Clinical Evaluation of a Diagnostic Test

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Cardiac CT and CT Angiography Techniques
Clinical Applications
Ethan J Halpern, MD Director, Cardiac CT Thomas
Jefferson University
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Cardiac Imaging Technique

• Patient Preparation
• Contrast Injection
• Scan Positioning
• mAs and kVp
• ECG Gating
• Multicycle Reconstruction
• Editing of ECG Gating
• ECG Gated Dose Modulation
• Image reconstruction

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Patient PreparationPrior to CT

• Ask patient to refrain from stimulants (i.e.
  coffee) on the day of the scan
• No solid food for 4 hours prior to the study
• Premedicate for asthma allergic history
• Medrol 32mg po 12hrs and 2 hrs prior to study
• Patient should have good IV access (18G
  antecubital)
• Adequate EKG tracing good contact

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Patient Preparation - Heart Rate

• IV Beta Blockade (preferred)
• 2.5 30 mg Metoprolol
• Titrate to heart rate of 55-60
• Monitor BP while giving metoprolol
• If asthmatic, consult physician
• No more than 10mg metoprolol
• Consider calcium channel blockers
• Diltiazem (bolus 0.25mg/kg)
• Oral Beta Blocker
• 50 100 mg Metoprolol
• 1 hour prior to examination
• Who will monitor the patient ?

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Objective of the Contrast Injection

• Uniform enhancement of the left heart to greater
  than 300 HU
• Minimize streaking due to contrast in SVC and RV

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Impact of Iodine Concentration

• 140cc injection
• HU in aorta

Cademartiri F et al. Intravenous Contrast
Material Administration at Helical 16Detector
Row CT Coronary Angiography Effect of Iodine
Concentration on Vascular Attenuation. Radiology
236661-665, 2005
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Contrast Injection

• Use high iodine density contrast ? 350 mgI/mL
• We use Optiray 350 (Mallinckrodt Inc.)
• 16 detector system (25-30 second scan)
• 100-150 cc contrast _at_ 4 cc/s
• 40 cc _at_ 4 cc/s
• 40 detector system (15-20 second scan)
• 100 cc contrast _at_ 5-5.5 cc/s
• 40 cc saline _at_ 5 cc/s
• 64 detector system (15 second scan)
• 75 cc contrast _at_ 5-5.5 cc/s
• 40 cc saline _at_ 5 cc/s
• Start scan 5 seconds after the contrast reaches
  the left heart
• Contrast volume scan duration injection rate
• Want sufficient contrast to enhance PDA at end of
  scan

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Scan Start Position

• Native coronary arteries
• Begin above carina
• Tortuous aorta or prominent upper left heart
  border begin scan 1-2cm higher
• Bypass Grafts
• Veins top of arch
• LIMA above clavicles

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Scan Ending Position

• Need to image PDA
• Note overlap of heart diaphragm
• Observe contour of heart
• Extend scan 2cm below the caudal extent of the
  heart
• Position of heart will change with inspiratory
  effort

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Center the Scan on the Heart

• Maximize spatial resolution for coronaries
• CT resolution is greatest in the center of scan
  field
• Set left-right position on AP scout view
• Move table up-down to center on aortic root and
  Left ventricle

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Voltage kV
90 kV, 120 kV, 140 kV
Cardiac protocols
These values determine the Peak value of X-ray
photons. The effective energy is about half of
these values

• A higher voltage means
• Lower contrast
• Less noise
• Higher Patient dose dose proportional to kV
  2.7
• Longer recovery time between scans (shorter life)

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Tube Current mA/mAs

• Axial mAs mA x Rotation-time/slice
• Helix mAs mA x (Rotation-time/360)/ Pitch
• For most scanners tube provides 300-500mA
• A higher mAs means
• Less noise noise proportional to 1/(mAs)0.5
• Higher Patient dose dose proportional to mAs
• Larger X-ray tube damage/scan
• Longer recovery time between scans

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Scan Parameters

• kVp
• Generally set at 120kVp
• For heavy patients (gt200lbs) use 140kVp
• For patients with calcified arteries and stents
  also use 140kVp
• mAs
• Effective mAs mA x (rotation time / pitch)
• Effective mAs in the range of 700-900
• Increase for heavy patients to minimize noise
• Pitch
• Generally 0.2-0.3, but adjust for heart rate

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EKG Gating

• Coronary CTA requires EKG gating to overcome
  cardiac motion
• Heart is most quiescent in mid-diastole and
  end-systole

• Best time for reconstruction
• 70-80 of R-R interval for LAD, CRX
• 70-80 or 40 for RCA
• Single cycle vs. multicycle

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EKG Based Techniques

• Fixed time offset
• Example 500 ms after R peak
• Window centered at 500 ms
• Percentage of R-R interval
• Example 60 of R-R interval
• For 60 bpm, R-R interval 1000 ms
• Window centered at 600 ms

0
500
0
600
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Heart rate variation during CTADiastole varies
in length
58 bpm r-r interval 1021 msec r-t interval
258 msec
70

• Timing of Intervals in
• Different Heart Rates
• Systole remains stable
• Changes in heart rate primarily effect diastole

79 bpm r-r interval 757 msec r-t interval 230
msec
104 bpm r-r interval 576 msec r-t interval
204 msec
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Consistent Phase SelectionBeat-to-Beat Variable
Delay Algorithm
58 bpm r-r interval 1021 msec r-t interval
258 msec
70

• Fixed time and percent of R-R may not pick a
  consistent phase
• Beat-to-Beat variable delay algorithm
• Always pick same percentage delay in diastole
• Improves image quality

79 bpm r-r interval 757 msec r-t interval 230
msec
104 bpm r-r interval 576 msec r-t interval
204 msec
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Single Cycle Reconstruction
Single Heart beat Uses 180o per heart
beat Temporal Res (rot time)/2
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Multi-Cycle Reconstruction

• Combine a portion of projections from one heart
  cycle with a portion of projections from another
  to make the full 1800.
• Improves temporal resolution, because each
  segment of data covers the same (smaller) region
  in time.

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Single Cycle vs. Multicycle
Toshiba Aquilion 16-slice 27/34 patients with
HRgt65
Dewey et al. Investigative Radiol 39223-229, 2004
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Temporal Window Heart Rate
---- 50 phase ____ 80 phase __
multicycle reconstruction
Hoffmann MHK Radiology 23486-97, 2005
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Image Quality Heart Rate
Hoffmann MHK Radiology 23486-97, 2005
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Correction of Gating Errors
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EKG Dose Modulation

• Best images obtained at mid-diastole
• RCA sometimes is best at end-systole
• Dose modulation can achieve dose reduction of
  40-50
• Use only with stable heart rate
• Limitations
• Cannot review coronary anatomy at end-systole
• Cannot correct for errors in gating

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Image Reconstruction

• Reconstruction slice thickness
• 3mm for function
• 0.5-0.8mm for coronary arteries
• 1.0-1.2mm for photon limited scans
• Reconstruction kernel
• Sharper kernel noisier image, but may be
  required to visualize coronary lumen with stents
  and calcified vessels

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Slice thickness vs. noise
0.8mm
1.0mm
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Reconstruction filter vs. noise
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Reconstructions

• Choose appropriate filter
• Sharper filter for patients with heavy coronary
  calcium or stents
• Perform targeted reconstructions
• 3mm reconstruction of contiguous slices _at_ 10
  phases for cardiac function analysis
• 0.8mm reconstruction of overlapping slices _at_ 40,
  70, 75 and 80 for coronary anatomy. 1.0mm
  recons for heavy patients.

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Clinical Application of Coronary CTA

• Indications
• Rendering display modes
• Characterization of Plaque
• Grading of stenosis

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Cardiac Indications

• The MDCT angiography of the chest for cardiac
  assessment (0146T-0149T) is indicated for the
  following signs or symptoms of disease
• Emergency evaluation of acute chest pain
• Cardiac evaluation of a patient with chest pain
  syndrome (e.g. anginal equivalent, angina), who
  is not a candidate for cardiac catheterization
• Management of a symptomatic patient with known
  coronary artery disease (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

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Rendering Modes

• MIP slab MIP
• Surface Display
• Vessel tracking
• Curved MIP
• Globe view

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Plaque Characterization

• Calcified vs. Soft
• Positive remodeling
• Irregularity
• Ulceration

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Grading of Stenosis
Leber AW et al. Quantification of Obstructive and
Nonobstructive Coronary Lesions by 64-Slice
Computed Tomography A Comparative Study With
Quantitative Coronary Angiography and
Intravascular Ultrasound JACC 46(1)147-54, 2005
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Bland-Altman Analysis of Stenosis Grading
Dashed lines --- 95 CI
Hoffmann JAMA, Volume 293(20).May 25,
2005.24712478
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Impact of Calcified Vesselson detection of
stenosis gt50

• Calcium score
• Cutpoint 55
• CTA 1310 segs
• Low CS pts
• Sens 90
• Spec 92
• High CS pts
• Sens 97
• Spec 91

Cademartiri F et al. Impact of coronary calcium
score on diagnostic accuracy for the detection of
significant coronary stenosis with multislice
computed tomography angiography. American Journal
of Cardiology. 95(10)1225-7, 2005
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Impact of Coronary Calcium
Kuettner A et al. Noninvasive detection of
coronary lesions using 16-detector multislice
spiral computed tomography technology initial
clinical results. JACC 44(6)1230-7, 2004.
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Proximal versus Distal Segments
Hoffmann F et al., Predictive value of 16-slice
multidetector spiral computed tomography to
detect significant obstructive coronary artery
disease in patients at high risk for coronary
artery disease patient-versus segment-based
analysis. Circulation 110 26382643.
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Non-coronary Assessment

• Valvular assessment
• Cardiac morphology
• Cardiac function
• EP planning