DOI:10.2214/AJR.05.0148
AJR 2006; 187:154-163
© American Roentgen Ray Society
Value of Delayed Imaging in MDCT of the Abdomen and Pelvis
Shreyas S. Vasanawala1 and
Terry Desser1
1 Both authors: Department of Radiology, Stanford University, 300 Pasteur Dr.,
Stanford, CA 94305-5105.
Received January 28, 2005;
accepted after revision March 25, 2005.
Address correspondence to S. S. Vasanawala
(vasanawala{at}stanford.edu).
Abstract
OBJECTIVE. Our objective was to illustrate the benefits of obtaining
delayed CT images.
CONCLUSION. There are several clinical scenarios in which delayed CT
images may improve diagnostic specificity.
Keywords: abdominal imaging • CT angiography • dynamic CT • MDCT • pelvic imaging • trauma
Introduction
MDCT protocols specifying contrast injection rate and timing are designed
to answer the expected clinical question underlying the study request.
Occasionally, additional images beyond the planned protocol must be obtained
when unexpected pathology is visualized. One useful problem-solving tool is
delayed imaging. Delayed images can show the passage of contrast material into
or out of mass lesions and may show the leakage of contrast material from the
vascular system or urinary tract. With the relatively long scanning times of
first- and second-generation CT scanners, all imaging was effectively delayed
imaging; with today's fast scanners, a deliberate effort is required to obtain
delayed images. This article illustrates our experience with delayed imaging
as a problem-solving tool in MDCT of the abdomen and pelvis.
Our institution uses an 8-MDCT scanner and a 16-MDCT scanner, with the
routine use of IV iohexol (350 mg/mL; 2 mL/kg; injection rate, 2 mL/sec).
Patients with renal insufficiency who require IV contrast material receive
iodixanol (320 mg/mL). Scanning begins 70 seconds after the initiation of
contrast injection. However, for CT angiography, bolus monitoring is used and
injection rates are increased to 4-5 mL/sec. The approach we take is physician
monitoring of scans to ensure adequacy of the study. Our default protocol for
all abdominopelvic imaging includes delayed images of the kidneys obtained at
approximately 3 minutes. Several other studies have routine delays, as will be
detailed.
Vascular Imaging and Acute Bleeding
Of the applications of delayed imaging, vascular problem solving is the
most common at our institution. A central clinical issue in the trauma setting
is determining whether a patient has active arterial extravasation, which
generally mandates embolization or surgery. Numerous reports have confirmed
that active extravasation can often be confidently diagnosed on
contrast-enhanced CT if a focus in a hematoma or adjacent to trauma has
attenuation similar to that of the adjacent arteries
[1].
However, at times this finding may not be present because of the short
delay between contrast administration and scanning
[2], or there may be
uncertainty as to whether the high-attenuation focus actually represents
extravasated contrast agent. In these situations, delayed imaging may confirm
extravasation by showing either an interval increase in the size of a
high-attenuation focus or an interval increase in the size of a hematoma
(Figs. 1A,
1B,
2A,
2B,
3A,
3B,
4A,
4B,
4C,
4D,
5A, and
5B).
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Fig. 1A —26-year-old woman 5 days after cesarean section for breech
presentation. Patient had laboratory evidence of hemolysis, elevated liver
enzymes, and low platelet count (HELLP). Initial contrast-enhanced image shows
uterus (arrow) with hemoperitoneum anteriorly.
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Fig. 1B —26-year-old woman 5 days after cesarean section for breech
presentation. Patient had laboratory evidence of hemolysis, elevated liver
enzymes, and low platelet count (HELLP). Image delayed by 5 minutes again
shows uterus (black arrow) with anterior pooling of contrast material
in a hematoma (white arrow), suggesting active extravasation. These
findings were confirmed during embolization of left inferior epigastric artery
later that day.
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Fig. 2B —18-year-old man who experienced rollover motor vehicle
collision. Image delayed by 7 minutes shows active extravasation as area of
high attenuation lateral to spleen (white arrows). Despite active
extravasation from splenic laceration (black arrow), patient required
only 2 U of packed RBCs.
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Fig. 3A —80-year-old woman pedestrian was struck by motor vehicle and
suffered multiple pelvic fractures (arrowhead, A). Initial
contrast-enhanced CT scan shows evidence of active extravasation
(arrow) just lateral to right pubic symphysis and hint of
extravasation just posterior to left pubic body.
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Fig. 3B —80-year-old woman pedestrian was struck by motor vehicle and
suffered multiple pelvic fractures (arrowhead, A). Image
delayed by 2 minutes confirms right (black arrow) and contralateral
(white arrow) extravasation. Patient required bilateral hypogastric
artery embolizations.
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Fig. 4C —70-year-old woman after motor vehicle accident. Arterial
phase image (C) also shows retrosternal high-attenuation foci
(arrow), and delayed image (D) confirms this to also be active
hemorrhage (arrow).
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Fig. 4D —70-year-old woman after motor vehicle accident. Arterial
phase image (C) also shows retrosternal high-attenuation foci
(arrow), and delayed image (D) confirms this to also be active
hemorrhage (arrow).
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Fig. 5A —58-year-old man with previous replacement of aortic valve
with St. Jude Medical valve. Arterial phase image (A) shows ascending
aortic aneurysm (black arrow) and adjacent bilobed collection
(white arrow) in pulmonic recess. Image delayed by 5 minutes
(B) shows intense enhancement of portion of collection from active
extravasation, indicating a contained rupture, which was confirmed at
surgery.
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Fig. 5B —58-year-old man with previous replacement of aortic valve
with St. Jude Medical valve. Arterial phase image (A) shows ascending
aortic aneurysm (black arrow) and adjacent bilobed collection
(white arrow) in pulmonic recess. Image delayed by 5 minutes
(B) shows intense enhancement of portion of collection from active
extravasation, indicating a contained rupture, which was confirmed at
surgery.
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Another vascular application of delayed imaging is monitoring of aneurysms
and endovascular stent-grafts (Figs.
5A,
5B,
6A, and
6B). Residual blood flow into
the aneurysm sac external to the stent-graft is termed an
"endoleak." Endoleaks indicate the potential for continued
aneurysm growth and possible rupture. Many authors have advocated delayed
imaging for the evaluation of endoleaks. Rozenblit et al.
[3] reported an increase in
sensitivity for the detection of endoleaks from 91% to 100% when delayed
imaging is added to an unenhanced arterial phase study. At our institution, we
routinely acquire a second scan for stent-graft cases 2 minutes after the
administration of contrast material.
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Fig. 6A —84-year-old woman with aortobiliac stent-graft. Arterial
phase image (A) shows stent-graft (black arrow) and aneurysm
sac (white arrow), revealing interval increase in size. Delayed image
(B) shows region of sac external to stent-graft with attenuation
similar to that within graft (arrowhead, B) and reveals an
endoleak, likely from lumbar arteries.
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Fig. 6B —84-year-old woman with aortobiliac stent-graft. Arterial
phase image (A) shows stent-graft (black arrow) and aneurysm
sac (white arrow), revealing interval increase in size. Delayed image
(B) shows region of sac external to stent-graft with attenuation
similar to that within graft (arrowhead, B) and reveals an
endoleak, likely from lumbar arteries.
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Hepatobiliary Imaging
Delayed imaging may also refine the differential diagnoses of hepatic
lesions. Common hepatic focal lesions are hemangiomas, which are present in up
to 20% of the population. Typically, these lesions show a characteristic
peripheral nodular enhancement
[4]. However, atypical
presentations include flash-filling, which may simulate a hypervascular
metastasis or a hepatocellular carcinoma, and an incomplete rim of
enhancement, which may simulate a melanoma metastasis
[4]. In addition, diffuse
hemangiomatosis may lack peripheral enhancement
[5], thus simulating a diffuse
or multifocal hepatocellular carcinoma. In these cases, delayed imaging will
confirm a benign entity by showing persistent uniform enhancement of the
lesion that parallels that of the vasculature (Figs.
7A,
7B, and
7C).
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Fig. 7C —30-year-old man with hepatitis B. Delayed image at 160
seconds after contrast administration again shows lesion enhancement
(arrow) paralleling that of aorta at 120 H. These findings suggest
lesion is a hemangioma.
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A diagnostic dilemma in hepatobiliary imaging is distinguishing between a
cholangiocarcinoma and a hepatocellular carcinoma or a metastasis.
Cholangiocarcinomas (Figs. 8A
and 8B) show increasing
contrast enhancement on delayed imaging
[6]. This phenomenon is
attributed to the desmoplastic nature of these tumors.
Genitourinary Imaging
In the context of genitourinary imaging, delayed images are of particular
value in four circumstances: suspected collecting system injuries,
hydronephrosis, transitional cell carcinomas, and adrenal masses. With
collecting system injury of the upper urinary tract, perinephric fluid may be
identified on contrast-enhanced images before the contrast agent has
accumulated in the collecting system. Delayed imaging may then show
extravasation of high-attenuation material, confirming a urine leak (Figs.
9A and
9B). Bladder rupture may be
similarly diagnosed (Figs. 10A
and 10B). Alternatively,
delayed images may help distinguish between hydronephrosis and peripelvic
cysts (Figs. 11A and
11B) because cysts will not
opacify. For cases of suspected urine extravasation, we typically use a delay
of 5-10 minutes, depending on patient stability and backlog. Alternatively,
the patient may return to the scanner after a longer delay.
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Fig. 9B —52-year-old man with back pain. Image delayed by 80 minutes
confirms collecting system rupture (arrow). Patient eventually
underwent left nephroureterectomy, which revealed undifferentiated malignant
neoplasm (sarcoma or spindle cell carcinoma).
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Fig. 10A —Young adult male struck by automobile while riding bicycle.
Initial delayed image at 4 minutes (A) shows blood clot (black
arrow) in bladder. Further delayed image at approximately 10 minutes
(B) shows contrast extravasation (white arrow, B),
consistent with extraperitoneal bladder rupture.
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Fig. 10B —Young adult male struck by automobile while riding bicycle.
Initial delayed image at 4 minutes (A) shows blood clot (black
arrow) in bladder. Further delayed image at approximately 10 minutes
(B) shows contrast extravasation (white arrow, B),
consistent with extraperitoneal bladder rupture.
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Fig. 12A —71-year-old man with hematuria. Contrast-enhanced image shows
either high-attenuation or enhancing mass (arrow) in right renal
pelvis. Without delayed images, this may represent hematoma or neoplasm.
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Fig. 12B —71-year-old man with hematuria. Washout of 20 H on 4-minute
delayed image indicates neoplasm (arrow). Subsequent right
nephroureterectomy revealed moderately differentiated transitional cell
carcinoma.
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Fig. 13C —67-year-old man with incidental adrenal lesion. Image
obtained 15 minutes after contrast administration shows 33-H lesion
(arrow), indicating greater than 40% washout. This lesion is likely
an adenoma; it was stable in size for more than 1 year.
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For suspected renal cell carcinoma, our routine abdominal CT protocol is
modified to include preliminary unenhanced images. In a case of suspected
transitional cell carcinoma, our protocol requires unenhanced images. These
are followed by the injection of 40 mL of contrast material at 2 mL/sec,
abdominal compression at 4 minutes, and then 80 mL of contrast material
administered at 2 mL/sec. After an additional 2 minutes, images are acquired
from the diaphragm to the iliac crest, immediately followed by release of
compression and imaging from the iliac crest to the pubic symphysis. If a
lesion is seen, further delayed images of this region may be acquired. The
goal of this protocol is to obtain images of an opacified urinary collecting
system similar to a conventional excretory urogram by delaying image
acquisition.
When an unsuspected renal mass is discovered and no preliminary unenhanced
images are available, delayed images are particularly useful. First, malignant
renal lesions of high attenuation (> 30 H) on initial contrast-enhanced
images show washout [7] on
delayed imaging. Urinary collecting system filling defects may represent
neoplasm, blood clot, or fungus. All of these lesions may show high
attenuation on contrast-enhanced images. However, with delayed imaging, a
decrease in attenuation will be seen with a neoplasm (Figs.
12A and
12B) as the contrast material
washes out. Second, dense contrast material filling the collecting system on
delayed images will increase the conspicuity of these lesions.
Adrenal lesions are present in approximately 9% of the population. CT
without IV contrast material may be able to distinguish benign from malignant
lesions. A meta-analysis by Boland et al.
[8] indicated that a cutoff of
10 H gives 71% sensitivity and 98% specificity. Several studies have suggested
that washout kinetics of adrenal lesions may increase the sensitivity for
adenomas (Figs. 13A,
13B, and
13C). Various delays have been
explored ranging from 15 to 60 minutes, with various attenuation upper limits
for adenoma ranging from 25 to 37 H. One study indicated 100% specificity for
adenoma using an upper limit of 25 H on 15-minute delayed images. The same
study concluded that a 40% washout at 15 minutes is 96% sensitive and 100%
specific for adenoma [9]. Our
standard delay for adrenal lesion imaging is 15 minutes.
Conclusion
Delayed images can increase confidence in diagnosing active arterial
extravasation in the setting of trauma or active bleeding. They can supplement
arterial phase imaging of vascular stent-grafts. Delayed images may confirm
urinary leaks. In characterizing mass lesions, wash-in or washout properties
of masses on delayed imaging can lead to more accurate lesion
characterization.
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Abstract
Introduction
