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Case Study
Use of MDCT Coronary Angiography
Imaging of coronary arteries with selective conventional
angiography based on severity of luminal stenosis remains the basis for planning
and guiding catheter-based and surgical myocardial revascularisation treatment
in the assessment of advanced stages of CAD
Dr Kapisoor Singh
Consultant Cardiac & Interventional Radiologist
KG Hospital, Coimbatore |
Since its introduction in the 1960s, catheter-based cine Coronary
Angiography (CAG) has largely defined our understanding of normal and pathologic
coronary anatomy. The precise angiographic depiction of luminal narrowing remains
the basis for catheter-based or surgical revascularisation of the myocardium
in the distribution of significantly diseased coronary arteries. However, it
is now appreciated that the accumulation of atherosclerotic plaque in the coronary
arterial wall begins much earlier than the development of luminal stenosis.
In fact, most acute coronary syndromes are initiated by sudden disruption of
atherosclerotic plaques that are not causing significant stenosis. Therefore,
the assessment of these early stages of Coronary Artery Disease (CAD) has emerged
as an important goal in preventing both CAD progression and complications of
atherosclerotic CAD (for example, myocardial infarction). Because early atherosclerotic
plaque accumulation is typically associated with compensatory vessel expansion
(positive arterial remodeling), the description of these important early changes
of CAD on the basis of alterations in coronary luminal dimensions (evaluated
by CAG-luminograms) alone, is not sufficient.
Approximately 50 per cent of all acute coronary syndromes occur in previously
asymptomatic subjects, hence there obviously is a need to identify these subjects
before coronary atherosclerosis clinically manifests and irreversible damage
occurs by progression to myocardial infarction or cardiac death.
While invasive techniques will remain vital to the diagnosis and treatment of
significantly stenotic coronary lesions, the comprehensive and serial assessment
of asymptomatic or minimally symptomatic stages of CAD for preventive purposes
will eventually need to rely on non-invasive imaging techniques. Cardiovascular
imaging with tomographic modalities, including Computed Tomography (CT) and
Magnetic Resonance Imaging (MRI), has great potential for providing this information.
Intravascular Ultrasonography (IVUS), Optical Coherence Tomography (OCT), virtual
histology using backscatter radiofrequency, thermography (infrared imaging)
are some of the techniques requiring selective coronary catheterisation that
have allowed excellent visualisation of the coronary arterial wall and direct
assessment of atherosclerotic plaques. However, these are invasive and scarcely
available.
Experience with 128 slice MDCT Angiography of Coronary Arteries
CT Angiographic Coronary Anatomy in Symptomatic Patients: Advanced CAD is characterised
by multiple stenotic lesions of variable severity, causing chronically reduced
coronary blood flow and potentially gradual or sudden vessel occlusion. Consequently,
advanced disease is clinically manifested by a wide variety of disease states,
including stable angina pectoris, chronic heart failure or acute coronary syndromes.
In these clinical situations with a high pre-test likelihood of significant
stenotic atherosclerotic disease, the definitive identification and accurate
quantification of luminal stenoses in the entire epicardial coronary tree are
necessary. Selective conventional angiography provides this information with
high spatial (0.1-0.2mm) and temporal (4-7m sec) resolution and allows simultaneous
catheter-based therapeutic interventions (angioplasty, stent placement). Therefore,
selective angiography will remain vital for guiding interventional or surgical
(bypass grafting) treatment of significantly stenotic coronary lesions.
However, in other clinical situations, the comprehensive assessment of obstructive
and non-obstructive CAD will eventually increasingly rely on non-invasive imaging
techniques. CCTA can demonstrate the presence of atherosclerotic plaque over
a decade earlier than other tests including invasive cardiac catheterisation.
Coronary angiography with use of multi-detector row CT is typically performed
in a spiral scan mode, which permits three-dimensional scanning of the entire
heart in a single breath hold. Data is retrospectively referenced to the ECG
signal to reconstruct images during the diastolic phase or other phases of the
cardiac cycle.
Adjacent overlapping transverse images are reconstructed with a minimum section
thickness of as little as 0.6 mm and a maximum in-plane spatial resolution of
approximately 0.3 x 0.3mm. Temporal resolution is 40-160mm depending on image
reconstruction algorithm/ protocol and whether a single source or a dual source
scanner is being used. Patient radiation dose with current high-end multi-detector
row CT scanners is approaching that at conventional coronary angiography (3.5-6
milli Severt).
A well-recognised limitation in the assessment of coronary
stenosis with CT angiography is related to 'beam hardening artefact' produced
by dense objects in coronary walls/ their vicinity - like densely calcified
plaques, metallic stents, surgical clips and implants. The resulting 'blooming'
effect of coronary arterial calcification causes difficulties in assessing adjacent
plaque structures, potentially resulting in a false-positive detection of stenosis
or overestimation of the degree of narrowing. Similarly, surgical clips and
coronary stent struts produce streaky artefacts which preclude confident detection
and grading of adjacent coronary artery lumen or in-stent restenosis. However,
differentiation between stent patency and occlusion, as well as evaluation of
stenosis at the leading or trailing ends of a stent is often directly or indirectly
(aided by assessment of flow distal to the stent) possible.
Curved MPR 128-slice MDCTA image showing soft and mixed plaques causing diffuse
disease in right coronary artery
3D volume rendered 128-slice MDCTA image in a post bypass graft (CABG) patient
In spite of the surgical clips, CT angiography has advantage in assessment of
bypass grafts because of unknown/ variable positions of the graft ostia pose
difficulty in their selective catheterisation.
An important advantage of Multi Detector row CT Angiography (MDCT-CA) over conventional
angiography is that additional information about cardiac and non-cardiac anatomy
is simultaneously provided. This information is of great clinical importance
because, for example, the demonstration of an anomalous artery tracking between
the aorta and the pulmonary artery may define an indication of surgical correction.
Additional non-atherosclerotic coronary abnormalities detectable at multi-detector
row CT angiography include the intra-myocardial course of a coronary artery
due to a myocardial bridge. In acute onset severe chest pain CTA is the one-stop
shop for "triple rule-out" of aortic dissection, pulmonary embolism
and acute coronary syndrome.
MDCT-CA seems to be effective as a pre-operative screening test prior to non-coronary
cardiac surgery. In this era of cost containment and optimal care of patients,
MDCT-CA is able to provide coronary vessel and ventricular function evaluation
and may become the method of choice for the assessment of a cardiovascular risk
profile prior to major surgery.
It is important to understand that the lower spatial resolution of CT angiography
is more problematic in the assessment of highly stenotic lesions, a subtotal
coronary occlusion with a lumen size below the resolution of CT cannot be differentiated
from a total occlusion. Also, very small coronary arterial branches and sub-branches
that are visualised on CAG may not be visualised on CTA because of lower spatial
and temporal resolution.
Also, about five to ten per cent of CTA studies are suboptimal because of an
unexpected rise in heart rate or an unexpectedly wide beat-to-beat variability
at the time of acquisition in spite of an optimal heart rate just before the
start of contrast injection during CTA. This results in motion artefacts rendering
many arterial segments non-evaluable.
On the other hand, CT angiography has the advantage of vessel wall and plaque
depiction in addition to its ability to enable assessment of luminal dimensions.
The assessment of size and composition (calcification and fat content) of coronary
arterial lesions and the associated changes in vessel architecture, for instance,
arterial remodeling may have important clinical implications.
Plaque Burden Assessment and Plaque Characterisation in Asymptomatic or Minimally
Symptomatic Patients: The number of significant stenoses in the coronary arterial
tree as assessed with selective conventional angiography has prognostic value.
However, serial angiographic observations of existing severe stenoses do not
allow the assessment of the effect of risk-factor modification. Generally, changes
in atherosclerotic plaque size are not well reflected in luminal dimensions,
because plaque progression and regression are associated with arterial expansion
(positive remodeling) and shrinkage (negative remodeling). This limitation of
angiographic lesion assessment has become evident from prevention trials with
lipid-lowering medications that showed a statistically significant reduction
in clinical events but only minimal change in the severity of existing angiographic
stenoses.
In addition, findings of several studies have shown that most acute coronary
syndromes are initiated by sudden changes of mildly stenotic lesions, commonly
found in positively remodelled arterial lesions, rather than from progression
of lesions already causing significant luminal narrowing. Therefore, it has
been postulated that the identification of mildly stenotic but vulnerable atherosclerotic
lesions and the overall plaque burden could provide better markers of coronary
risk than do measures of luminal stenosis. This hypothesis is currently being
examined in intravascular US studies. Multi-detector row CT has a potential
to address several important parameters about early atherosclerotic changes
in the coronary arteries.
Calcium Scoring
It is a very low radiation-dose CT scan (<1 mSv) that does not involve injecting
of intravenous contrast medium. It has been shown that the negative predictive
value of a zero calcium score in asymptomatic individual is 97-99 per cent and
in patients with atypical angina symptoms, it is 90-95 per cent. The calcium
score has been shown to predict cardiac events independently of standard risk
factors and enhance Framingham risk stratification categories. The score provides
a more accurate rationale for determining the necessity for LDL-lowering therapy
and risk factor management. Calcium scoring helps identify the calcified portion
of the overall atherosclerotic plaque burden. However, non-calcified atherosclerotic
plaque (which in fact could have micro-calcifications below the threshold of
detection with calcium scoring techniques) may be more unstable and prone to
rupture, causing acute coronary syndromes.
The morphologic characteristics of unstable or vulnerable plaques are being
understood. Differences between stable and unstable coronary plaques have been
examined with invasive catheter-based methods like coronary intravascular US
(hypoechoic plaque has been associated with the clinical presentation of unstable
angina), optical coherence tomography (capable of differentiating lipid and
fibrous components as well as calcification at higher resolution than IVUS),
intravascular thermography and with non-invasive methods like nuclear imaging
(PET-CT, SPECT-CT) using multiple radiopharmaceutical approaches.
On CTA, the vulnerable plaques appear as soft plaques with excessive (thick)
lipid content (hypodense, <30 Hounsfield unit) and thin fibrous cap (fibrous
tissue is more dense, 50-80 HU) with or without calcification (>300HU).
According to a recent Korean study published in Journal of the American College
of Cardiology July, 2008; screening coronary angiography (64-slice MDCT) of
1,000 asymptomatic middle aged (50+9 yrs) Korean subjects was done to detect
occult CAD as part of a general health evaluation. Atherosclerotic plaques were
identified in 22 per cent individuals, four per cent had only non-calcified
plaques, five per cent subjects had significant (>50per cent ) diameter stenosis
and 2per cent had severe (>75per cent ) stenosis. Midterm follow-up (17 +
2 months) revealed 15 cardiac events only in those with CAD on CTA: 1 unstable
angina requiring hospital stay and 14 revascularization procedures. 87per cent
events occurred within 90 days of index CTA.
Thus, the prevalence of occult CAD in apparently healthy individuals was not
negligible and CTA has a potential to provide a better insight about the occult
CAD in this population.
The problem in applying CTA as a screening tool for asymptomatic population
is the radiation exposure at CT Angiography. The Food and Drug Administration
announced that there is a small chance (one in 2,000) of developing a fatal
cancer due to a 10-mSv CT study. However, with newer MDCT-scanners allowing
coronary CTA in 3.5-8 mSv radiation dose (depending on the protocol chosen),
persons with low and intermediated risk for heart attack (risk factors include
diabetes, obesity, smoker (past or current), high blood pressure, family history
of heart disease, high cholesterol and age above 45 years for males and age
above 50 years for females can be considered for screening CTA examination.
With the chances of having a heart attack in this subgroup much greater than
developing breast and other cancer following a coronary CTA, screening can save
many lives.
 With
rapidly evolving MDCT technology, the radiation dose for CTA would likely be
soon minimised to less than 3 mSv (less than the level of annual background
natural radiation exposure).
Overview
Imaging of coronary arteries with selective conventional angiography based on
severity of luminal stenosis remains the basis for planning and guiding catheter-based
and surgical myocardial revascularisation treatment in the assessment of advanced
stages of CAD. In contrast, prevention of coronary events requires identification
of early stages of atherosclerosis and the associated abnormalities in coronary
architecture before the development of luminal stenosis. Here comes the role
of coronary plaque imaging methods like IVUS, OCT, MDCT, PET/CT, SPECT/CT. Catheter-based
diagnostic techniques, like IVUS and OCT allow high-resolution visualisation
of a wide range of coronary lesions/ plaques, independent of their luminal dimensions
but they are invasive, complicated and scarcely available.
Non-invasive simple and rapid MDCT coronary imaging technique is rapidly gaining
ground for the routine examination of asymptomatic low-risk or minimally symptomatic
patients with moderate risk despite all the politics involved. MDCT-CA can be
applied to both the assessment of significant luminal stenosis and the identification
of non-stenotic atherosclerotic plaques. It could become complementary to conventional
angiography in the assessment of selected patients with stenotic atherosclerotic
or non-atherosclerotic coronary disease providing an overall assessment of cardiac
anatomy beyond coronary imaging, including cardiac function assessment, paralleling
capabilities of cardiac MR imaging.
It has already a well-established role in non-coronary causes of chest pain.
The non-invasive characterisation and quantification of atherosclerotic plaque
burden has important implications for risk stratification and prevention of
CAD progression and/ or its complications. In centres where both angiographic
and tomographic imaging modalities are available, the integration of tomographic
image information will facilitate comprehensive non-invasive imaging of patients
with early and those with late stages of CAD.
The clinical impact of non-invasive tomographic imaging technologies, such as
multi-detector row CT, on imaging of coronary arteriosclerosis will need further
elucidation, but the prospects are exciting.
kapisoor@gmail.com
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