1. CARDIAC CT BASIC PRINCIPLES AND CT CAG
DR RAJESH K F
CARDIAC CT BASIC PRINCIPLES AND CT CAG

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

3. 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)

4. 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

5. SDCT
Single detector row helical/spiral CT
MDCT
Electronically acquire multiple adjacent sections simultaneously

6. Full Scan Reconstruction
Full rotation (3600) reconstruct one image
Half-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

7. RECENT ADVANCES Temporal resolution Gantry rotation time decreased
Temporal resolution correspond to half rotation time
Maximum gantry rotation time 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)

8. RECENT ADVANCES Spatial resolution
Decreased slice collimation (thickness)
Approximately 0.5 mm3
Strengthened X-ray tubes - Reduce image noise
Multislice
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

9. 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

10. 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

11. EVOLUTION OF COMMON MULTIDETECTOR COMPUTED TOMOGRAPHY TECHNICAL PARAMETERS
4-ROW
16-ROW
64-ROW
320-ROW
Temporal resolution (half-scan reconstruction)
250 msec
210 msec
165 msec
175 msec
Spatial resolution
1.25 mm
1 mm
0.4 mm
Volume coverage
0.5-3 cm
1-2 cm
2-4 cm
15 cm
Breath-hold
30-40 sec
20 sec
10 sec
2 sec

12. 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

13. CARDIAC COMPUTED TOMOGRAPHY
Thin-slice cardiac CT reconstructions
Displayed in any imaging plane

14. Multiplanar imaging
Oblique planar views
Images displayed in orthogonal planes (axial, coronal, sagittal) or nonstandard planes
Analysis of cardiac chambers

15. 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)

16. Curved multiplanar reformations
Curved structures can be viewed in planar oblique multiplanar reformats
Can be used to evaluate entire coronary tree in one view

17. Volume rendered reconstructions
Useful for revealing general structural relationships but not for viewing details of coronary anatomy

18. 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

19. 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

20. CALCIUM VOLUME SCORING
Area = 8 mm2
Peak CT = 290
Score = 8 x 2 = 16
Area = 15 mm2
Peak CT = 450
Score = 15 x 4 = 60
AGATSTON SCORE = Sum
Hn x-factor
(Agatston Scoring)
>

21. CALCIUM SCALE 4 calcium score categories
Calcium score correlates directly with risk of events and likelihood of obstructive CAD
Interscan variability of 10% to 20%
none
1–99
mild
100–400
moderate
>400
severe

22. 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

23. 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)

24. Five-Year Mortality Rates in Framingham Risk Subsets by Coronary Calcium Score*
*p<0.001 * *
Shaw et al. Radiology 2003; 228:

25. 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
Intermediate
High U

26. 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

27. Myocardial morphology - wall thinning, calcification or fatty replacement (negative HU densities)
Atrial morphology and volume

28. 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
Assessment of RV morphology in suspected ARVD

29. 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

30. 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

31. 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

32. PERICARDIUM
Embedded in epicardial and pericardial fat-can be delineated in CT
Normal thickness-1to 2mm
Can clearly delineate pericardial calcification

33. 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
Evaluation of pericardial anatomy
Evaluation of pulmonary vein anatomy prior to RFA for AF
Noninvasive coronary vein mapping prior to biventricular pacemaker

34. 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

35. 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 Microgm
Breath hold of 6 to 20 s
Depend on scanner generation and dimensions of heart
50 to 120 ml of contrast IV

36. 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%

37. TYPICAL DATASET AS ACQUIRED BY CTA AFTER INTRAVENOUS CONTRAST AGENT

38. Transaxial image

39. 2D image reconstruction
Maximum intensity projections
Facilitate data interpretation
Only maximal density values at each point in 3-D volume are displayed

40. 2D image reconstruction
Curved multiplanar reconstruction
Evaluate entire coronary tree in one view

41. 3 Dimensional display
Visually pleasing
Rarely helpful to evaluate data

42. IMAGE QUALITY AND ARTIFACTS
Motion artifact
Irregular and fast HR
Respiration
Limit temporal and spatial resolution
Blurr contours of coronaries
RCA - most frequently affected

43. 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

44. Streaks and low-density artifacts
Adjacent to regions of very high CT density
e.g., metal or calcium

45. 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

46. 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

47. 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

48. 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

49. CTA Limitations Rapid (>80 bpm) and irregular HR
High calcium scores (> )
Stents
Contrast requirement
Small vessels, distal vessels (<1.5 mm) and collaterals
Obese
Radiation exposure

50. 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
 5. ECG uninterpretable or unable to exercise
 6. ECG uninterpretable or unable to exercise

51. 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

52. ACUTE SYMPTOMS WITH SUSPICION OF ACUTE CORONARY SYNDROME
Normal ECG and cardiac biomarkers
Low/Intermediate A
High U
ECG uninterpretable
Nondiagnostic ECG or equivocal cardiac biomarkers
Acute chest pain of uncertain cause (differential diagnosis includes pulmonary embolism, aortic dissection, and acute coronary syndrome [triple rule-out])

53. 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
Stress Imaging Procedures
 Discordant ECG exercise and imaging results
 Stress imaging results: equivocal
Diagnostic Impact of Coronary Calcium in Symptomatic Patients
  Coronary calcium score <100
Coronary calcium score
Coronary calcium score > U

54. 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

55. RISK ASSESSMENT POST CABG
Symptomatic (Ischemic Equivalent)
Evaluation of graft patency after coronary bypass surgery A
Asymptomatic
Localization of grafts and retrosternal anatomy prior to reoperative chest or cardiac surgery

56. 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)