Cardiovascular computerized tomography Basics and evaluation of CAD

Cardiovascular computerized tomography Basics and evaluation of CAD Frijo Jose A

Scanner Generations First Second Third Fourth • Third generation most popular since detector geometry is simplest – collimation is feasible which eliminates scattering artifacts

• 1 st generation- 4. 5 min/image – series of exposures - X-ray source moved laterally – tube then rotated to a different position - sequence repeated • 2 nd generation - 2. 5 min/image – fan beam architecture – activate multiple detectors before moving to a new position • 3 rd generation - 18 s/image – rotating X-ray source coupled to rotating detectors • 4 th generation - 2 s/image – rotating X-ray source with a fixed detector array

Principle • a fan-shaped, thin Xray beam passes through the body at many angles to allow for cross-sectional images • The corresponding X-ray transmission measurements are collected by a detector array. • The data recorded by the detectors are digitized into picture elements (pixels) with known dimensions. • The gray-scale information contained in each individual pixel is reconstructed according to the attenuation of the Xray beam along its path • Gray-scale values for pixels within the reconstructed tomogram are defined with reference to the value for water and are called “Hounsfield units” or simply “CT numbers. ”

Cardiac CT • Needs both spatial and temporal resolution • EBCT has the best temporal resolution, MDCT the best spatial resolution • 2 biggest problems - MDCT and EBCT angiography - dense calcifications and stents

MDCT • Rapidly rotating X-ray tube and several rows of detectors, also rotating • The tube and detectors are fitted with slip rings that allow them to continuously move through multiple 360° rotations

MDCT v/s single detector-row helical/spiral CT • principally by the design of the detector arrays and data acquisition systems, which allow the detector arrays • configured electronically to acquire multiple adjacent sections simultaneously

Single-slice Multi-slice

• One of the advantages of MDCT over EBCT is the variability of the m. A settings, thus increasing the versatility for general diagnostic CT in nearly all patients and nearly all body segments

MDCT modes • Sequential mode (prospective triggered) • Helical mode (retrospective gating)

Sequential mode

Spiral mode

Prospective Triggering • “step and shoot” system, similar to EBCT • obtains images at a certain time of cardiac cycle, which can be chosen in advance, only 1 image/detector/cardiac cycle obtained • reduces contrast requirements • does not allow for CT angiographic images, as motion artifacts may plague these images

Retrospective Gating • A simultaneous ECG recorded during the acquisition of cardiac images • The ECG retrospectively used to assign source images to the respective phases of the cardiac cycle (ECG gating) • Best imaging time to minimize coronary motion from 40% - 80% of cardiac cycle (early to middiastole). • There is a continuous model of the volume of interest from base to apex • Increase in radiation dose

EBCT

• Non-mechanical X-ray source – fixed X-ray source, which consists of a 210° arc ring of tungsten, activated by bombardment from a magnetically focused beam of electrons fired from an electron source “gun” • Allows for image acquisition on the order of 50– 100 ms • Prospective ECG triggering

Comparisons of EBCT and MDCT Scanners • • Spatial Resolution X-ray flux Speed/Temporal Resolution Radiation Dose

Spatial Resolution • Current CAC scanning protocols use 3 mm thickness for EBCT and betw 2. 5 mm to 3. 0 mm for MDCT • For CT angiography, EBCT utilizes 1. 5 mm slices, and MDCT most often obtains images with 0. 5– 0. 75 mm per axial slice

X-ray flux • EBCT limited to 63 m. As (100 m. As for e-Speed) • m. As for MDCT angiography is 300 to 400 • MDCT can increase the m. As (and k. V) to help with tissue penetration, while EBCT is more limited in this clinical setting

Speed/Temporal Resolution • Current CT system images for measuring calcified plaque at 50– 100 ms (EBCT) and 180– 300 ms (prospectively gated MDCT) cannot totally eliminate coronary artery motion in all individuals • The temporal resolution – determines degree of motion suppression • Dependent on – Pitch factor – Gantry rotation time – Patient’s heart rate

Radiation Dose

Clinical Applications-Cardiovasc CT • Non-contrast studies can accurately identify and quantify coronary calcification (a marker of total plaque burden) • Contrast-enhanced studies can define ventricular volumes, EF, and RWMA and wall thickening • Can visualize the coronary artery, including the lumen and wall

ECG Triggering

ECG Triggering

Cardiac Anatomy by CT

Performing the Cardiac CT Angiogram 1. one or more planar scout images 2. non-contrast cardiac gated CT 3. a timing scan (can be avoided with certain new scanners) 4. a contrast-enhanced CT angiogram

cardiac CT angio- reconstruction • • Shaded Surface Display Volume Rendering Maximal Intensity Projections Multiplanar Reformatting (MPR)

Shaded Surface Display • Discards all pixels below a certain HU threshold • Remaining pixels shaded according to depth and lighting • No “upper limit” or threshold • Visualize selectively the contrast-enhanced lumens, while automatically deleting vessel wall and connective tissue • The picture looks much like a plaster cast of the heart • The overestimation of luminal patency in the presence of calcium • Partial volume effects

Volume Rendering

Maximal Intensity Projections • only the maximal density values at each point in the 3 -D volume are displayed • Osseous structures are also removed • Relying on density differences and avoiding image smoothing, allows the visualization of smaller vessels and better visualization of LIMA • Does not convey depth relationships • Doesn’t depict overlapping vessels • The lack of 3 -D in a single MIP is minimized by displaying multiple MIPs through a single axis • Metal and calcium are much brighter than contrast-enhanced lumens • Superior accuracy in detecting significant obstructions in the coronary arteries as compared to other techniques

Multiplanar Reformatting (MPR) • Otherwise known as “curved surface reformation, ” multiplanar reformatting • Can be used to evaluate the entire coronary tree in one view

demonstrates a volume-rendered image, which does not well distinguish the calcification from the lumen in the left anterior descending artery (white arrow).

demonstrates a maximal intensity projection, displaying the sequential calcifications in the LAD distribution. Since MIP is more transparent, the overlapping vessels (particularly the septal perforators in this image) become more problematic.

maximal intensity projection of the left anterior descending demonstrating a severe luminal stenosis (non-calcific or “soft”) of the proximal LAD (arrow

CTA - Clinical Applications

– Does not have the same spatial or temporal resolution as CAG – cannot be performed in all pts (e. g. arrhythmias) – Purely diagnostic -doesn’t provide option for immediate intervention • Thus, clinical application of CTA in pts with a high pretest likelihood for CAD is of limited value • If the predicted necessity for an intervention is reasonably high, the patient should proceed directly to CAG

• Routine “screening” of asymptomatic individuals by CTA will not be beneficial, since treatment of an asymptomatic stenosis is generally not expected to alter pt’s prognosis

• Symptomatic with atypical chest pain and have positive or equivocal stress test results, so that invasive coronary angiography is deemed necessary to rule out the unlikely presence of stenoses. Such patients with low or intermediate pretest likelihood are, forexample, younger men and women with atypical chest pain---- CTA

• CTA -information complementary to CAG • Study- In CTO, for intervention, occlusion length & degree of calcification by CT are more accurate predictors of success than CAG • CTA may provide information prior intervention of bifurcation lesions or other challenging subsets of coronary stenoses – plaque burden, extent of calcification, 3 -D anatomy of vessels

• CTA- preferred modality to investigate pts with known or suspected congenital coronary artery anomalies

MSCT Coronary Angiography Accuracy • Accuracy of testing: – For 64 slice CTA: (for clinically significant occlusions, >50%) • Sensitivity: 89 % – (If cardiac cath positive (gold standard), CTA will be positive 89% of the time) • Specificity: 96% – (if cath negative, the CTA will not find it 96% of the time. ) • Negative Predictive Value: 99% – (if negative CTA, cath is negative 99% of time) • Positive Predictive Value: 78% – (if positive CTA patient has positive cath 78% of the time) » Clinical Cardiology vol. 30 , 9/07

Comparison of different Screening tests Test Sensitivity Specificity CTA 89% (76 -99) 96% (95 -97) Nuc Med Spect 70 -80% Stress Echo 70 -80% ETT 60 -65% 70 -75% Cardiac Cath 100%

Cardiac Indications – Emergency evaluation of acute chest pain – Cardiac evaluation of a pt with chest pain syn (e. g. anginal equivalent, angina), who is not a candidate for cardiac cath – Management of a symptomatic pt with known CAD (e. g. , post-stent, post CABG) when the results of the MDCT may guide the decision for repeat invasive intervention – Assessment of suspected congenital anomalies of coronary circulation

Assessment of Cardiac Structure & Function Computed Tomography

Vertical long axis (Long A) end-diastolic (ED) and end-systolic (ES) images in a patient with normal LVEF, but apical hypokinesis.

Apical aneurysm without thrombus

Completed transmural infarction with apical thinning (arrows)

Assessment of Cardiovascular Calcium

CAC • Pathognomonic for atherosclerosis • Mönckeberg’s calcific medial sclerosis doesn’t occur in coronaries • Atherosclerosis is the only vascular disease known to be associated with cor calcification • Calcium phosphate (in the hydroxyapatite form) • Arterial calcium in athero is a regulated active process similar to bone formation, rather than a passive precipitation of calcium phosphate crystals

• Coronary calcium is defined as a lesion above a threshold of 130 HU, with an area of ≥ 3 adjacent pixels (at least 1 mm 2)

• Calcium score developed by Agatston product of the calcified plaque area and max calcium lesion density (1 -4 based upon HU) • 1– 10 - minimal, 11– 100 - mild, 101– 400 – moderate, >400 - severe • Calcium volume score • Parameter of choice for serial studies to track progression or regression of atherosclerosis

Calcium percentile - index of the prematurity of atherosclerosis

• While the presence of CAC is nearly 100% specific for atherosclerosis, it is not specific for obstr disease since both obstr and non-obstr lesions have calcification present in the intima • Comparisons with pathology specimens have shown that the degree of luminal narrowing is weakly correlated with the amount of calcification on a site-by-site basis, whereas the likelihood of signi obstr increases with the total CAC score

• CAC scanning is not routinely recommended in patients with classical angina symptoms. • However, in those with atypical chest pain, a 0 or very low CAC score would render obstructive disease very unlikely

Annual event rates and relative risks for cardiac events in 5585 asymptomatic patients at different levels of coronary artery calcium (St Francis Heart Study). The solid line indicates the 2%/year event rate consistent with secondary prevention risk

1. A -ve EBCT test - atherosclerotic plaque, including unstable plaque- unlikely 2. A -ve test - highly unlikely in the presence of signi luminal obstructive dis 3. -ve tests - majority of pts - angiographically normal cor arteries 4. A -ve test - low risk of a cardiovascular event in the next 2 to 5 yrs 5. A +ve EBCT confirms the presence of a cor atherosclerotic plaque

6. The greater the calcium, the greater the likelihood of occlusive CAD, but there is not a 1 -to -1 relationship, and findings may not be site specific 7. The total amount of calcium correlates best with the total amount of atherosclerotic plaque, although the true “plaque burden” is underestimated 8. A high calcium score may be consistent with moderate to high risk of a cardiovascular event within the next 2 to 5 years

ENHANCEMENT IN AMI (EARLY DEFECT, LATE ENHANCEMENT, RESIDUAL DEFECT) • Early defect (ED) is observed as a myocardial perfusion • defect (dark zone) in the early image (30– 60 s) • Residual defect (RD) is observed as smaller dark regions observed in the subendocardium, surrounded by a partially hyperenhanced zone of late enhancement (LE) in the late image (5– 10 min) • The density of the ED < normal myocardium • The LE (112. 9 ± 18. 5 HU) presented with higher density than RD (59. 3 ± 11 HU)

In AMI patients after successful PCI • Depth (subendocardial or transmural) of the ED can predict wall thickness and wall motion in the chronic phase (1 mo).

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