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Christoph Becker is associate
professor and section chief of the
CT department at the University of
Munich. Prior to this, he completed
his graduate studies at Ludwig-
Maximilians-University in Munich
before taking up a residency in
diagnostic radiology at the
University of Munich followed by a
fellowship in cardiovascular radiology.
Dr Becker’s previous publications
cover CT angiography of cerebral
aneurysms, investigation of different
CT detectors and radiation exposure
and comparison of cardiac
investigations between electron
beam CT and conventional CT for
detection and quantification of
coronary calcium. Dr Becker’s
current focus is on CT angiography
of coronary arteries and
characterization of
atherosclerotic plaques. |
The impact of new CT technology on clinical practice
Christoph Becker, MD
Division of Cardiology, Uniformed Services University of
the Health Sciences, Bethesda, MD, USA
Christoph R Becker, MD
Associate Professor
Section Chief Body CT
Department of Clinical Radiology
University Hospital Grosshadern
D-81377 Munich, Germany
Tel: +49 89 7095-3620 Fax: +49 89 7095-8832
Email: christoph.becker@med.uni-muenchen.de
Abstract
With the introduction of multi-detector-row CT
(MDCT), new applications such as cardiac CT
angiography, lung and colon screening and scanning of
polytrauma patients became feasible. Coronary artery
disease can reliably be ruled out by MDCT and therefore
the technique surpasses conventional invasive
angiography in some patients. In screening for lung and
colon cancer, special post-processing for dedicated
analysis of the MDCT dataset is required for comfortable
and safe exclusion of early tumour disease. Scanning
has already replaced conventional X-ray as the first line
diagnostic tool for polytrauma patients in the
emergency department.
All these applications require CT image acquisition with
highest temporal and spatial resolution of a certain
volume within shortest period of time. These special
acquisition conditions also entail special considerations
about the contrast medium and form of administration.
With most modern CT scanners, CT angiography is
performed with high iodine delivery rates. Heat
sensations, blood volume expansion and contrastinduced
nephropathy are less frequent with iso-osmolar
contrast media. In many instances, a highly
concentrated (more than 300 mgI/ml) isosmolar
contrast media appears to be the optimal contrast
agent for MDCT investigations.
Introduction
Multi-detector-row CT (MDCT) scanners were first
introduced into clinical practice in 1998. Four-detectorrow
CT with 500 ms gantry rotation and detector
collimation between 5 and 1 mm allowed for both high
scan speed and improved spatial resolution. A number
of different clinical applications were gained from these
new scanning features, such as emergency radiology, CT
screening (CTS) and CT angiography (CTA). Indeed, for
the first time, CTA of the coronary arteries became
feasible with MDCT by a new technique called
retrospective ECG gating. For this technique, the spiral CT scan is acquired with a slow table feed per gantry
rotation (pitch<0.3) in conjunction with the ECG signal,
which is recorded during scanning. Image reconstruction
is usually performed in the slow motion diastole phase
of the cardiac cycle.1
CT screening
CT has often been described as a sensitive tool for the
early detection of morphological changes.With the
increased spatial resolution offered by MDCT, it began
to be more widely used to screen for lung and colon
cancer as well as for coronary artery disease. All screening
applications share low radiation exposure and no
requirement for contrast medium administration.
Dedicated post-processing software has become
mandatory to support the detection of early stages
of diseases by MDCT.2
The potential of coronary calcium screening to predict
the risk of unheralded myocardial infarction has been
investigated for years. Until now, CT calcium screening
has failed to provide an incremental predictive value
over conventional risk factors, but prospective studies
are currently being performed to support this
hypothesis.3
In lung cancer screening, multi-detector-row CT is far
more sensitive than conventional X-ray imaging for the
detection of intrapulmonary lesions. However, in the
early days, many of these detected lesions turned out
to be benign, and a number of unnecessary biopsies
were performed. Meanwhile, it has been shown that
lesion growth as detected by a change of lung nodule
volume is a better predictor for malignancy (Figure 1).
As yet, no studies are available to indicate whether
detection by MDCT improves outcome in lung cancer.4
 |
| Figure 1. If a lung nodule is detected by CT lung cancer screening with a
size below 5 mm, repeated scanning is performed within 6 months. The
presented case demonstrates nodule growth from 37 to 152 mm2 within
this period of time. Surgical resection confirmed the diagnosis of stage I
lung cancer. |
Virtual colonoscopy seems to be gaining wider
acceptance in comparison with optical colonoscopy
probably because of better patient comfort. In the
future, faecal tagging may even allow this application to be performed without bowel cleansing. Reliability for
the detection of polyps has improved with the
improved spatial resolution and faster scanning offered
by the more advanced MDCT generations, explaining
the difference in many of the studies published so far
(Figure 2). However, data are also lacking to support the
widespread use of virtual colonoscopy for screening,
since colon polyps and flat lesions that may also turn
into colon cancer may still be missed.5
 |
| Figure 2. The newest CT scanners allow for 3D display of the inner colon
wall after distension with air. In a clean bowel, it is easy to visualize any
polyps that may turn into cancer over time. |
CT angiography
The investigation of any vascular territory from head
to toe has improved significantly with MDCT scanner
generation (Figure 3). The shorter scan time and the
higher spatial resolution has led to remarkable
improvements in the visualization of even the smallest
peripheral arterial branches. The type of contrast injection
that is used for CTA basically depends on the
duration of the CT scan and the delay time between contrast injection and the CT scan.With longer scan
times, as for the four-detector row CT, a dual-phase
contrast protocol is an advantage for compensating for
contrast recirculation in the later phase of the injection
and maintaining a constant and homogenous
enhancement for the entire scan time. Since the MDCT
scan time became shorter than 20 s with the newest
CT scanner generation, only mono-phase contrast
injection protocols are used. In short scans, a certain
time delay appears necessary in order to allow the
contrast medium to reach the targeted vascular system
and to allow for the breath-hold command.
 |
| Figure 3. Peripheral run-off studies are easily performed by MDCT
angiography with highest resolution. Ideally the table feed is as fast as the
contrast bolus travels along the arteries. |
The arrival time of the contrast medium depends on
different factors and may best be determined by
tracking the arrival of the contrast bolus in the target
vessel, e.g. the ascending aorta. Shorter scan times also
require less contrast medium for achieving a certain
level of enhancement.6
Iodinated contrast media are available with different
concentrations ranging between 140 and 400 mgI/ml.
Contrast media with higher concentrations may allow
the achievement of higher vessel enhancement than
lower concentration contrast media with the same
flow rate. However, increasing the flow rate with low
concentration contrast media will also allow a similar enhancement to be achieved as with high-density
contrast media. For this reason, it seems reasonable to
deal with the iodine delivery rate (grams of iodine per
second) in order to calculate the ideal flow rate for a
certain contrast concentration. The iodine delivery rate
simply corresponds to the product of the contrast
media flow and concentration and should ideally be in
the range of 1.5–2 g of iodine per second for a
CTA study.
Highly concentrated contrast media may be preferable
in order to achieve iodine delivery rates with low flow
rates. However, highly concentrated contrast media
tend to have a higher viscosity compared to less dense
contrast media. As a result of the high viscosity, the
contrast medium does not mix well with the blood and
the final enhancement may therefore vary more often
from patient to patient and may more often be related
to physiological factors such as the blood volume and
circulation time.Warming to body temperature (37 °C)
prior to administration will significantly reduce the
viscosity of the contrast media.7
Non-ionic low-osmolarity monomeric contrast media
are most commonly used for CT and angiography. As an
alternative, non-ionic iso-osmolar dimeric contrast
media are available for intravenous administration.
In general, iso-osmolar contrast media may be
preferable in any vascular territory where the endothelium
is sensitive to the osmolarity such as the brain, heart
and kidneys. In the brain, for instance, it has been
shown that iso-osmolar contrast media affect the
blood–brain-barrier to a lesser degree than low-osmolar
contrast media.8 The rhythm and the function of the
heart may also be influenced to a lesser degree by
isosmolar as compared to low-osmolar contrast media.
Temporary impairment of renal function may occur
after the use of contrast media. Contrast-induced
nephropathy (CIN) has been observed less commonly with a isosmolar contrast medium than with a
low-osmolar contrast medium in patients at risk
undergoing cardiac catheterization.9 Hydration prior to
the administration of contrast media and a reduction in
the amount of iodine appears mandatory for
preventing CIN.
In relation to the osmolarity of the contrast media in
use, water may be pulled out of the interstitium into the
vascular system, resulting in blood volume expansion.
Patients with impaired cardiac function may be at risk
of decompensation caused by right heart volume
overload. Therefore, iso-osmolar contrast media may be
preferable in these patients.10 In addition, patients less
commonly complain about pain or heat sensations
related to the administration of iso-osmolar contrast
media. On the other hand, some studies have reported
that late allergic skin reactions may be more commonly
associated with the administration of isosmolar
contrast media.
Cardiac CT
 |
| Figure 4. Higher iodine delivery rates (A = 1 g iodine per sec; B = 1.5 g iodine per sec) results in better visualization of the smallest vessels and branches
of the coronary arteries, as well as of the pulmonary arteries. |
The vessel enhancement should be as high as possible
for the assessment of the coronary artery lumen and its
patency. The brightest enhancement may also help in
visualizing the smallest branches in the periphery of the
coronary artery tree (Figure 4). A very recent publication
comparing coronary 64-slice CT angiography with
conventional cardiac catheterization described
remarkably high sensitivities, specificities and positive
and negative predictive values in the range of 87–99%.11
Coronary CT angiography is also well suited to
visualizing the anatomy in patients with coronary
anomalies and identifying patients in whom the
aberrant coronary artery runs between the ascending
aorta and the pulmonary outflow tract. In this particular
location, the coronary artery is at risk of being
squeezed in between the two major vessels, which may
subsequently result in myocardial ischaemia. Coronary CT angiography may also provide valid and useful
information in patients with vasculitis and aneurysms,
fistulas or dissection.
 |
| Figure 5. For scanning the chest with bright contrast in the pulmonary
arteries, aorta and coronary arteries simultaneously, the contrast bolus
needs to be stretched and low iodine delivery rates (1 g iodine per sec)
must be used. |
If the potential of coronary CT angiography for the
direct detection of vulnerable plaques in patients with
acute coronary syndromes is realised, new strategies
will need to be considered for the appropriate
treatment of these patients. Non-invasive vulnerable
plaque detection may probably justify intensive medical
treatment or may lead to invasive approaches such as
plaque sealing.
CT angiography may soon serve as a tool for a
complete diagnostic work-up of patients presenting
with atypical chest pain. A single CT angiography
investigation will allow pulmonary emboli, aortic
dissection or coronary thrombus to be either ruled in
or out (Figure 5).
CT emergency
MDCT scanning provides a rapid overview even in
critically ill patients that need observation at the same time. It is therefore quite logical that MDCT scanners
have been installed in some institutions dedicated to
emergency treatment, replacing conventional imaging
almost completely where available. Any spine, bone or
joint trauma as well as any vessel injury and parenchymal laceration can be displayed within a few
seconds. MDCT simultaneously provides important
information about the localization, severity and extent
of bleeding or ischaemia.
Future technical and clinical potentials
The use of MDCT for emergency investigation has
become a routine application. Non-invasive CTA imaging
of the coronary arteries is the main driver of further
developments in MDCT. Some technical improvements
in MDCT are foreseeable within the next few years, such
as shorter exposure times and higher spatial resolution.
Exposure times will become short enough to scan the coronary arteries at any heart rate without the necessity
of administering b-blockers and the higher spatial
resolution will reduce artifacts caused by metal or
calcium. Once these requirements are fulfilled, coronary
CT angiography performed with very little contrast
media may replace cardiac catheterization for the triage
of patients for conservative, interventional or surgical
therapy. Thus, it may reduce the use of invasive
diagnostic procedures in pre-selected patients in whom
coronary interventions are essentially required.
The widespread adoption of MDCT for screening will
mainly depend on the results of studies currently
underway investigating the reliability and efficiency of
this new application.
References
1. Flohr T, Stierstorfer K, Raupach R, et al. Performance evaluation of a
64-slice CT system with z-flying focal spot. Rofo 2004; 176: 1803–10.
2. Schoepf UJ, Becker CR, Obuchowski NA, et al. Multi-slice computed
tomography as a screening tool for colon cancer, lung cancer and
coronary artery disease. Eur Radiol 2001; 11: 1975–85.
3. Schmermund A, Mohlenkamp S, Stang A, et al. Assessment of clinically
silent atherosclerotic disease and established and novel risk factors for
predicting myocardial infarction and cardiac death in healthy middleaged
subjects: rationale and design of the Heinz Nixdorf RECALL Study.
Risk Factors, Evaluation of Coronary Calcium and Lifestyle. Am Heart J
2002; 144: 212–8.
4.Wormanns D, Fiebich M, Saidi M, et al. Automatic detection of
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with virtual computed tomographic colonography. Am J Surg 2002; 183:
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6. Fleischmann D. Use of high concentration contrast media: principles
and rationale vascular district. Eur J Radiol 2003; 45: S88–S93.
7. Becker CR, Hong C, Knez A, et al. Optimal Contrast Application for
Cardiac 4-Detector-Row Computed Tomography. Invest Radiol 2003; 38:
690–4.
8. Doerfler A, Fiebach J,Wanke I, et al. Iodixanol in cerebral computed
tomography: a randomized, double-blind, phase-III, parallel study with
iodixanol and iohexol. Eur Radiol 1999; 9: 1362–5.
9. Aspelin P, Aubry P, Fransson SG, et al. Nephrotoxic effects in high-risk
patients undergoing angiography. N Engl J Med 2003; 348: 491–9.
10. Matsumoto M, Kodama N, Sakuma J, et al. 3D-CT arteriography and
3D-CT venography: the separate demonstration of arterial-phase and
venous-phase on 3D-CT angiography in a single procedure. AJNR Am J
Neuroradiol 2005; 26: 635–41.
11. Leschka S, Alkadhi H, Plass A, et al. Accuracy of MSCT coronary
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