Experiences.html
Experiences of using a single post-contrast CT scan of the chest after biphasic contrast injection
P C Pretorius, FCRad (Diag) SA
Drs Visser, Erasmus, Vawda & Partners, Port Elizabeth
Corresponding author: P Pretorius (ppret@telkomsa.net)
Computed tomographic (CT) chest
investigations can be enhanced; in many cases, the arterial phase of a
post-contrast arterial and delay (60 seconds) study can be omitted when
planning the contrast injection and scanning technique carefully. A
biphasic contrast injection was used before starting a single 60-second
post-contrast scan.
S Afr J Rad 2012;16(2):56-60.
Introduction
While experimenting with single post-contrast computed tomographic
(CT) scans of the urinary tract after a triphasic contrast injection,1
I considered whether a similar technique could be used to cut down on
unnecessary scan series in our patients referred for chest CT scans. A
precontrast scan and then a biphasic contrast injection followed by a
single scan of the thorax was therefore considered and implemented to
test the reliability and quality of this technique.
Protocols for chest CT surveys for mediastinal pathology, lung
masses and ‘rule out pathology’ chest scans traditionally
dictate a post-contrast scan beginning 60 s after initiating an
intravenous contrast injection. In numerous practices, 100 - 120 cc of
contrast is traditionally used. This process usually results in
adequate contrast throughout the vascular structures in the mediastinum
and lungs and enhancement of pathologic masses or lymph nodes.
When there are clinical indications of vascular pathology (i.e.
aortic or pulmonary arteries and their respective branch vessels), it
is imperative to add a dedicated arterial phase scan before the 60 s
scan series. If there are no clinical indications to include an
arterial phase, a single 60 s post-contrast scan only should be
performed.
Unfortunately, clinical information from referring physicians
is not always clear and, even when clear, many radiologists feel
insecure without including an arterial series. We felt that this
biphasic injection followed by a single post-contrast scan technique
improved the diagnostic information and also alleviated the insecurity
of excluding the arterial series when performing chest CT scans for
non-vascular indications.
After scanning a few test cases using a single post-contrast series
after a biphasic contrast injection, we were pleasantly surprised with
the results, and we have now introduced this technique in most of our
practice, when a ‘general rule out’ or ‘mass lesion
characterisation’ scan is requested. This requires radiologists
to shift their comfort zones, to rely on a single post-contrast scan
replacing either a post-contrast arterial, 60 s, or both arterial and
60 s scans.
I have included a pictorial representation of a variety of
pathologies while using the newer technique, to share our experiences
over a wide range of pathologies, which will assist our colleagues in
making this shift of technique with more confidence, and will hopefully
address some of the queries that may be raised.
Injection plan
1. 80 cc contrast @ 3 cc/s (27 s) – contributes to ‘venous phase’ of the scan
2. 20 cc saline @ 3 cc/s (7 s) – pushes contrast into system.
3. Delay (11 s)
4. 40 cc contrast @ 2.5 cc/s (16 s) – contributes to the ‘arterial phase’ of the scan
5. Followed by 20 cc saline @ 2.5 cc/s
6. The scan is started at 60 s, which is 15 s into the
arterial phase of the injection, 1 s before the end of the injection.
Most scanners require a 4 - 5 s delay from the time the scan is
initiated until the first ‘cuts’ are taken – see
comments in the discussion below.
Case studies
Normal arterial phase scan (Fig. 2)
Arterial phase-contrast scans of the chest show good contrast
enhancement of pulmonary arteries and veins, heart chambers, the aorta
and its branch vessels. There is good differentiation of contrast
between the central venous and arterial structures (dense enhancement
v. no enhancement). The returning systemic veins contralateral to the
side injected, from the head and neck, from infradiaphragmatic veins
(especially the azygos vein) and any mediastinal and hilar masses and
lymph nodes and lung pathology are not enhanced at this early
post-contrast phase. Consequently, there is often difficulty in
differentiating the unenhanced dilated or anomalous venous structures
from the unenhanced lymph nodes and masses in the chest.
Normal 60 s post-contrast scan (Fig. 3)
The contrast enhancement of the vascular structures, mass lesions
and lymph nodes within the chest is usually adequate, but
differentiation between central arterial and venous structures and
returning systemic venous structures relies on anatomical
identification, as the contrast density of these structures is usually
very similar. The contrast in the pulmonary arteries and branches is
often mediocre, and incidental pulmonary arterial emboli may not be
well seen.
Post biphasic contrast injection scan (Fig. 4)
After a biphasic contrast injection, there is dense enhancement of
central pulmonary arteries and veins and the aorta and its branches.
Returning systemic veins also show enhancement, but less dense than the
central vessels, allowing further clarity to confirm the anatomical
differentiation. Lung and mediastinal masses and lymph nodes will also
enhance, usually to a lesser degree than systemic veins, and will show
their enhancement patterns.
Lung carcinoma after biphasic contrast injection (Fig. 5)
Despite the scan beginning at 60 s, there is dense enhancement of
the pulmonary arteries, veins and aorta, without compromising
enhancement and characterisation of the lung mass and lymph nodes.
Mycetoma after biphasic contrast injection (Fig. 6)
It is interesting to compare this case with the preceding lung
carcinoma case (Fig. 5), where there was enhancement of the mass and
the typical peripheral enhancement of infiltrated lymph nodes, unlike
the non-enhancement of this mass and the uniform node enhancement.
Lymphoma after biphasic contrast injection (Fig. 7)
Having dense contrast in all the vascular structures, with
enhancement of the mass, all on one post-contrast series, helped to
‘unpack’ all the pathological processes going on in this
case, which initially appeared quite complex because of the gross
changes.
Goitre after biphasic contrast injection (Fig. 8)
Although not a particularly challenging diagnosis to make, following
the enhanced thyroid from the neck, into the mediastinum, and
differentiation from adjacent vascular structures all on one
post-contrast series was made easier in this case.
Sarcoid after biphasic contrast injection (Fig. 9)
There is clear differentiation of the enhancing lymph nodes from the
adjacent, more densely enhancing vascular structures. Again, this is
not a particularly challenging call to make – but compare with
the next case below (Fig. 10).
Interstitial lung disease after biphasic contrast injection (Fig. 10)
The lymphadenpathy was more subtle than the sarcoid case
above, but can be confidently called because of the adjacent azygos
vein and aortic enhancement.
Opportunistic infection after biphasic contrast injection (Fig. 11)
The pulmonary arterial emboli might not have been as clearly seen on
a single standard post-contrast 60 s scan, and the splenic
micro-abscesses might also have been missed on a standard post-arterial
scan. These were all seen on the single post-contrast series.
Atelectasis after biphasic contrast injection (Fig. 12)
The collapsed lung is enhanced, with dense enhancement of vascular structures.
Discussion
The contrast load traditionally used for a survey chest CT is
100 - 120 cc, to gain adequate contrast density in vascular structures
in the chest. A ‘rule of thumb’ is that, from the time the
contrast injection into an antecubital vein begins, it takes 9 s to
reach the heart, by 12 s it is through the lungs back to the heart, and
by 15 s has reached the descending thoracic aorta.
Contrast injection rates vary between 2 - 5 cc/s, depending on
the sequence chosen. A contrast injection of 100 cc at 4 cc/s. will
ensure a 25 s bolus of contrast. Since modern multislice scanners can
scan the whole thorax in 10 - 16 s and the resolution is so high, it is
conceivable that, if one could get the timing perfect in every patient,
one could theoretically perform a good arterial phase diagnostic scan
of the chest with only 16 s x 2.5 cc/s = 40 cc of contrast.
Naturally, there are numerous variable factors in patients
such as body morphology, cardiac function, degree of valsalva with
breath hold, etc. ‘Perfect timing’ of the scan in every
case is consequently not feasible but, as can be seen from the above
description, a theoretical 60 - 80 cc of contrast (100 - 120 cc minus
40 cc) is non-contributory to the diagnostic information, if one were
looking purely at the arterial filling of vessels. This
‘extra’ contrast can be used to enhance the contrast
appearances in the other vessels by splitting the bolus delivery.
Using the above logic, we designed an injection sequence to
try to maximise the benefit of this ‘non-contributory’
portion of contrast that we inject. The logic is as follows:
• The initial 80 cc starting 60 s before the scan allows
adequate contrast filling of the returning systemic veins, particularly
the brachiocephalic and azygos veins. The injection rate is kept
similar to that of a ‘traditional’ chest survey CT scan.
This contrast is ‘pushed’ by a saline chaser injection at
the same rate, followed by a calculated delay.
• A second contrast injection begins 15 s before the CT
scan starts. This will enhance the pulmonary arterial and pulmonary
venous vessels and the aortic vessels in the chest. This timing allows
the end of the second phase bolus of contrast to densely enhance the
central vessels (pulmonary veins and arteries), while the lead portion
of this second bolus will have reached the descending aorta in almost
all patients.
• In fit patients, this timing is invariably adequate.
• In patients with known
poor cardiac function, the contrast passage may be a bit slower. In
these cases, one may start the scan a little later – possibly at
65 s This is still usually adequate as the contrast bolus tends to
‘stretch out’ i.e. a tight bolus injected into a peripheral
vein over 15 s. will have spread out to possibly over 20 s by the time
it has gone through the heart, lungs and heart again, before entering
the aorta.
• The 60 s initiation of
the scan after the beginning of the contrast injection, means that the
scan effectively starts at 64 s, because of the built-in delay for a
scan to initiate. This is 3 s after completion of the contrast
injection in the arm. I have found that this timing is adequate (as the
contrast bolus still has to travel up the arm and through the heart and
lungs and into the aorta) and the dense arterial phase of contrast in
the vessels from the superior vena cava, through the heart, pulmonary
arteries and veins and aorta is captured regularly with this timing.
• A final saline chaser injection pushes the contrast towards the chest.
This routine gave the following advantages:
• adequate enhancement of
returning systemic veins, to help differentiate them from mediastinal
and hilar masses and lymph nodes
• relatively dense contrast
enhancement of pulmonary arteries, veins and thoracic aorta and its
branches, which allowed clear differentiation between systemic
returning veins and mediastinal and hilar masses and lymph nodes
• enhancement of masses and lymph nodes allowed their characterisation
• all the post-contrast
information was seen on one set of images, negating the need to run
both an arterial and a 60 s post-contrast series (2 series). This made
interpretation quicker and in many cases easier (see case of lymphoma
above – Fig. 7).
In my opinion, the information gained is increased when compared with a single 60 s post-contrast scan.
Radiation exposure is reduced by omitting the arterial phase
scan in practices where both an arterial and 60 s post-contrast are run
for those cases where the arterial series is not indicated, but is
added for the radiologist’s comfort.
Other advantages from reducing unnecessary series of
post-contrast CT scans have been discussed in a previous article using
a single post-contrast CT scan of the urinary tract after triphasic
contrast injection.1 These advantages include:
• more efficient reading of the scans owing to more information presented on a single post-contrast series
• ease of viewing of a single scan post-contrast series for the referring clinician
• reduction in radiation to the patient from unnecessary extra scan series
• reduced wear and tear on scanner hardware
• reduction in the number of images needing electronic archiving.
It must again be emphasised that this technique is used to
replace chest CT scans where the clinical information dictates a
pre-contrast and a single post-contrast scan at 60 s and should not to
be used when a dedicated arterial series is indicated.
1. Pretorius PC. Experiences of using a single post-contrast CT scan of
the urinary tract after triphasic contrast injection. SA Journal of
Radiology 2011;15(4):140-145.
1. Pretorius PC. Experiences of using a single post-contrast CT scan of
the urinary tract after triphasic contrast injection. SA Journal of
Radiology 2011;15(4):140-145.
Fig. 1. Injection plan.
2a
2b
Fig. 2a. White arrows point to
unenhanced left brachiocephalic vein anteriorly and unenhanced lymph
nodes posteriorly. In Fig. 2b, the white arrow indicates the unenhanced
azygos vein.
3a
3b
3c
3d
Fig. 3. Scan of the chest 60 s
after contrast injection. Arrows in 3a and 3b point to the left
brachiocephalic vein and azygos vein, respectively.
4a
4b
4c
4d
Fig.
4. Biphasic contrast injection single post-contrast injection series.
The white arrow in 4a and 4b shows contrast-enhanced left
brachiocephalic and azygos veins. The white arrow in Figs 4c and 4d
show an enhancing ectopic thyroid nodule, with calcification, abutting
on the enhanced left brachiocephalic vein.
7e 7f
Fig. 7. Mass in right chest with
superior medistinal syndrome clinically. The mass seen on precontrast
(7a) shows vague enhancement (20HU) on the 60 s scan (7b and 7c). There
is tumour infiltration causing partial obstruction of SVC (7e), the IVC
(7f) and the left internal jugular vein (7d) (white arrows).
5a
5b
5c
5d
Fig. 5. There is a carcinoma mass
in the right lung (arrows in precontrast 5a and post-contrast 5b scans)
with peripheral enhancement of the mass. Infiltrated lymph nodes show
typical peripheral enhancement on the post-contrast series (arrowheads
in 5b, 5c and 5d).
6a
6b
6c
6d
Fig. 6. A lung mass showing
increased density of 52HU on the precontrast scan (6b) with no
significant enhancement on the 60 s post-contrast scan (6c). There are
inflammatory uniformly enhancing lymph nodes in the adjacent
mediastinum – arrow in 6d.
7a
7b
7c
7d
7e
7f
Fig. 7. Mass in right chest with superior medistinal syndrome
clincally. The mass seen on precontrast (7a) shows vague enhancement
(20HU) on the 60 s scan (7b and 7 c). there is tumour infiltration
causing partial obstruction of SVC (7e), the IVC(7f) and the left
internal jugular vein (7d) (white arrows).
8a
8b
8c
Fig. 8. Retrosternal extension of a
goitre (arrow), with cystic change in left lobe. Clear separation and
differentiation from adjacent aortic arch branches bilaterally and the
enhanced left brachiocephalic vein anteriorly noted in 8b.
9a
9b
9c
9d
Fig. 9. Sarcoid infiltration of
lung (arrow in 9a), with enlarged uniformly enhancing mediastinal and
hilar lymph nodes (arrows in 9b - d).
10a
10b
Fig. 10. Interstitial changes in
the right lung. There is mediastinal lymphadenopathy (arrow in 10b)
differentiated from the adjacent enhanced azygos vein (arrowhead in
10b).
11a
11b
11c
11d
Fig. 11. Opportunistic nodular lung
infection in immunocompromised patient. Unsuspected pulmonary arterial
emboli noted on the right (arrow in 11b and arrowhead in 11c). Lymph
node enlargement noted (arrow in 11c). Micro-abscesses also identified
in spleen (arrows in 11d) at the inferior range of the scan.
12a
12b
Fig. 12. Right pneumothorax (arrow in 12a) with atelectasis of underlying lung (arrow in 12b) and pleural fluid.