- Coronary CT angiography uses x-rays and contrast material to examine the coronary arteries with high spatial and temporal resolution. It is non-invasive compared to traditional coronary angiography. - Key factors for cardiac imaging include high temporal resolution (<250ms), spatial resolution (<0.75mm), and synchronization with the cardiac cycle using ECG gating. Prospective and retrospective gating, partial and multi-segment reconstruction, and low pitch values (<0.5) help achieve this. - Advances in multi-detector CT scanners, faster gantry rotation times (<330ms), and improved reconstruction algorithms now allow temporal resolutions as low as 80ms for coronary CT angiography.

1. CORONARY CT
ANGIOGRAM:
PHYSICS AND TECHINCAL
CONSIDERATION.
REMIX MAHARJAN
BSC.MIT, CMC

2. Introduction
 Coronary angiography is a procedure that’s uses
contrast material and x-ray to examine the blood
vessels supplying heart( coronary arteries) or the
chamber of heart.
 Primary and gold standard tool to evaluate and
treat Coronary artery disease (CAD)-
fluoroscopically guided.
- Invasive
- Longer examination time
- Pt. prep time and recouping time.

3. Cardiac Scanners: EBCT:
 1982, specifically for cardiac imaging, able to acquire an
image in less than 100msec (scan time as short as
50ms).

4.  Mostly used for non-invasive evaluation of coronary
artery calcium but other applications including
assessment of coronary artery stenosis have been
reported in limited cases.
 Expensive and widely not available

5. MDCT:
 Sub-millimeter spatial resolution (<0.75 mm),
 improved temporal resolution (80–200msec),
 electrocardiographically (ECG) gated or
triggered mode of acquisition, and current
generation of MDCT scanners (16–128 row
detectors) makes cardiac imaging possible.

6. Essentials for cardiac imaging
 High temporal resolution
 Virtually “freeze” the beating heart to image
coronary arteries
 Imaging is best if performed in diastole phase-
most quiescent part of cardiac cycle.
 ECG continuously recorded for
synchronization image acquisition and
reconstruction with heart motion.

7.  Diastolic phase narrows
with increasing heart rate
 Desired temporal
resolution :
• 250ms for heart rates upto
70 bpm
• Upto 150 ms for heart
rates greater than 100
bpm
• Ideally around 50 ms for
motion free imaging

8.  High spatial resolution- resolve very fine
structures eg. proximal coronary segments
which range from a few mm to a few sub-mm
in diameter as they traverse away from aorta
in all directions around heart.
 High CNR- to resolve small and low contrast
structures such as plaques.
 High low contrast resolution. But can
degrade with the increasing number of CT
detectors in the z- direction due to increased
scatter radiation.

9. Cardiac CT Physics : Temporal
resolution
 Number of factors influence the temporal
resolution
 Gantry rotation time
 Acquisition mode
 Type of image reconstruction
 Pitch

10. Gantry rotation time:
 amount of time reqd. to complete one full
rotation (360) of the tube and detector around
the pt.
 Advances in technology have decreased this
time to as low as 330-370 ms (250 ms
nowadays).
 Faster gantry rotation, greater temporal
resolution
 Increased gantry rotation, increase in stress on
gantry structure because of higher G forces.

11. Acquisition mode: Prospective ECG
Triggering:
 Similar to conventional step and shoot method
 cardiac functions are monitored continuously through
ECG signals
 Protocols so built to start exposure at a desired
distance from R-R peak. ( Eg: 60% or 70% of R-R
interval)
 Scanner starts the scan at the preset point in the R-R
interval period.
 Projection data are acquired for only part of the
complete gantry rotation ( i.e partial scan).
 Min amount of projection data required to construct a
complete CT image is 180 degree plus fan angle of
the detectors in the axial plane.

14.  Once the desired data are acquired, table translated to
next bed position, and after a suitable and steady heart
rate is achieved, acquisition of more projections.
 This cycle is repeated until entire scan length is covered,
typically 12-15cm
 Best TR for partial scan is slightly greater than half of
the gantry rotation time.
 Usual scan-cycle times of modern multi-slice CT
scanners are in the range of 0.8–1.5 s
 Thus, one heart beat has to be skipped in between
every scan for usual clinical examinations at heart rates
between 50 and 90 bpm with R-R interval times between
0.7 and 1.2 s.

15.  Advantages:
 reduced radiation exposure, Temporal
resolution range from 200 to 250 msec.
 used for calcium scoring studies, since
calcium scoring analysis is typically performed
in axial scan mode.
 Use of low tube current (mA) for a calcium
scoring protocol, since calcium has a high CT
number and is easily visible even with a
noisier background.

17. Retrospective ECG gating:
 Main choice of data acquisition in MDCT
 ECG signals are monitored continuously and data
is acquired continuously (simultaneously) in
helical mode.
 Both the scan projection data and the ECG
signals are recorded.
 The information about the patient’s heart cycle is
then used during image reconstruction, which is
performed retrospectively, hence the name
retrospective gating.
 The image reconstruction is performed either with

19.  In segmented reconstruction, data from different parts
of the heart cycle are chosen, so that the sum of the
segments equates to the minimal partial scan data
required for image reconstruction.
 This results in further improvements in temp
resolution(TR) Can range from 80 to 250 msec.
 Disadvantage: increased radiation dose, even
though partial data are actually used in the final image
reconstruction.
 Also, since this scan is performed helically and the pitch
factor is quite low, indicating excessive tissue overlap
during scanning, increases radiation dose to the
patients.

20. RECONSTRUCTION METHOD:
Partial scan reconstruction:
 most practical solution is the partial scan
 can be used for both prospective triggering and
retrospective gating acquisitions.
 partial-scan fan beam data set has to cover a projection-
angle interval αP (angle interval between tube positions
at the start and end points of tube rotation) of 180° plus
the breadth of the X-ray fan: αP = π + βf.
 The breadth of the X-ray fan-beam (βf) depends strongly
on the diameter of the scan field of view (usually 50 cm)
and the distances of the focal spot and detector from the
center of the scan field of view.

21.  The equation αP = π + βf states that a minimum data
segment of 180° has to be available for every fan angle
β.
 for a gantry rotation of 500msec, the scan time
for acquiring data for partial scan reconstruction
is around 260 to 280 msec.
 To date, the fastest commercially available
gantry rotation time is 330 msec. In such
scanners, the partial scan reconstruction
temporal resolution can be as high as 170–180
msec.

23.  Better temporal resolution can be achieved with
special reconstruction algorithms that use the
minimum required amount of scan data, referred
to as "half-scan" reconstruction, can be best
explained using parallel-beam geometry.
 The fan-beam geometry of the partial- scan data
set is transformed to parallel-beam geometry
using "rebinning" techniques.
 The rebinning of a partial scan fan-beam data set
provides 180° of complete parallel projections,
including chunks of incomplete parallel projections
that consist of redundant data.

25. Multi-segment reconstruction:
 Principle- The scan projection data required to
perform a partial scan reconstruction are selected
from various sequential heart cycles instead of from
a single heart cycle.
 This is possible only with a retrospective gating
technique and a regular heart rhythm.
 This is done by using projection data from two
separate segments of the heartbeat cycle for image
reconstruction.
 selecting projection data from three or four different
heart cycles, resulting in temporal resolution as low
as 80 msec.

26.  In general, temporal
resolution can range
from a maximum of
TR/2 to a minimum of
TR/2M, where TR is the
gantry rotation time
(seconds), and M is the
number of segments in
adjacent heartbeats
from which projection
data are used for image
reconstruction.
 Usually, M ranges from
1 to 4.

27.  Advantage- Possibility to achieve high
temporal resolution.
 Disadvantage- Because projection data sets
are obtained from different heartbeat cycles, a
misregistration due to rapid motion can result in
the degradation of image spatial resolution.

30. Synchronization with the ECG and cardiac
motion
 With both prospective ECG triggering and
retrospective ECG gating, the starting points of data
acquisition or the start points of data selection for
reconstruction have to be defined within each cardiac
cycle during the acquisition.
 Start points are determined relative to the R-waves of
the ECG signal by a phase parameter.
 The relative delay and absolute reverse approaches
are most frequently used.
 End-diastolic reconstruction is feasible with the
absolute reverse approach, while the absolute delay
approach allows for most consistent reconstruction in
end-systolic phase.

31.  Relative delay: A delay relative
to the onset of the previous R-
wave is used for determining the
start point of the ECG-triggered
 Absolute reverse: Fixed times
prior to the onset of the next R-
wave define the start point of the
ECG-triggered acquisition For
ECG triggering, the position of
the next R-wave has to be
prospectively estimated based
on the prior RR interval times
 Absolute delay: Fixed delay
times after onset of the R-wave
define the start point of the ECG
triggered acquisition.

32. PITCH
 Defined as the ratio of table increment per gantry
rotation to the total x-ray beam width.
 Pitch values less than 1 imply overlapping of the
x-ray beam and higher patient dose; greater than
1 imply a gapped x-ray beam
 Cardiac imaging demands low pitch values
because higher pitch values result in data gaps,
which are detrimental to image reconstruction.
Also, low pitch values help minimize motion
artifacts
 Typical pitch values used for cardiac imaging
range from 0.2 to 0.4. (<0.5)

33.  For single segment( partial scan) reconstruction ,
pitch factor is influenced by patients heart rate.
 At higher pitch, there are substantial data gaps.
 When the subject’s heart rates are rapid and
difficult to control, the diastolic ranges are smaller,
so images are reconstructed using multi-segment
reconstruction in order to improve temporal
resolution.
 With multiple-segment reconstruction, the number
of segments used in the reconstruction further
restricts the pitch factors.
 radiation dose is inversely proportional to the pitch

35. Spatial Resolution
 Influenced by detector size in the z-direction,
reconstruction algorithms and patient motion
 Z-axis spatial resolution ranges from 1to10mm in
non-helical and in helical single-row detector CT,
while further reduced to sub-millimeter in side with
MDCT.
 Reconstruction interval: degree of overlap
between reconstructed images
 Independent of x-ray beam collimation, image
thickness and has no effect on scan time or
patient exposure
 Improve z-axis resolution and improve lesion
visibility in 3D and MPR images

36.  For routine MPR and 3D applications, a 30%
image overlap is sufficent (1mm thickness with
0.7mm interval)
 For cardiac CT 50% overlap is desirable
(0.5mm thickenss with 0.25mm interval)
 Too much overlap-larger number of images,
increases recon time and can put undue strain
on image handling overhead cost (image
transfer, display and archiving etc) with no
significient gain in image quality.

38. ARTIFACTS IN CARDIAC CT
 Due to factors such as tachycardia,
arrhythmia, or inappropriate scanning delay
with retrospective electrocardiographic gating.
 Cardiac pulsation artifacts
 Motion artifacts
 Misalignment and slab artifacts
 Blooming artifacts
 Respiratory artifacts

39. Cardiac pulsation artifacts:
 Occurs due to cardiac pulsation, shows
disconnect in lateral reconstructed images.
 Minimized by multi-segment reconstruction or
scanning at higher temporal resolution (50ms)

40. Motion Artifacts/Banding Artifacts:
 Motion artifacts occur at high heart rates and
most often in the mid-segment of the right
coronary artery.
 RCA has highest-velocity movement and
greatest positional change in the x and y
planes
 Remedy: lower the heart rate. (administration
of beta –blocker)

42. Misalignment and Slab Artifacts:
 Occur especially in patients with high heart
rates, heart rate variability, and the presence
of irregular or ectopic heart beats (e.g.,
premature ventricular contractions [PVCs] and
atrial fibrillation)
 can be best recognized in a sagittal or coronal
view.
 Often limit the diagnostic assessment of
coronary artery segments.
 One option is to reconstruct the dataset at
different phases of the cardiac cycle.

44. Blooming Artifacts/streak artifacts:
 High-attenuation structures, such as stents,
calcified plaques or calcium deposition appear
enlarged (or bloomed) because of partial
volume averaging effects and obscure the
adjacent coronary lumen.
 Sharper filters or kernels and thinner slices
(0.5–0.6 mm) may reduce these artifacts and
may enable an improved assessment of stent
patency.

46. Respiratory Artifacts:
 Produce ‘‘stair-step’’ artifacts
through the entire dataset.
 Can be recognized easily as
inward motion of the sternum
in a large sagittal view.
 Adequate patient preparation
with training of the breath-
hold commands is mandatory
to avoid such artifacts.

48. RADIATION RISK DUE TO
CARDIAC CT
 Radiation doses are higher with MDCT compared with
the doses delivered with EBCT and fluoroscopically
guided diagnostic coronary angiography (3 to 6 mSv)
and similar procedures.
 Highly dependent on the protocol used in cardiac CT.
 Calcium scoring:1–3 mSv.
 For retrospective gated CT angiography: 8–22 mSv and
higher.
 One approach to reduce the high dose associated with
retrospective gating is called ECG dose modulation.
 A 10%–40% dose reduction can be achieved.

50. Radiation dose reduction strategies
 ECG gated tube current modulation
 Minimize scan range
 Heart rate reduction
 Reduced tube voltage in suitable patients
 Perform Ca scoring only if needed
 Sequential scanning- prospective triggering
 Iterative reconstruction methods

51. ECG gated dose modulation:
 The nominal tube output (mA) is only required
during those phases of the cardiac cycle that will
be reconstructed (during diastolic phase).
 Within every cardiac cycle, tube output is raised to
the nominal level during diastolic phase in which
the data are most likely to be reconstructed with
thin slices and a high signal-to-noise ratio.
 During the remaining part of the cardiac cycle
(during systolic phase) , the tube output can be
reduced by about 80% by a corresponding
decrease of the tube current enabling dose
reduction.

52. Automatic tube current modulation
(ATCM)
 Temporal modulation: based on modulating
tube current(mA) at specified time period of
ECG signal
 Spatial modulation: based on modulating
tube current(mA) at different spatial
projections.

54. DUAL SOURCE CCTA
 Two x-ray tubes
positioned at 90°
apart from each
other
 Principles: benefit of
improved temp
resolution
 MSCT- temp
resolution : ½ gantry
rot.
 DSCT- temp

55.  In a DSCT scanner a complete data set of 180
of parallel beam projections can be generated
from two 90 data sets (quarter scan segments)
that are simultaneously acquired by the two
independent measurement systems
 As both quarter scan segments are acquired
simultaneously within a quarter (one fourth) of
a rotation, the total acquisition time and
temporal resolution of the resulting half scan
data set are a quarter of the rotation time.

58. POST ERA OF CARDIAC CT: 64-Detector
Row, Dual-Source, Dual Focal Spot
 Second-generation dual-source MDCT (Somatom
Definition FLASH) introduced at the end of 2008
 gantry rotation time is 280 ms, with temporal
resolution of approx 75 ms when the scanner
operates with both x-ray tubes collecting data at
the same energy.
 The pitch required for multiphase acquisition
ranges from 0.2 to 0.4 (depending on the heart
rate).
 With the high-pitch acquisition mode, only one
“phase” is acquired, which gradually increases with
the z-axis table translation.
 Scan the entire heart (12 cm) in 270 ms, with a

59. 128-Detector Row, Single-Source, Dual
Focal Spot
 Philips introduced the 256-slice MDCT
(Brilliance iCT) in 2007, a 128×0.625-mm
detector row system with dual focal spot
positions to double the number of slices within
the 8-cm (width) z-axis gantry coverage.
 270-ms gantry rotation time, which translates
to an approximate temporal resolution of 135
ms
 Prospectively ECG gated cardiac CT typically
covers the entire heart in two axial acquisitions
over three heartbeats.

60. 320-Detector Row, Single-Source, Single
Focal
Spot
 This hardware (Aquilion One Dynamic Volume
CT) currently has the largest z-axis detector
coverage.
 Each detector element is 0.5 mm wide,
yielding a maximum of 16-cm z-axis coverage
 This configuration allows three dimensional
volumetric whole heart imaging during the
diastole of one R-R interval.
 Temporal resolution of approximately 175 ms,
one half the gantry rotation time.

62. LIMITATIONS OF CARDIAC CT
 Extensive calcifications
 Stents : spatial resolution
 Heart rate: temporal resolution
 Radiation risk
 Small branches/ septal branches

63.  How dose MDCT makes cardiac imaging possible?
 What are the key issues in cardiac imaging?
 What do you mean by temporal resolution and why it should be higher for
cardiac imaging?
 Which part of cardiac cycle is quiescent for cardiac imaging and why?
 What are the factors that influence temporal resolution?
 Describe prospective ECG triggering acquisition mode?
 Describe retrospective ECG gating mode of acquisition?
 What do you mean by partial scan and multi-segment reconstruction?
 Describe type of artifacts arise in cardiac CT and their remedy.
 How can you reduce radiation dose to patients in cardiac CT?
 What do you mean by ECG gated dose modulation?
 What are the new technological advancement in cardiac CT?

64. Thank You !!!