Thoracic Imaging 2003 - Society of Thoracic Radiology

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sive revision of contrast material injection protocols. Faster scan
acquisition times allow scan acquisition during maximal contrast
opacification of pulmonary vessels 40 but pose an
increased challenge for precise timing of the contrast bolus.
Strategies that have the potential to improve the delivery of contrast
media for high and consistent vascular enhancement during
CT pulmonary angiography include use of a test bolus or automated
bolus triggering techniques 76 . Saline chasing 77, 78 has
been used for effective utilization of contrast media and for
reduction of streak artifacts arising from dense contrast material
in the superior vena cava. Use of multi-phasic injection protocols
has proven beneficial for general CT angiography 79, 80
but has not been scientifically evaluated for the pulmonary circulation.
Another limitation that in some instances results in suboptimal
diagnostic quality of CT pulmonary angiography are
motion artifacts due to patient respiration or transmitted cardiac
pulsation. Shorter breath-hold times that are feasible with multidetector-row
CT should facilitate investigation of dyspneic
patients 43 and reduce occurence of respiratory motion artifacts.
Similarly, artifacts arising from transmitted cardiac pulsation
appear amenable to decreased temporal resolution with fast CT
acquisition techniques 40 . ECG-synchronization of CT scan
acquisition allows for effective reduction of cardiac pulsation
artefacts that might interfere with the unambiguous evaluation
of cardiac structures, the thoracic aorta and pulmonary structures
81, 82 . However, the spatial resolution that could be
achieved e.g. with retrospectively ECG gated technique using
the previous generation of 4-slice multidetector-row CT scanners
was limited by the relatively long scan duration inherent to
data oversampling 82 . Thus, high-resolution acquisition could
only be achieved for relatively small volumes, e.g. the coronary
arterial tree, but not for extended coverage of the entire chest.
The advent of 16-slice scanners now effectively eliminates these
previous tradeoffs. With 16-slice multidetector-row CT it is now
possible to cover the entire thorax with sub-millimeter resolution
in a single breath-hold with retrospective ECG gating,
effectively reducing transmitted pulsation artefacts. This way,
potential sources of diagnostic pitfalls arising from cardiac
motion can be effectively avoided.
Radiation Dose:
Use of high resolution multidetector-CT protocols was
shown to improve visualization of pulmonary arteries 45 and the
detection of small subsegmental emboli 46 . In suspected PE,
establishing an unequivocal diagnosis as to the presence or
absence of emboli or other disease based on a high-quality multidetector-row
CT examination may reduce the overall radiation
burden of patients, since further work-up with other tests that
involve ionizing radiation may be less frequently required.
However, if a 4-slice multidetector-row CT protocol with 4x1mm
collimation is chosen to replace a single-detector CT protocol
based on a 1x5-mm collimation, the increase is radiation
dose ranges between 30% 83 and 100% 41 . Similar increases in
radiation dose, however, are not to be expected with the introduction
of 16-slice multidetector-CT technology with sub-millimeter
resolution capabilities. The addition of detector elements
should improve tube output utilization compared to current 4slice
CT scanners and reduce the ratio of excess radiation dose
that does not contribute to actual image generation 84 . As
sophisticated technical devices move into clinical practice, that
modulate and adapt tube output relative to the geometry and xray
attenuation of the scanned object, i.e. the patient 85-87 , substantial
dose savings can be realized without compromising
diagnostic quality 88 . The most important factor, however, for
ensuring responsible utilization of multidetector-row CT’s technical
prowess is the increased awareness of protocols used by
technologists and radiologists. It has been shown that diagnostic
quality of chest CT is not compromised, if tube output is adjusted
to the body type of the individual patient 89 . Also, with multidetector-row
CT radiologists are more and more adapting to
the concept of volume imaging. There is a trade-off between
increased spatial resolution and image noise, when thinner and
thinner sections are acquired with fast CT techniques. Given the
great flexibility and diagnostic benefit that a high-resolution,
near-isotropic multidetector-row CT data set provides radiologists
are increasingly willing to compromise on the degree of
image noise in an individual axial thin-section image that they
are willing to accept in order to keep radiation dose within reasonable
limits.
Data Management:
Multidetector-row CT increases our diagnostic capabilities,
however, the massive amount of data, which is generated by this
technique puts significant strain on any image analysis and
archiving system. A high-resolution 16-slice multidetector-row
CT study in a patient with suspected pulmonary embolism routinely
results in 500 – 600 individual axial images. 3D visualization
of multidetector-row CT data in suspected PE may aid
diagnosis in some instances and help avoid diagnostic pitfalls
for example for the correct interpretation of hilar lymphatic tissue
adjacent to central pulmonary arteries 90 . However, in contrast
to focal lung disease, which can be accurately diagnosed
by use of maximum intensity projection reconstructions that
beneficially “condense” large volume multidetector-row CT data
sets 91 , a diagnosis of pulmonary embolism is usually most
beneficially established based on individual axial sections.
Interpretation of such a study is only feasible by use of digital
workstations that allow viewing in “scroll-through” or “cine”
mode. Development of dedicated computer aided detection
algorithms 92 may be helpful in the future for the identification
of pulmonary emboli in large volume multidetector-row CT data
sets. Large and accessible storage capacities are an essential
requirement for successful routine performance of multidetector-row
CT in a busy clinical environment. Adapting this environment
to the new demands which are generated by the introduction
of ever-faster scanning techniques is not a trivial task.
New modalities for data transfer, data archiving and image
interpretation will have to be devised in order to make full use
of the vast potential of multidetector-row CT imaging.
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