Thoracic Imaging 2003 - Society of Thoracic Radiology

5. The volume of contrast medium required is equivalent to
the injection rate (ml/sec.) multiplied by scan duration
(sec.). A bolus of 120-150 ml of the contrast medium is
administered by power injector if scanning only the thoracic
aorta, but it is increased to 200 ml if scanning both
thoracic and abdominal aorta.
6. The injection rate is 3 ml/sec. via a 22 gauge intravenous
angio-catheter.
7. A 30 second delay time is used between the start of injection
and onset of scanning. Although timing scan is not
routinely obtained at our institution, it may be acquired in
patients with cardiac dysfunction to determine time delay
for peak aortic enhancement. In this procedure, a bolus of
20 ml of contrast material is injected via a peripheral vein
at a rate of 3 ml/sec. Beginning 10 seconds after the start
of injection, 20 dynamic images are obtained at the level of
aortic arch at 2-second intervals (i.e., 1 sec./section scan
acquisition time, 1 second interscan delay). A time attenuation
curve is generated at the arch level by placing a
region of interest (ROI) and delay time is calculated from
the start of injection to peak aortic enhancement.
Alternatively, the manufacturer’s automated technique such
as “Smart Prep” (General Electric Medical Systems,
Milwaukee, WI) or “Care Bolus” (Siemens, Erlangen,
Germany) can be used where a series of low milliamperage
(mA) single scans are obtained until the desired threshold
density is reached and the high mA spiral aortic scan is
performed.
8. 120kV, 180-280 mAS, 0.8-second time per gantry rotation.
9. The patient is hyperventilated 3 times prior to scanning and
the scan is acquired in suspended inspiration.
10. The field of view for the CT Angiogram is approximately
20-25 cm, detector collimation is 2.5 mm, slice thickness is
2.5 mm and the table speed is 15mm per gantry rotation
(high speed HS mode). The images are acquired caudocranial
from the aortic hiatus in the diaphragm to 2 cm
above the arch.
11. The images are reconstructed at 1.5 mm interval with standard
reconstruction filter.
12. Saggital oblique reformations (MPR) are performed
through aortic arch at the scanner. Data is transferred to
PACS workstation for interpretation. In selected cases,
data is also transferred to clinical 3D laboratory for performing
shaded surface display, curved planar reformations
or maximum intensity projection. Every other image is
printed on film as 20:1 format, axial contrast sequence at
vascular window. Additionally the oblique saggital reformation
images of aorta are also printed. (Although reformations
do not improve sensitivity for the disease detection,
they do better display the anatomic relationships and
extent of disease in a format that is familiar to most clinicians.)
UCLA has just installed a 16 channel Siemens Sensation 16
scanner. The protocols for the clinical applications are being
developed currently.
Pitfalls in interpretation of Thoracic Aortic Angiography
A variety of pitfalls are encountered when interpreting CTA,
particularly for aortic dissection and aneurysms. These are
attributed to technical factors, streak artifacts, periaortic structures,
aortic wall motion, aortic variations such as congenital
ductus diverticulum or acquired aortic aneurysm, and penetrating
atherosclerotic ulcer.
Technical factors: Optimal contrast enhancement is critical
to the diagnosis of aortic pathology, for example aortic dissection.
Improper timing of contrast material administration relative
to image acquisition or slow rate of injection may fail to
opacify the lumen sufficiently to demonstrate intimal flap resulting
in false - negative diagnosis.
Streak artifacts: High attenuation material within or outside
the patient may generate streak artifacts on both non-contrast
and contrast enhanced images. These appear as straight parallel
lines that radiate from a single point and generally exceed the
confines of aorta. Conversely, intimal flaps are thin and
smooth, slightly curved and restricted to aortic lumen.
Strategies to reduce these artifacts include the use of diluted
contrast material, injection of contrast material in right antecubital
vein and acquiring scan in caudo-cranial direction.
Periaortic Structures: Several vessels such as origins of arch
vessels, left brachiocephalic, left superior intercostal and left
pulmonary veins may simulate double lumen or intimal flap.
Residual thymus, superior pericardial recess, atelectasis, pleural
thickening or effusion, periaortic fibrosis or lymphoma can also
simulate aortic dissection. These structures can be identified on
reviewing contiguous images, and by the knowledge of normal
anatomy.
Aortic wall motion and normal aortic sinuses
The pendular motion of ascending aorta produces curvilinear
artifacts in the left anterior and right posterior aspect of aortic
root. The combined pendular and circular motion of the aorta
on the other hand produces a circumferential artifact over the
ascending aorta. The motion artifact is restricted to only one or
two adjacent scans. The artifact can be minimized by reconstructed
images generated with 180° linear interpolation algorithm
or by electrocardiographic gating of the scan.
Normal aortic sinuses are recognized by their characteristic
location at the same level as the proximal left coronary artery.
Congenital aortic diverticulum and acquired aortic aneurysm
Congenital aortic diverticulum appears as a smooth focal
bulge with obtuse angles with the aortic wall. It is located at
the aortic isthmus.
A fusiform aneurysm with intraluminal thrombus may simulate
aortic dissection with thrombosed false lumen. However,
on non-contrast images high attenuation thrombosed false
lumen and internal displacement of intimal calcification is seen
in dissection. Calcification occurring on the surface of an intraluminal
thrombus in an aneurysm is difficult to distinguish from
aortic dissection.
Penetrating atherosclerotic ulcer
It is an atheromatous lesion where the ulcer, a focal contrast
material filled out-pouch protudes from the intima into the
media and produces intramural hemorrhage. It generally occurs
in descending aorta and an intimal flap is absent.
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