Cardiac CT

A Norouzi MD

Jan. 2014

• Computed tomography (CT) A technique that can noninvasively fully evaluate

cardiac structure function

• The basic principle of CT technology is the use of ionizing radiation within a gantry rotating around the patient in which x-rays are detected on a detector array and converted through reconstruction algorithms to images.

• MDCT ( Multidetector CT) The most notable technical advance is progressive

increase in the number of detector rows (or slices). Each row is a narrow channel, approximately 0.625 mm

in width, through which x-rays are detected on scintillation crystals. The number of detector rows aligned in an array has increased from a single detector to 4, 16, and 64 (present standard technology) and now on to “wide” detectors of 256 to 320 rows.

The increase in the number of rows leads to wider coverage (more of the heart viewed simultaneously, e.g., 64 rows of 0.625-mm width produces approximately 4 cm of coverage, ) leading to shorter scan acquisition times and consequently reduced radiation exposure and contrast requirements.

4 to 64 Slice ScansFive Heart Beats

10 mm detectorPitch ~0.25

3 cm in 5 sec

20 mm detectorPitch ~0.25

6.2 cm in 5 sec

40 mm detectorPitch ~0.25

12.5 cm in 5 sec

Scan Modes There are two basic scan modes in cardiac CT, helical (spiral) and axial (sequential, step & shoot) scanning.

Helical (spiral) scanning : Most current MDCT scanners use spiral, retrospectively gated acquisition techniques. Helical scanning involves continuous radiation exposure and table movement (the patient is moved through the rotating x-ray beam), during which the detector arrays receive projection data from multiple contiguous slices of the patient.

Axial (sequential, step & shoot) scanning : axial imaging involves sequential scanner “snapshots,” in between which the x-ray tube is turned off and the table is moved to a different position for the next image to be acquired.

EC

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Diagram showing effect of ECG dose modulation.

In Fig 1- continuous scanning throughout the cardiac cycle with full tube current , resulting in high radiation dose.

In Fig 2- ECG dose modulation is turned on and full tube current is applied only during 40-80% of cardiac cycle, where cardiac motion is least.

In Fig 3- To further decrease radiation, a single phase of cardiac cycle is selected for scanning during which full tube current is applied.

PROSPECTIVE ECG GATING

Scan acquisition is triggered by the ECG signal at the prospected mid-diastolic phase of the cardiac cycle.

Between 40% and 80% of the R-R interval

Benefits: Smaller patient radiation dose

Limitation: Reconstruction of image in different cardiac phase for functional analysis of ventricle is not feasible

ECG gating

a. Prospective triggering: The trigger signal is derived from the patient’s ECG based on a prospective estimation of the RR interval. The scan is usually triggered to begin at a defined point after the R wave, usually allowing image acquisition to occur during diastole.

Advantage: ● dose efficient (80% reduction in x-ray exposure) Disadvantage : ● limited portion of cardiac cycle data set obtained

● greatly depends on the regularity of patient’s heart rate

b. Retrospective gating : Collects data during the entire cardiac cycle. Once scan is complete , data from specific periods of the cardiac cycle are used for image reconstruction by retrospective referencing to the ECG signal.

Advantage : allows assessment of cardiac function via four-dimensional reconstruction.

Disadvantage : higher radiation dose exposure.

Contr

ast

Mediu

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Optimal coronary artery

opacification depends on

1. The iodine medium concentration (300-400 mg iodine/ ml is used)

2. The volume and rate of contrast administration

3. Timing of the contrast medium delivery.

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Volu

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Using 64 detector MDCT technology:

80ml of contrast agent is injected at 6 ml/sec f/b 40ml saline solution at 4ml/sec

Using 16 detector MDCT technology:

100- 120 ml of contrast agent at 4 to 5 ml per sec.

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Delivery of contrast medium s/b timed to ensure that the scan of cardiac region will occur at the peak of opacification of the coronary tree.

It can be assessed by two techniques-

1. Automated contrast bolus tracker technique- the ROI is placed on ascending aorta. When ct value of ROI is greater than predetermined threshold of 100- 150 HU, the scan begins.

2. Test bolus scan – here a small bolus of contrast is injected to determine contrast transit time. The time from the start of the injection to the peak contrast enhancement in the ascending aorta determines the scan delay after the initiation of contrast material administration.

After contrast administration, CT is obtained in single breath-hold

Scan volume covers the entire heart from the proximal ascending aorta (approximately 1–2 cm below the carina) to the diaphragmatic surface of the heart

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Indications

and Guidelines

for cardiac CT

ACCF/SCCT/ACR/AHA/ASE/

ASNC/NASCI/SCAI/SCMR 2010

Appropriate Use Criteria for

Cardiac Computed Tomography

A Report of the American College of Cardiology Foundation Appropriate Use Criteria Task Force ,

the Society of Cardiovascular Computed Tomography ,

the American College of Radiology ,

the American Heart Association,

the American Society of Echocardiography ,

the American Society of Nuclear Cardiology ,

the North American Society for Cardiovascular Imaging ,

the Society for Cardiovascular Angiography and Interventions ,

and the Society for Cardiovascular Magnetic Resonance

In general, use of CCT angiography for diagnosis and risk assessment in patients with low or intermediate risk or pretest probability for coronary artery disease (CAD) was viewed favorably, whereas testing in high-risk patients, routine repeat testing, and general screening in certain clinical scenarios were viewed less favorably.

Use of noncontrast computed tomography (CT) for calcium scoring was rated as appropriate within intermediate- and selected low-risk patients.

In total, 35 of 93 indications were judged to be appropriate, and 58 were judged to be either inappropriate or uncertain.

uncertain indications often require individual physician judgment and understanding of the patient to better determine the usefulness of a test for a particular scenario.

As such, the ranking of an indication as uncertain (4 to 6) should not be viewed as limiting the use of CCT imaging for such patients.

It should be emphasized that the technical panel was instructed that the uncertain designation was still designed to be considered as a "reimbursable" category.

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Table. Appropriate Indications (Median Score 7–9)

Indication Appropriate Use Score (1–9) Detection of CAD in Symptomatic Patients Without Known Heart Disease Symptomatic—Nonacute Symptoms Possibly Representing an Ischemic Equivalent

1. ECG interpretable AND Able to exercise

Intermediate pretest probability of CAD A (7)

2. ECG uninterpretable or unable to exercise

Low pretest probability of CAD A (7)

3. ECG uninterpretable or unable to exercise

Intermediate pretest probability of CAD A (8)

Detection of CAD in Symptomatic Patients Without Known Heart Disease Symptomatic—Acute Symptoms With Suspicion of ACS (Urgent Presentation)

4. Normal ECG and cardiac biomarkers

Low pretest probability of CAD A (7)

5. Normal ECG and cardiac biomarkers

Intermediate pretest probability of CAD A (7)

6. ECG uninterpretable

Low pretest probability of CAD A (7)

7.ECG uninterpretable. Intermediate pretest probability of CAD A (7)

8. Nondiagnostic ECG or equivocal cardiac biomarkers, Low pretest probability of CAD A (7)

9. Nondiagnostic ECG or equivocal cardiac biomarkers .Intermediate pretest probability of CAD A (7)

Detection of CAD/Risk Assessment in Asymptomatic Individuals Without Known CAD—Noncontrast CT for CCS

10. Family history of premature CHD

Low global CHD risk estimate A (7)

11. Asymptomatic ,No known CAD

Intermediate global

risk estimate A (7)

Detection of CAD in Other Clinical Scenarios—New-Onset or Newly Diagnosed Clinical HF and No Prior CAD

12. Reduced left ventricular ejection fraction. Low pretest probability

of CAD A (7)

13. Reduced left ventricular ejection fraction. Intermediate pretest probability of CAD A (7)

Detection of CAD in Other Clinical Scenarios—Preoperative Coronary Assessment Prior to Noncoronary Cardiac Surgery

14. Coronary evaluation before noncoronary cardiac surgery

Intermediate pretest probability of CAD A(7)

Use of CTA in the Setting of Prior Test Results—Prior ECG Exercise Testing

15. Normal ECG exercise test

Continued symptoms A (7)

16. Prior ECG exercise testing

Duke Treadmill Score—intermediate risk findings A (7)

Use of CTA in the Setting of Prior Test Results—Sequential Testing After Stress Imaging Procedures

17. Discordant ECG exercise and imaging results A(8)

18. Stress imaging results: equivocal A (8)

Use of CTA in the Setting of Prior Test Results—Prior CCS

19. Diagnostic impact of coronary calcium on the decision to perform contrast CTA in symptomatic patients ,CCS <100 A (8)

20. Diagnostic impact of coronary calcium on the decision to perform contrast CTA in asymptomatic patients ,CCS 100–400 A (8)

Use of CTA in the Setting of Prior Test Results—Evaluation of New or Worsening Symptoms in the Setting of Past Stress Imaging Study

21. Previous stress imaging study normal

A (8)

Risk Assessment Postrevascularization (PCI or CABG)—Symptomatic (Ischemic Equivalent)

22. Evaluation of graft patency after CABG

A (8)

Risk Assessment Postrevascularization (PCI or CABG)—Asymptomatic—Prior Coronary Stenting

23. Prior left main coronary stent with stent diameter =3 mm A(7)

Evaluation of Cardiac Structure and Function—Adult Congenital Heart Disease

24. Assessment of anomalies of coronary arterial and other thoracic arteriovenous vessels A (9)

25. Assessment of complex adult congenital heart disease

A (8)

Evaluation of Cardiac Structure and Function—Evaluation of Ventricular Morphology and Systolic Function

26. Evaluation of left ventricular function ,Following acute MI or in HF patients.Inadequate images from other noninvasive methods

A (7)

27. Quantitative evaluation of right ventricular function A(7)

28. Assessment of right ventricular morphology Suspected arrhythmogenic right ventricular dysplasia A (7)

Evaluation of Cardiac Structure and Function—Evaluation of Intra- and Extracardiac Structures

29. Characterization of native cardiac valves ,Suspected clinically significant valvular dysfunction .Inadequate images from other noninvasive methods A (8)

30. Characterization of prosthetic cardiac valves Suspected clinically significant valvular dysfunction, Inadequate images from other noninvasive methods A (8)

31.Evaluation of cardiac mass (suspected tumor or thrombus) .Inadequate images from other noninvasive methods A (8)

32. Evaluation of pericardial anatomy A (8)

33. Evaluation of pulmonary vein anatomy,Prior to radiofrequency ablation for atrial fibrillation

A (8)

34. Noninvasive coronary vein mapping,Prior to placement of biventricular pacemaker A (8)

35. Localization of coronary bypass grafts and other retrosternal anatomy,Prior to reoperative chest or cardiac surgery A (8)

CCT was felt to be appropriate primarily for situations involving a low or intermediate pretest probability of obstructive CAD.

Scenarios involving high-probability CAD patients were rated as uncertain

Screening asymptomatic patients using coronary CT angiography was considered inappropriate, as was repeat coronary calcium testing.

Repeat CT angiography in asymptomatic patients or patients with stable symptoms with prior test results was broadly considered inappropriate.

The evaluation of coronary stents was considered as a function of patient symptom status, time from revascularization, and stent size.

Only with larger stents (=3 mm in diameter) after long time periods (=2 years) was stent imaging considered uncertain, and only with left main stents was imaging of stents considered appropriate.

A strength of cardiac CT imaging is the evaluation of cardiac structure and function.

Appropriate indications include coronary anomalies, congenital heart disease, evaluation of right ventricular function, evaluation of left ventricular ejection fraction when images from other techniques are inadequate, or evaluation of prosthetic heart valves.

New to this document is the use of CCT for evaluation of myocardial viability when other modalities are inadequate or contraindicated (uncertain), and in suspected arrhythmogenic right ventricular dysplasia (appropriate).

Indications:

A. Evaluation of chest pain in patients at low to intermediate pretest probability of disease and persistent chest pain after an equivocal stress test.

B. Suspicion of coronary artery anomalies. MDCT has very high sensitivity and specificity for coronary anomalies.

C. Pulmonary vein evaluation can be performed, often before or after pulmonary vein isolation for atrial fibrillation.

D. Evaluation of cardiac masses when other modalities such as TTE, TEE, or MRI are unrevealing.

E. Evaluation of pericardial disease when other modalities such as TTE, TEE, or MRI are unrevealing.

F. Assessment of anatomy in complex congenital heart disease.

G. Presurgical evaluation, particularly before redo open heart surgery. MDCT can aid in describing prior bypass graft location, identifying safe sites for surgical approach.

H. Assessing graft patency after prior bypass surgery is feasible in many cases, though sometimes limited by artifacts related to calcium and surgical clips.

I. Evaluation of aortic disease. MDCT is the test of choice for evaluating aortic aneurysm and suspected aortic dissection.

J. Evaluation of suspected pulmonary embolism

CONTRAINDICATIONS:Unlike with cardiac MRI, few absolute contra indications exist for cardiac CT. However, there are important risks associated with radiation and/or contrast exposure that must be weighed against the benefits of the scan.

Absolute contraindications :

A. Renal insufficiency. Given the potential for contrast nephropathy, patients with significant renal insufficiency (i.e., Cr > 1.6 mg/dL) should not undergo contrast-enhanced CT unless the information from the scan is critical and the risks/benefits are thoroughly discussed with the patient.

B. Known history of anaphylactic contrast reactions A prior anaphylactic response to contrast is generally felt to be an absolute contraindication to intravenous iodinated contrast administration at many institutions.

C. Pregnancy

D. Clinical instability

Relative contraindications

A. Contrast (iodine) allergy. Patients with allergic reactions to contrast should be pretreated with diphenhydramine and steroids before contrast administration.

B. Recent intravenous iodinated contrast administration. Patients who have received an intravenous dose of iodinated contrast should avoid contrast-enhancedCT scanning for 24 hours to reduce the risk of contrast nephropathy.

C. Hyperthyroidism. Iodinated contrast is contraindicated in the setting of uncontrolled hyperthyroidism due to possible precipitation of thyrotoxicosis.

D. Atrial fibrillation or any irregular heart rhythm, is a contraindication to coronary CT angiography due to image degradation from suboptimal ECG gating.

E. Inability to breath hold for at least 10 seconds. Image quality will be significantly reduced due to respiratory motion artifact if the patient cannot comply with breath hold instructions.

F. Morbid obesityG. Severe coronary calcium

CTA

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Rapid (>80 bpm) and irregular HR

High calcium scores (>800-1000)

Stents

Contrast requirements (Cr > 2.0 mg/dl)

Small vessels (<1.5 mm) and collaterals

Obese and uncooperative patients

RADIATION EXPOSURE

Patients are optimally suited for CCT under the following conditions: Regular heart rate and

rhythm including a heart rate at a level commensurate with the temporal resolution of the available scanner.

Body mass index below 40 kg/m2.

Normal renal function.

For CT angiography, patient requirements may include the ability to: Hold still and follow

breathing instructions.

Tolerate beta blockers.

Tolerate sublingual nitroglycerin.

Lift both arms above the shoulders.

SAFETY

A. Radiation exposure : Radiation doses of cardiac CT scans vary greatly depending on the scan parameter settings, scan range (cranial-caudal length of the scan), gender (women receive more radiation due to breast tissue), and patient body habitus (obesity increases exposure).

● Chest x-ray is 0.04 to 0.10 mSv,

● Average annual background radiation 3 to 3.6 mSv.

● Invasive diagnostic coronary angiography 2.1 to 4 mSv.

● Coronary CT angiography 4 to 11 mSv. With use of prospective-ECG triggering, axial imaging modes, and software adaptations, recent studies have reported the feasibility of CT coronary angiography with comparable image quality and substantially reduced radiation doses (i.e., 1.1 to 3.0 mSv). This remains an area of active investigation.

Effective Dose of Selected Radiologic Examinations

PA/Lateral CXR 0.04-0.06 mSv

Head CT 1-2 mSv

Chest CT 5-7 mSv

Abd/Pelvis CT 8-11 mSv

Diagnostic Cor Angiogram 3-5 mSv

MSCT angiography 9.3-11.3 mSv

Morin et al. Circulation 2003;107:917-22.

*Average annual background radiation in U.S ~ 3.6 mSv

Radiation Risks

Exact quantification of harmful effects of radiation difficult to ascertain

For a child under age 15, the risk of cancer death from a single CT scan is approximately 1 in 500

For a 45 year old adult, the risk of death from cancer from a single CT exam is about 1 in 1,250

Brenner et al. Radiology, 231(2):440-445.

RADIATION DOSE

Ranges between 12-16 mSv depending on CT scanner and type of ECG gating used.

ECG-controlled dose modulation systems allows reduction of radiation exposure by upto 50%

Lower the KVP to 100 causes significant dose reduction.

A prospective gate window of 20% over diastole in patients with HR of 60, can reduce total dose by 80%.

There is a significant difference between calculated effective doses for prospective CT angiography (mean, 4.1 mSv ± 1.8; median, 3.2 mSv; 95% CI: 3.5 mSv, 4.5 mSv) and retrospective CT angiography (mean, 20.0 mSv ± 3.5; median, 19.5 mSv; 95% CI: 18.9 mSv, 21.1 mSv) (P < .001).

The radiation dose of prospective CT angiography was reduced by 79% compared with that of retrospective CT angiography.

The mean effective radiation dose per examination in the RGH technique group was 18.4 mSv ± 2.4 (range, 8.7–22.0 mSv), and the mean effective dose in the PGT technique group was 2.8 mSv ± 1.3 (range, 0.75–6.7 mSv) (P < .001)

Figure 1: PGT technique relies on combined approach with transverse data acquisition, adaptive ECG triggering, incrementally moving table, and multiphase image reconstructions. Table is stationary during acquisition of 40-mm coverage (64 detector rows and 0.625-mm section thickness [64 × 0.625 mm]) group of transverse scans and then moves 35 mm, allowing a 5-mm overlap of image groups, to the next location for another scan that is initiated by the subsequent normal cardiac cycle. An adaptive prediction algorithm is used for dynamic prediction of the heart rate for the next cardiac cycle instead of using a single heart rate for the entire study.

Radiology, http://pubs.rsna.org/doi/abs/10.1148/radiol.2463070989

Published in: James P. Earls; Elise L. Berman; Bruce A. Urban; Charlene A. Curry; Judith L. Lane; Robert S. Jennings; Colin C. McCulloch; Jiang Hsieh; John H. Londt; Radiology  2008, 246, 742-753.© RSNA, 2008

One PowerPoint slide of each figure may be downloaded and used for educational not promotional purposes by an author for slide presentations only. The ATS citation line must appear in at least 10-point type on all figures in all presentations. Pharmaceutical and Medical Education companies must request permission to download and use slides, and authors and/or publishing companies using the slides for new article creations for books or journals must apply for permission. For permission requests, please contact the Publisher at .

Figure 4: Distribution of effective radiation doses for RGH and PGT technique groups. Effective dose in RGH group was 8.7–22.0 mSv (mean, 18.4 mSv ± 2.4). Effective dose in PGT group was 0.75–6.7 mSv (mean, 2.8 mSv ± 1.3). This difference represents a mean 83% reduction from the dose with RGH technique to the dose with PGT technique; the difference was significant (P < .001).

Radiology, http://pubs.rsna.org/doi/abs/10.1148/radiol.2463070989

Published in: James P. Earls; Elise L. Berman; Bruce A. Urban; Charlene A. Curry; Judith L. Lane; Robert S. Jennings; Colin C. McCulloch; Jiang Hsieh; John H. Londt; Radiology  2008, 246, 742-753.© RSNA, 2008

One PowerPoint slide of each figure may be downloaded and used for educational not promotional purposes by an author for slide presentations only. The ATS citation line must appear in at least 10-point type on all figures in all presentations. Pharmaceutical and Medical Education companies must request permission to download and use slides, and authors and/or publishing companies using the slides for new article creations for books or journals must apply for permission. For permission requests, please contact the Publisher at .

CLINICAL APPLICATIONS

A. Coronary calcium scoring Coronary calcium is a surrogate marker for coronary

atherosclerotic plaque. Coronary artery calcium score is directly proportional to the overall extent of atherosclerosis, although typically only a minority (approximately 20%) of plaque is calcified.

Complete absence of coronary artery calcium makes the presence

of significant coronary luminal obstruction highly unlikely and indicates a very low risk of future coronary events.

Men, CKD, diabetics tend to have higher coronary calcium scores.

Contrast is not necessary because calcium is readily identified secondary to its very high x-ray attenuation coefficient (high Hounsfield unit score).

The Agatston coronary artery calcium (CAC) score is the most frequently used scoring system. It is derived by measuring the area of each calcified coronary lesion and multiplying it by a coefficient of 1 to 4, depending on the maximum CT attenuation within that lesion.

Volume score

Mass score

A coronary calcium coverage score : multivessel coronary calcium, the number of calcified lesions and diffuse spotty pattern (small foci <3 mm) are associated with a higher clinical risk.

The CAC score can be classified into five groups: 1) zero, no coronary calcification;2) <100, mild coronary calcification; 3) > 100 to 399, moderate calcification;4) >400 to 999, severe calcification; 5) > 1000, extensive calcification.

Coronary calcium score (CCS) and all* coronary

disease events Relative risk ratio (95%CI)

n† (weighted) Event rate %

CAC SCORE

1.0 (reference) 1504 1.0 0

1.9 (0.8-4.2) 1973 2.6 1-99

10.2 (4.8-21.6) 686 3.8†† 100-399

26.2 (12.6-53.7) 450 6.3 >400

*Included coronary death, nonfatal MI, coronary bypass surgery and percutaneous coronary angioplasty .

†n=4,613 subjects with follow-up data ††1.3%/year for total CHD events, .58%/year for hard CHD events (nonfatal MI and coronary

death) **33% of the participants had a score of 0

Hn x-factor(Agatston Scoring)

130-199 1

200-299 2

300-399 3

>400 4

Area = 15 mm2

Peak CT = 450Score = 15 x 4 = 60

Area = 8 mm2

Peak CT = 290Score = 8 x 2 = 16

Total Score = S

Calcium Volume Scoring

Ethnic Differences in Coronary CalcificationThe Multi-Ethnic Study of Atherosclerosis (MESA)

Bild DE et al. Circulation. 2005;111:1313-1320.

6814 men and women aged 45-84 years

In comparison with a CAC score of zero, the presence of any CAC is associated with a fourfold risk of coronary events over 3 to 5 years.

In patients at intermediate clinical risk for coronary events (e.g., by Framingham score), the CAC score can help to reclassify patients to a higher or lower risk group. For instance, a CAC score of zero confirms low risk of events. Conversely, a CAC score of greater than 400 is observed with a significant cardiac event rate (greater than 2 %/year) in patients who appear to be intermediate risk by Framingham score.

Because statins have no documented effect on CAC progression, there is no value in repeating CAC in persons with a score of greater than 100 or the 75th percentile.

However, not every atherosclerotic plaque is calcified, and even the detection of a large amount of calcium does not imply the presence of significant stenoses.Therefore, it adds only incrementally to traditional risk assessment and shouldnot be used in isolation. The test is most useful in intermediate risk populations,in which a high or low score may reclassify individuals to a higher or lower riskgroup. Unselected screening is not recommended.

CAC and Stenosis Severity : Significant coronary artery stenosis (>50%) by angiography is frequently associated with the presence of coronary artery calcium. However, the severity of angiographic coronary artery stenosis is not directly related to the total CAC.

CAC and Myocardial Ischemia : Good correlation between CT and myocardial perfusion SPECT for identifying both subclinical CAD and silent myocardial ischemia in a generally asymptomatic population who had risk factors for CAD development. Few patients with CACS <400 had a perfusion defect, whereas nearly half of the patients with CACS >400 had an abnormal SPECT.

Five-Year Mortality Rates in Framingham Risk Subsets by Coronary Calcium Score

Shaw et al. Radiology 2003; 228:826-833

*

*

**p<0.001

Coronary Disease Progression

Role for CTA >60% stenosis (+)

stress/imaging

Calcified Plaque Detected by CT

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Presence of any detectable calcium

Implies presence of

CAD

More aggressive BP control, lipid lowering

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.… Patients with high scores

(>400)

likelihood of harboring a

significant stenosis

Should undergo stress testing to evaluate for inducible ischemia

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Patients with intermediate scores

Require further testing based on other factors like age etc.

Score of zero

No need for further imaging tests

for Coronary disease

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Adva

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calc

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sc

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Gives an idea of whether CAD

is present, despite a lack of symptoms or is likely to develop in next few years develop in next few years.

Non invasive and less time consuming.

No contrast required needed.

The examination can suggests the presence of CAD even when the coronary arteries are <50% narrowed.

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Not all calcium deposits mean there is a blockade and not all blocked arteries contain calcium.

The earliest form of CAD soft plaque, cannot be detected by cardiac CT.

A high heart rate interferes with the test.

Men <35 yrs and women <40 yrs are not likely to benefit from cardiac CT for calcium scoring unless there is risk factors such as diabetes or a strong family history of heart disease.

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Association for the Eradication of Heart Attacks (AEHA.org)

Coronary CT angiography :

The primary clinical application of cardiac CT is the performance of noninvasive coronary CT angiography among patients with symptoms suggestive of myocardial ischemia. The overall accuracy of 64-row CT angiography included a sensitivity of 87% to 99% and specificity of 93% to 96%.Coronary CT angiography for evaluating CAD is most useful in low- to intermediate-risk patients with angina or anginal equivalent. The negative predictive value of coronary CT angiography is uniformly high in studies, approaching 93% to 100%; in other words, coronary CT angiography is an excellent modality for ruling out coronary disease.

Noncalcified plaque appears as a low to intermediate attenuation irregularity in the vessel wall. Calcified plaques are bright, high-attenuation lesions in the vessel wall and may be associated with positive remodeling of the vessel. Densely calcified plaques are often associated with calcium blooming artifact, which can lead to overestimation of luminal stenosis severity.

The accuracy of coronary CT angiography is highest in the larger proximal to medium vessels, which are more likely to benefit from an invasive management strategy. Coronary stenoses are generally categorized as mild (less than 50% diameter stenosis), moderate (50% to 70% stenosis), or severe (greater than 70% stenosis). Similar to results with invasive coronary angiography, the determination of an anatomic stenosis is only modestly predictive of inducible ischemia. A 50% or greater stenosis on cardiac CT has a 30% to 50% likelihood of demonstrable ischemia on myocardial perfusion imaging .

Detection of Noncalcified Plaque

Defined as any coronary arterial wall lesion with an x-ray attenuation detectably below the iodine contrast medium but higher than surrounding tissue.Such plaque is difficult to quantify, with limited accuracy and reproducibility. Detection requires maximal spatial and temporal resolution and minimized image noise with higher radiation exposures. Compared with intravascular ultrasound, the sensitivity of coronary MDCT is approximately 80%.

Dete

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Plaque features proposed to be associated with greater risk for plaque rupture or acute coronary syndromes include

Low-attenuation plaque (plaque <30 HU),

Outward arterial remodeling (artery diameter ratio of the involved segment to a proximal reference of 1.1 or greater), and

A spotty pattern (<3 mm in size) of calcification.

In particular, both low-attenuation plaque and outward arterial remodeling have been associated with increased risk of acute coronary events.

A threefold worse cardiovascular prognosis has been found in the setting of a greater number of coronary vessels and of coronary artery segments involved with plaque.

Bypass graft imaging :

1. Graft location : MDCT can accurately characterize the origin, course, and touchdown of prior bypass grafts

2. Graft patency : Using a protocol similar to that used for coronary artery assessment (less than 1 mm slice thickness), patency of both arterial and venous bypass grafts can be assessed. Recent studies have suggested that the sensitivity and specificity of MDCT for detecting stenosis or occlusion of bypass grafts, when compared with invasive angiography, is 97% and 97%, respectively. Occasionally, artifacts related to metallic clips can interfere with assessment of the distal anastomosis of an arterial graft (internal mammary or radial artery graft).

Stent patency:Image artifact from metallic stents limits the application in patients with prior coronary stent procedures. Small stents are difficult to evaluate and prone to noninterpretability. However, 90% accuracy can be obtained in stents 3 mm or greater in diameter with the use of sharp kernel and wide display window. Quantitative assessment of within-stent contrast density may assist in the diagnosis.

Coronary artery anomalies :

MDCT is an excellent modality for assessing patients with known or suspected coronary artery anomalies. MDCT can accurately assess the origin and course of anomalous coronaries, and can describe the relationship of the coronary artery to neighboring structures. Although MRI can also be used to assess anomalous coronaries without the need for radiation exposure, the spatial resolution, ease of data acquisition, and reliable image quality of MDCT make it a reasonable first choice. Intramyocardial bridging can also be detected with high sensitivity.

Cardiac morphology/function :

Contrast-enhanced MDCT can provide high resolution morphologic images of the cardiac chambers as well as accurate assessment of right and left ventricular systolic function. However, other imaging modalities such as echocardiography or MRI, which do not require radiation exposure, are generally preferred initially for assessing cardiac morphology.1. Patients with prior myocardial infarction can have fibrous

replacement of myocardium with or without calcification, ventricular wall thinning, aneurysm formation and cavitary thrombus.

2. Ventricular dysplasia is characterized by: fibrous and/or fatty replacement of myocardium, ventricular wall thinning and/or focal aneurysm formation, and ventricular cavity dilation with regional or global wall motion abnormalities.

3. Mass : CT provides somewhat less information about tissue type than

cardiac MRI.

Pericardial diseases :The pericardium appears as a thin line (1 to 2 mm) surrounding the heart, usually visible with a small amount of adjacent pericardiaI fat. The pericardium normally enhances with contrast administration; hyperenhancement of the pericardium in the appropriate clinical setting is characteristic of pericarditis.1. By CT, congenital absence of the pericardium is easily diagnosed.2. Findings of pericardial constriction on CT include irregular pericardiaI thickening and calcification, conical or tubular compression of one or both ventricles, enlargement of one or both atria, dilation of the IVC, and a characteristic diastolic bounce of the interventricular septum.3. Pericardial effusions can be reliably detected by CT. Pericardial tamponade is better evaluated by echocardiography, however, due to its ability to provide hemodynamic information.4. A pericardiaI cyst will appear as a well circumscribed paracardiac mass with characteristic water attenuation (H.U. = 0), usually in the right costophrenic angle.5. Both primary neoplasms and, more commonly, metastatic neoplasms can be visualized in the pericardium.

Congenital heart disease :

MDCT may be used in selected patients in whom echocardiography is non-diagnostic or inadequate and MRI is not available. The ability to evaluate cardiovascular anatomy in multiple planes is often helpful for delineating cardiac morphology in congenital heart disease, particularly with regard to the relationship of the great vessels, pulmonary veins, and coronary arteries.

Specific situations in which MDCT is helpful include1. "hard-to-find" adult shunt detection (sinus venosus atrial septal

defect, patent ductus arteriosus);

2. visualization of pulmonary arteries in cyanotic congenital heart disease;

3. precise definition of aortic anatomy in Marfan's syndrome or coarctation;

4. definition of partial or total anomalous pulmonary venous drainage.

Diseases of the aorta constitute a common and important indication for CT examinations.

Contrast-enhanced MDCT is nearly 100% sensitive and specific forevaluating acute aortic syndromes. 1. Acute aortic dissection is characterized on CT by visualization of a dissection flap (i.e., separation of the intima from the media) that forms true and false lumens. The CT study can characterize the origin and extent of the dissection, classify it as Type A or B, assess for concomitant aneurysmal aortic dilatation, and identify branch vessels involvement.2. Aortic intramural hematomas are believed to be caused by spontaneous hemorrhage of the vaso vasorum into the medial layer. They appear as crescent-shaped areas of increased attenuation with eccentric aortic wall thickening. Unlike dissections, hematomas do not spiral around the aorta.3. Aortic aneurysm is a permanent dilation of 150% of the normal aortic caliber(usually greater than 5 cm in the thoracic aorta and greater than 3 cm in the abdominal aorta). 4. Penetrating atherosclerotic ulcer. These tend to be focal lesions of the descending thoracic aorta that appear as contrast-filled irregular outpouchings of the aortic wall.

Evaluation of pulmonary veins :

In the context of electrophysiology interventions such as pulmonary vein isolation (PVI), preprocedural MDCT can be used to define pulmonary venous anatomy and identify supernumerary veins, and postprocedural MDCT can be used to evaluate for pulmonary vein stenosis. Additionally, in the setting of congenital heart disease, CT can be used to identify anomalous pulmonary venous return.

Pulmonary Vein Stenosis

Vasamreddy et al. Heart Rhythm (2004) 1, 78-81.

Valvular heart disease :

Visualization of the valve leaflets, particularly the aortic valve, is feasible with newer-generation scanners due to their improved temporal resolution. Nonenhanced MDCT is also useful for assessing prosthetic mechanical valve leaflet motion.

Surgical planning :

The utility of MDCT in surgical planning before cardiothoracic surgery, particularly for reoperations, is increasingly recognized. Preoperative scans can evaluate the proximity of mediastinal structures to the sternum (i.e., aorta, right ventricle, bypass grafts); the degree of aortic calcification (i.e., to guide cannulation sites); and concomitantly provide information about cardiac morphology (e.g., presence of a ventricular aneurysm). Ongoing studies are evaluating whether this added information might reduce intraoperative and perioperative complications.

The Great Promise of MSCT

The “Triple Rule-Out”