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Cardiac MRI and Cardiac CT: An indispensable Tools for the Diagnosis of CAD

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Dr Bhavin Jankharia, Partner and Consultant, Jankharia Imaging Centre and Dr Parang Sanghavi, Consultant Radiologist, Jankharia Imaging Centre elaborate on the different ways to evaluate coronary artery diseases using CT and MRI

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Dr Bhavin Jankharia

Coronary artery disease (CAD) affects 7- 13 per cent of urban and 2- 7 per cent of the rural populations in India and produces a significant disease burden [1].  While, accurate evaluation of CAD is one way to improve outcomes, early diagnosis of CAD can help institute aggressive measures to control disease and prevent progression to ischemic heart disease [2].

Cardiac CT in the last two decades has made great technological leaps that has allowed it to make a difference in the evaluation of CAD.

Cardiac CT

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Dr Parang Sanghavi

For many years, this was the holy grail of imaging. While electron beam CT scanners (EBCT) had the temporal resolution to allow evaluation of the calcium content of the coronary arteries [3]. It was with the advent of four slice CT scanners [4], with a slow heart rate of around 60, that it was possible to achieve a temporal resolution that could summate the coronary arteries over a few heart-beats and allow visualisation of the coronary arteries. The current 256 and 320 slice scanners allow even faster acquisition of images, though good quality studies still need a low, steady heart rate and can be achieved with 64-slice CT scanners as well.

Ca scoring

This was the first modality to evaluate CAD risk. Outcome data over more than two decades has shown that a calcium score of  zero is associated with an extremely low coronary event risk [5]. As the calcium score increases, the event risk rises. It is an independent risk factor for coronary events and event-free survival. It is a modality ideally situated for mass screening

  • Plain scan
  • Low radiation
  • Easy to interpret

Coronary angiography (CCA)

This requires intravenous contrast administration, a low heart rate as far as possible and a steady heart rate for the best images.

CCA is performed in the following situations.

To rule out coronary artery disease

In patients with medium to high risk of coronary artery disease, who are otherwise asymptomatic or have equivocal symptoms or results of ECG or stress test, CCA is the modality of choice to evaluate the status of the coronary arteries. Outcome data is now available and shows that a normal coronary angiogram has a negligible coronary event risk with a negative predictive value approaching 100 per cent [6] (Figure 1).

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Figure 1: Volume rendered coronary CT angiogram (CTA) shows normal coronary arteries

CCA is also used in emergency rooms to triage chest pain. A ‘triple rule-out’ study helps rule out coronary artery disease, pulmonary thromboembolism and aortic aneurysm with dissection [7].

Stents

In-stent evaluation is still an issue. The larger the stent, the  easier it is to view the lumen (Figure 2). Faster scanners with iterative reconstructions have improved the ability to see the in-stent lumen (Figure 3), though with small stents, there are still issues [8]. While CCA is used in some instances to evaluate in-stent lumen, especially in patients with equivocal symptoms, often the reason to do CCA is to evaluate the rest of the vessels, with the same clinical indication as above.

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Figure 2: Maximum intensity projection (MIP) CTA of the left anterior descending artery (LAD) using an iterative reconstruction algorithm shows in-stent occlusion (arrow)

Plaque evaluation

Cardiac CT is an excellent modality to evaluate plaque composition. The newer dual energy scanners with iterative reconstruction techniques have made plaque analysis (Figure 3) more robust, though clinical utility is still suspect [9]. The analysis of plaque-at-risk using CCA is being evaluated in multiple clinical trials, but clinical utility may still be a few years away [10].

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Figure 3 (a-c): MIP images in cross section (a) and in two perpendicular longitudinal planes (b, c) shows a complex plaque (arrows) with a lipid core (arrow in a) with significant stenosis

Perfusion

The newer scanners allow perfusion studies to be performed, but given the increased radiation and the availability of other equally good or better modalities with tested outcome data, it is unlikely that CT perfusion will assume an important role at least in the near future.

Following bypass surgery

Cardiac CT allows accurate evaluation of grafts, both venous and arterial and can serve as the first modality to evaluate graft patency, anastomotic site pathology and abnormalities of the post-graft vessel. A recent meta-analysis shows a sensitivity and specificity of 99 per cent each for graft occlusion and 98 per cent each for evaluation of >50 per cent graft stenosis [12] (Figure 4).

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Figure 4: VRT CT angiogram shows 2 occluded venous grafts (fat white arrows). The saphenous vein graft (SVG) to the posterior descending artery (PDA) shows focal severe stenosis (arrow) with another 50 per cent stenosis more distally (arrow)

References:
1. Krishnan MN. Coronary heart disease and risk factors in India – on the brink of an epidemic? Indian Heart Jour 2012;64:364.
2. Shah N, Soon K, Wong C, Kelly A-M. Screening for asymptomatic coronary heart disease in the young ‘at risk’ population: Who and how? IJC Heart Vascul 2015;6: 60.
3. Sechtem U. Electron beam computed tomography: on its way into mainstrem cardiology? Eur Heart Jour 2000;21:87
4. Achenbach S, Giesler T, Ropers D et al. Detection of coronary artery stenoses by contrast-enhanced, retrospectively electrocardio-graphically gated, multislice spiral computed tomography. Circulation 2001;103:2535.
5. Greenland P, Bonow RO, Brundage BH et al. ACCF/AHA 2007 clinical expert consensus document on coronary artery calcium scoring by computed tomography in global cardiovascular risk assessment and in evaluation of patients with chest pain: a report of the American College of Cardiology Foundation Clinical Expert Consensus Task Force (ACCF/AHA Writing committee to update the 2000 Expert Consensus Document on Electron Beam Computed Tomography) developed in collaboration with the Society of Atherosclerosis Imaging and Prevention and Society of Cardiovascular Computed Tomography. JACC 2007;49:378.
6. Mowatt G, Cook JA, Hillis G et al. 64-Slice computed tomography angiography in the diagnosis and assessment of coronary artery disease: systematic review and meta-analysis. Heart 2008;94:1386.
7. Frauenfelder T, Appenzeller P, Karlo C et al. Triple rule-out CT in the emergency department: protocols and spectrum of imaging findings. Eur Rad 2009;19:789.
8. Taylor AJ, Cerqueira M, Hodgson JM et al. 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. Jour Cardiovasc Computed Tomogr 2010;4:407e1-33.
9. Obaid DR, Calvert PA, Gopalan D et al. Dual-energy computed tomography imaging to determine atherosclerotic plaque composition: a prospective study with tissue validation. Jour Cardiovasc Computed Tomogr 2014;8:230.
10. Latif MA, Cury R, Akhlaq M et al. A systematic review and meta-analysis: prevalence of coronary plaque high-risk features (low attenuation, enlarged diameter or positive remodeling, napkin ring, and spotty calcification (lens) in acute coronary syndrome as assessed by coronary computed tomographic angiography (CTA). JACC 2016;67:1736.
11. Varga-Szemes A, Meinel FG, De Cecco CN et al. CT myocardial perfusion imaging. Am J Roentgenol 2015 Mar;204:487.
12. Barbero U, Innaccone M, d’Ascenzo F et al. 64 slice-coronary computed tomography sensitivity and specificity in the evaluation of coronary artery bypass graft stenosis: a meta-anaylsis. Int J Cardiol 2016;216:52.

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