regions

where the deep venous system lies

near the skin (1,22,24-32). Alterna-

tively, anechoic venous thrombus

may be missed in regions where ye-

nous compression is difficult, specifi-

cally the distal superficial femoral

vein as it courses through the adduc-

tor canal and the deep calf veins

(1,22,24-32,35).

Because CDFI vividly displays ye-

nous blood flow, this technique al-

lows direct imaging of the entire ye-

nous system of the lower extremities.

Unlike other noninvasive methods,

CDFI allows calf veins to be readily

examined in most patients (37,38).

The diagnostic accuracy of CDFI for

detection of above-knee DVT in our

series (99%) compares favorably with

results achieved with other noninva-

sive modalities, particularly gray-

scale sonography. Although iliac

vein patency could not be evaluated

in nine patients, this difficulty was of

no consequence because evaluation

was rendered unnecessary by the si-

multaneous presence of sonographi-

cally identified common femoral

vein thrombus. Since neither isolated

nor nonocclusive iliac vein thrombo-

sis occurred in this series of patients,

the ability of CDFI to enable detec-

tion of such infrequent but potential-

ly important thrombus remains un-

tested. The large caliber and relative-

ly superficial course of the femoro-

popliteal veins allowed a technically

adequate study of the thigh in all pa-

tients but one.

The lower diagnostic accuracy (81%

overall) of CDFI for detection of in-

frapopliteal DVT in this series was

due to a relatively high frequency of

technically limited calf studies (40%).

The most common cause of subopti-

mal calf sonograms was poor sound

penetration due to profound calf

swelling, multiple collateral vessels,

or obesity. When these calf veins

were adequately imaged (in 60% of

extremities), diagnostic accuracy was

98%, which is superior to the results

of other noninvasive techniques. The

clinical importance of DVT in the

calf remains controversial. Some evi-

dence has been presented suggesting

that calf vein thrombosis may giverise to clinically significant pulmo-

nary emboli (39-43). In approximate-

ly 20% of cases of infrapopliteal DVT,

the thrombosis has been shown to ex-

tend proximally above the knee (44).

The presence of above-knee DVT

greatly increases the risk for pulmo-

nary embolism and eventual post-

phlebitic syndrome (45-47). There-

fore, noninvasive modalities that do

not demonstrate isolated calf DVT

with high sensitivity, such as imped-

ance plethysmography, must be re-

peated serially over 7-10 days to de-

tect proximal extension of thrombus.

If our results are confirmed by other

investigators, it may be feasible to

omit noninvasive serial follow-up

testing in patients with clinically sus-

pected DVT but with technically ade-

quate negative CDFI studies.

Although Polak et al were able to

image the anterior tibial veins in 65%

of CDFI studies in the calf (38), we do

not routinely image these vessels.

The fact that all patients with veno-

graphic anterior tibial vein thrombo-

sis had associated, readily detectable

femoropopliteal DVT may support

the omission of this portion of the

CDFI study in patients without anter-

olateral calf symptoms.

A potential diagnostic pitfall of

CDFI is its poor sensitivity for detec-

tion of small nonocclusive thrombi.

CDFI demonstrated only one of four

such thrombi in our patients. In our

series of symptomatic patients, isolat-

ed small nonocclusive thrombi oc-

curred infrequently (two of 69 pa-

tients). We are aware, however, that

this pitfall may represent a major

limitation in screening for asympto-

matic DVT in high-risk patients (eg,

those immobilized for a prolonged

period after major hip or knee sur-

gery), a population shown to have a

high prevalence of nonocclusive

thrombosis (48-49).

One unanticipated result of our

study was the high level of accuracy

of CDFI (89%) for detection of throm-

botic occlusion in the subset of pa-

tients with a history of previous DVT

involving the same extremity. This

favorable outcome was probably the

result of a preponderance of above-

knee DVT in this population. The

high frequency of above-knee throm-

bosis in our series offset the fact that

58% of patients with a history of pri-

or DVT had diagnostically limited

calf studies. Cronan and Leen dem-

onstrated that compression (gray

scale) US has limited value in detect-

ing acute DVT in patients with docu-

mented previous DVT: 53% of ex-

tremities with previous acute DVT

had sonographic abnormalities sug-

gestive of acute DVT on routine fol-

low-up sonograms 6-31 months after

the acute episode of thrombosis (50).

It remains to be seen whether CDFI

can enable differentiation of new

DVT from old.

With respect to our results in limbs

with potential isolated DVT in a calf

vein, it is noteworthy that CDFI tech-

nology continues to evolve. Substan-

tial improvements in image quality are

imminent. These improvements will

no doubt increase the proportion of

satisfactory calf vein studies and the

accuracy of the procedure in general.

In conclusion, on the basis of the

results presented here, we believe

that CDFI may be used as the prima-

ry imaging modality in patients with

lower extremity symptoms suggest-

ing DVT. Contrast venography, with

its attendant discomfort and risks,

can be reserved for (a) patients with-

out sonographic evidence of above-

knee DVT but with technically limit-

ed calf studies, and (b) patients with a

history of DVT and sonographic evi-

dence of DVT of indeterminate dura-

tion. U

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