CARDIAC CT BASIC PRINCIPLES AND CT CAG

• DR RAJESH K F

• Cardiac CT and CCTA has emerged as promising noninvasive imaging modality for coronary artery and cardiac structural and functional evaluation

Formation of CT image• Three phase process• Scanning phase -scan

data• Reconstruction phase -

processes acquired data and forms digital image(pixels)

• Digital to analog conversion phase - Visible and displayed analog image (shades of gray-Hounsfield units)

Sequential mode • First scanning mode • Scan and step • Prospective triggered• One complete scan around

body while body is not moving

Spiral or helical scanning • Retrospective gating• Body moved continuously

as x-ray beam scan around

• Higher radiation dose

SDCT• Single detector row

helical/spiral CTMDCT • Electronically

acquire multiple adjacent sections simultaneously

Full Scan Reconstruction• Full rotation (3600)

reconstruct one imageHalf-scan reconstruction • Commonly used in cardiac

CT• Data from 1800 sweep • Temporal resolution- half

gantry rotation time Multisegment reconstruction • For multidetector systems• Use <1800 rotation

RECENT ADVANCES

Temporal resolution• Gantry rotation time decreased• Temporal resolution correspond to half rotation

time• Maximum gantry rotation time - 270 to 330 msec• Temporal resolution is approximately 83 to 165

msec - half-scan reconstruction techniques• Image acquisition or reconstruction during periods

of limited cardiac motion (end systole to mid-late diastole)

RECENT ADVANCES

Spatial resolution• Decreased slice collimation (thickness)• Approximately 0.5 mm3Strengthened X-ray tubes - Reduce image noiseMultislice• Data in more slices simultaneously• From 4 to 64 to 320 per rotation• Decreases overall duration of data acquisition,

breath hold duration and amount of contrast

TECHNOLOGY OF CARDIAC CT

64-slice scanners • High temporal and

spatial resolution • Gantry rotation times

of 420 ms or shorter • Spatial resolution of

0.4 by 0.4 by 0.4 mm• “state-of-the-art”

equipment for CTA • Breath hold is 6 to 12 s

256 slice CT• Spatial and temporal

resolution remain unchanged

• Approx 0.5-mm collimation

• Increase volume coverage (number of slices)

• Image heart in single beat• Less vulnerable to

arrhythmia

EVOLUTION OF COMMON MULTIDETECTOR COMPUTED TOMOGRAPHY TECHNICAL PARAMETERS

  4-ROW 16-ROW 64-ROW 320-ROWTemporal resolution (half-scan reconstruction)

250 msec 210 msec 165 msec 175 msec

Spatial resolution 1.25 mm 1 mm 0.4 mm 0.4 mmVolume coverage 0.5-3 cm 1-2 cm 2-4 cm 15 cmBreath-hold 30-40 sec 20 sec 10 sec 2 sec

Dual-source CT• Number of slices - 64 • 2 X-ray tubes and

detectors in single gantry at 90°

• One-quarter rotation of gantry collect data from 180° of projections

• Temporal resolution is twice of single X-ray tube and detector

• Reduce motion artifact

CARDIAC COMPUTED TOMOGRAPHY

Thin-slice cardiac CT reconstructions • Displayed in any

imaging plane

Multiplanar imaging• Oblique planar

views• Images displayed in

orthogonal planes (axial, coronal, sagittal) or nonstandard planes

• Analysis of cardiac chambers

Maximal intensity projection• Thick-slice projections• Pixel within slab volume

with highest Hounsfield number is viewed

• Ability to view more structures in single planar view

• Can obscure details when high-density structures are present (coronary artery calcium)

Curved multiplanar reformations • Curved structures

can be viewed in planar oblique multiplanar reformats

• Can be used to evaluate entire coronary tree in one view

Volume rendered reconstructions• Useful for revealing

general structural relationships but not for viewing details of coronary anatomy

CORONARY ARTERY CALCIUM SCANNING• Non-contrast study• Refine clinically predicted

risk of CHD beyond that predicted by standard cardiac risk factors

• Used in asymptomatic patients

• Coronary calcium Present in direct proportion to extent of atherosclerosis

• Minority (20%) of plaque is calcified

• 3 mm non overlapping thick tomographic slices

• Average about 50–60 slices • From coronary artery ostia to inferior wall

of heart• Calcium score of every calcification in each

coronary artery for all of tomographic slices is summed

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

AGATSTON SCORE = Sum

CALCIUM VOLUME SCORING

CALCIUM SCALE4 calcium score categories

Calcium score correlates directly with risk of events and likelihood of obstructive CAD

Interscan variability of 10% to 20%

0 none

1–99 mild

100–400 moderate

>400 severe

• Coronary calcium presence and extent are dependent on age, gender, ethnicity, and standard cardiac risk factors

• Calcium scores are higher for age and male gender among whites

• Data from 13 studies (75,000 patients) during 4 years - calcium score of 0 is associated with a very high event-free probability (99.9% per year)

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

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

*

*

**p<0.001

DETECTION OF CAD/RISK ASSESSMENT IN ASYMPTOMATIC INDIVIDUALS WITHOUT KNOWN CAD

GLOBAL CHD RISK ESTIMATE

SCORE

Noncontrast CT for coronary calcium score

Low risk with a family history of premature CHD

A

Noncontrast CT—coronary calcium score Low I

Noncontrast CT—coronary calcium score Intermediate A

Noncontrast CT—coronary calcium score High U

VENTRICULAR MORPHOLOGY AND FUNCTION

• Helical scan • Provide CT data

from systole and diastole

• Can be displayed in cine-loop format

• Estimation of RVEF, LVEF, volumes and RWMA

• EF highly accurate

• Myocardial morphology - wall thinning, calcification or fatty replacement (negative HU densities)

• Atrial morphology and volume

EVALUATION OF VENTRICULAR MORPHOLOGY AND SYSTOLIC FUNCTION

Evaluation of LV function in acute MI or HF with inadequate images from other noninvasive methods

A

Quantitative evaluation of RV function A

Assessment of RV morphology in suspected ARVD

A

VALVULAR MORPHOLOGY AND FUNCTION• Anatomic evaluation of

cardiac valves and their motion

• Both native and prosthetic

• Lack of physiologic valve flow evaluation

• Prosthetic valve malfunction- size mismatch, tissue ingrowth, and valve thrombosis

• Severe AR- malcoaptation of leaflets >0.75 cm2

• AS- extent of valve calcification and planimetry

• Planimetry equalent to other invasive and noninvasive methods

• Aortic valve calcification is directly related to valve area and quantitated by area-density methods

CARDIAC MASSES • Less information concerning

tissue type than CMR• Lipomas-low CT numbers

(< 50 HU)• Cysts – water like density

(0 to 10 HU) • Intracardiac thrombi – (20

to 90 HU)• Density values overlap with

myocardium• Identify thrombi in LAA• Poor enhancement of LAA-

false-positive result common

PERICARDIUM

• Embedded in epicardial and pericardial fat-can be delineated in CT

• Normal thickness-1to 2mm

• Can clearly delineate pericardial calcification

EVALUATION OF INTRACARDIAC AND EXTRACARDIAC STRUCTURES

Characterization of native cardiac valves or prosthetic valves with clinically significant valvular dysfunction when other noninvasive methods are inadequate

A

Evaluation of cardiac mass (suspected tumor or thrombus) with inadequate images from other noninvasive methods A

Evaluation of pericardial anatomy A

Evaluation of pulmonary vein anatomy prior to RFA for AF A

Noninvasive coronary vein mapping prior to biventricular pacemaker

A

CORONARY CT ANGIOGRAPHY • Visualization of

coronary arteries and lumen

• Excellent tool to investigate coronary artery anomalies

Problems• Rapid motion• Small dimensions of

coronary arteries• Temporal and spatial

resolution of CT

DATA ACQUISITION FOR CORONARY CTA

Lower heart rate to 60 beats/min - Oral or intravenous BBs• Metoprolol 25 to 100 mg orally 1 hour before or IV 5 mg

rpt doses Dilate coronary arteries• Sublingual nitrates immediately before scanning • Nitroglycerin 400 to 800 MicrogmBreath hold of 6 to 20 s • Depend on scanner generation and dimensions of heart• 50 to 120 ml of contrast IV

RADIATION EXPOSURE (effective dose)

• 3 to 15 mSv, depending on scan protocol

• ECG-correlated tube current modulation

• Reduction of tube current in systole

• Can reduce radiation exposure by 30% to 50%

TYPICAL DATASET AS ACQUIRED BY CTA AFTER INTRAVENOUS CONTRAST AGENT

• Transaxial image

• 2D image reconstruction

• Maximum intensity projections

• Facilitate data interpretation

• Only maximal density values at each point in 3-D volume are displayed

• 2D image reconstruction

• Curved multiplanar reconstruction

• Evaluate entire coronary tree in one view

• 3 Dimensional display

• Visually pleasing• Rarely helpful to

evaluate data

IMAGE QUALITY AND ARTIFACTS

Motion artifact• Irregular and fast

HR• Respiration• Limit temporal and

spatial resolution • Blurr contours of

coronaries RCA - most frequently affected

Partial volume effect • e.g., metal, bone ,

calcifications• Appear bright on

image• Lead to overestimation

of dimensions of high-intensity objects

• Accuracy for detection of coronary stenoses is lower

Streaks and low-density artifacts • Adjacent to regions

of very high CT density

• e.g., metal or calcium

DETECTION OF CORONARY ARTERY STENOSES

64-row CTA • Overall accuracy • Sensitivity of 87% to 99%• Specificity of 93% to 96%• NPV -93 to 100%• ~4% uninterpretable• Specificity reduced in

calcium scores > 400 to 1000 or obesity (excess image noise)

• Best for ostial and first centimeter lesions

PROSPECTIVE MULTICENTER STUDIES FOR DIAGNOSTIC PERFORMANCE OF CCTA

Most studies are limited by selection of patients optimized for cardiac CT and analysis involves only more proximal coronary segments down to 1.5 mm

• Compared with grading by CAG, CT CAG stenosis severity tends to be worse and correlation is 0.5-0.6

• Correlates very well with IVUS (better visualization of arterial wall)

• >50% stenosis on cardiac CT has 30% to 50% likelihood of demonstrable ischemia on MPI

DIAGNOSTIC ACCURACY OF CCTA FOR MYOCARDIAL ISCHEMIA

• Identification of obstructive CAD did not successfully identify individuals with abnormal MPS

• Measures of perpatient coronary artery plaque burden, proximity, and location predictive of identifying individuals with abnormal MPS

CTA Limitations

• Rapid (>80 bpm) and irregular HR

• High calcium scores (>800-1000)

• Stents• Contrast requirement • Small vessels, distal

vessels (<1.5 mm) and collaterals

• Obese • Radiation exposure

RISK STRATIFICATION BY CCTA IN INDIVIDUALS WITH STABLE CHEST PAIN

Non-Acute Symptoms Possibly Representing an Ischemic Equivalent 1. ECG interpretable and able to exercise

Low U

 2. ECG interpretable and able to exercise

Intermediate

A

 3. ECG interpretable and able to exercise

High I

 4. ECG uninterpretable or unable to exercise

Low A

 5. ECG uninterpretable or unable to exercise

Intermediate

A

 6. ECG uninterpretable or unable to exercise

High U

USE OF CCTA IN THE EVALUATION OF ACUTE CHEST PAIN

• 2%-6% of patients are erroneously discharged with missed MI

• CCTA useful in this patient subgroup• Highlighting the NPV of CCTA• A successful triage tool that may allow safe early

discharge of low-risk patients

ACUTE SYMPTOMS WITH SUSPICION OF ACUTE CORONARY SYNDROME

Normal ECG and cardiac biomarkers Low/IntermediateA

Normal ECG and cardiac biomarkers High U

ECG uninterpretable Low/IntermediateA

ECG uninterpretable High U

Nondiagnostic ECG or equivocal cardiac biomarkers

Low/IntermediateA

Nondiagnostic ECG or equivocal cardiac biomarkers

High U

Acute chest pain of uncertain cause (differential diagnosis includes pulmonary embolism, aortic dissection, and acute coronary syndrome [triple rule-out])

U

Use of CTA in the Setting of Prior Test Results

ECG Exercise Testing Exercise testing and Duke Treadmill Score, intermediate-risk

A

 Normal exercise test with continued symptoms A

Stress Imaging Procedures

 Discordant ECG exercise and imaging results A

 Stress imaging results: equivocal A

Diagnostic Impact of Coronary Calcium in Symptomatic Patients  Coronary calcium score <100 A

Coronary calcium score 100-400 A

Coronary calcium score >401-1000 U

EVALUATION OF CORONARY BYPASS GRAFT PATENCY

• Sensitivity and specificity - nearly 100%

• Large size and limited mobility of grafts

• Limitation in native coronary artery evaluation (metallic clips and calcium)

• Cardiac structures adjacent or adherent to sternum and grafts cross midline can be seen

RISK ASSESSMENT POST CABG

Symptomatic (Ischemic Equivalent)Evaluation of graft patency after coronary bypass surgery

A

AsymptomaticLocalization of grafts and retrosternal anatomy prior to reoperative chest or cardiac surgery

A

CORONARY ARTERY STENTS

• Image artifact limits application

• Accuracy of 90% in stents >3 mm

• Small stents are difficult to evaluate

• Dependent on stent design

• Optimization of reconstruction techniques (sharp kernel) and display characteristics (wide display window)