This presentation deals with the image reconstruction, technique, artifacts and radiation risk associated with cardiac ct scanning.

1. CARDIAC
CT
Sudil Paudyal
M.Sc. MIT (12)
IOM, MMC
1

2. INTRODUCTION
 Coronary artery disease- major cause of death, enormous
economic burden on healthcare.
 Prime diagnostic tool- Coronary angiography
 Invasive
 Longer examination time
 Pt. prep time and recouping time.
 Advent of MDCT technology: sub millimeter spatial resolution,
improved temporal resolution and ECG triggered or gated mode
of acquisition
 Non-invasive imaging of heart and coronary arteries.
 Calcium scoring, CT angiography, and assessment of ventricular function.
 The American College of Cardiology Foundation and the American College of
Radiology formulated recommendations for “the use of CT coronary
angiography as an alternative to conventional catheter angiography in the
examination of patients who have symptoms but have inconclusive findings at
stress ECG or are unable to exercise.” 2

3. CARDIAC SCANNERS
 EBCT:
 1982, specifically for cardiac imaging, able to acquire an
image in 100 msec, suited for cardiac imaging at that time.
 Electrons accelerated in a vacuum funnel and are precisely
focused toward and swept across a 210º tungsten ring anode
placed under the patient. A cone beam of X-ray photons is
emitted which go through the patient and are captured by two
240º detector rows above the patient.
 Slice collimation is 3 mm, so 40 slices needed to cover the
entire heart (12 cm),for a total imaging time of 30 sec.
 One breath-hold.
 ECG-based triggering used for motion-free imaging during
diastole.
 Significant motion artifacts still remain.
3

4.  The spectrum of applications limited, and its physical setup
cumbersome.
 Mostly used for noninvasive 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.
4

5.  SSCT: Single-slice helical CT could be used for
cardiac imaging with ECG-synchronized protocols.
 Temporal resolution was improved over nonhelical
scanners but, remained insufficient for motion-free
cardiac imaging.
 Could not meet unique demands of imaging the
beating heart require optimal spatial resolution,
temporal and contrast resolution.
 MDCT:.
 submillimeter spatial resolution (0.75 mm),
 improved temporal resolution (80–200 msec), and
 electrocardiographically (ECG) gated or triggered mode
of acquisition, MDCT scanners (16–64-row detectors)
makes cardiac imaging possible.
5

6. KEY ISSUES IN CARDIAC IMAGING
 High temporal resolution
 Virtually “freeze” the beating heart to image coronary
arteries located close to heart muscles, which show
rapid movement during cardiac cycle.
 Imaging is best if performed in diastole phase- most
quiescent part of cardiac cycle.
 ECG continuously recorded for synchronisation with
image acquisition and reconstruction.
 High spatial resolution- resolve very fine structures eg.
proximal coronary segments which range from a few
mm in diameter (at the apex of aorta) and decrease to a
few submm in diameter as they traverse away from
aorta in all directions.
6

7.  Diastolic phase narrows with increasing heart rate.
7

8.  Desired temporal resolution :
 250 ms for heart rates upto 70 bpm
 Upto 150 ms for heart rates greater than 100 bpm
 Ideally around 50 ms for motion free imaging
 Standard of reference for comparing temporal resolution
is fluoroscopy, wherein motion is frozen during dynamic
imaging to a few ms (1-10 ms).
 Sufficient CNR- to resolve small and low contrast
structures such as plaques. CT- excellent low contrast
resolution. But can degrade with the increasing number
of CT detectors in the z- direction due to increased
scatter radiation.
8

9. CORONARY ARTERY
 Coronary artery is a vasa
vasorum that supplies the heart.
 The coronary artery arises just
superior to the aortic valve and
supply the heart
 The aortic valve has three cusps
–
 left coronary (LC),
 right coronary (RC)
 posterior non-coronary (NC)
cusps.
9

10.  Originates from right coronary
sinus of Valsalva
 Courses through the right AV
groove between the right
atrium and right ventricle to
the inferior part of the septum
10
RIGHT CORONARY ARTERY

11. 11
Right coronary artery
Conus artery
Sinu nodal artery
Marginal artery
Post. Descending IV artery
AV nodal artery-
Conus branch
SINU NODAL BRANCH
AV Nodal Branch
BRANCHES OF RCA

12.  Conus branch – 1st branch supplies the RVOT
 Sinus node artery – 2nd branch - SA node.(in 40%
they originate from LCA)
 Acute marginal arteries-Arise at acute angle and runs
along the margin of the right ventricle above the
diaphragm.
 Branch to AV node
 Posterior descending artery : Supply lower part of the
ventricular septum & adjacent ventricular walls. Arises
from RCA in 85% of case.
12

13. Right coronary anatomy
AO
LA
RCA
CONUS BR
RCA
SAN
1 2
3 4
RCA
AM
13

14. RCA
AM
AM
14

15. AREA OF DISTRIBUTION
RT CORONARY ARTERY:
1)Right atrium
2)Ventricles
i) greater part of rt. Ventricle except the area
adjoining the anterior IV groove.
ii) a small part of the lt ventricle adjoining posterior
IV groove.
3)Posterior part of the IV septum
4)Whole of the conducting system of the heart,
except part of the left br of AV bundle
15

16.  Arises from left coronary cusps
 Travels between RVOT
anteriorly and left atrium
posteriorly.
 Almost immediately bifurcate
into left anterior descending
and left circumflex artery.
 Length – 10-15mm
16
LEFT CORONARY ARTERY

17. Left coronary
artery
LAD
Diagonal
artery
Lt Conus
artery
Anterior Septal
br
Circumflex
artery
Obtuse
marginal
branches
Ventricular
branches
Atrial rami
17

18. LEFT CORONARY ARTERY 18

19. AREA OF DISTRIBUTION
LT CORONARY ARTERY
1) Left atrium.
2) Ventricles
i) Greater part of the left ventricle, except the area
adjoining the posterior IV groove.
ii) A small part of the right ventricle adjoining the
anterior IV groove.
3) Anterior part of the IV septum.
4) A part of the left br. Of the AV bundle.
19

20. 20

21. DOMINANCE
 Determined by the arrangement that which artery
reaches the crux & supply posterior descending
artery
 The right coronary artery is dominant in 85% cases.
 8% cases - circumflex br of the left coronary artery
 7% both rt & lt coronary artery supply posterior
IVseptum & inferior surface of the left ventricle-
codominance
21

22. SOME PHYSICS
TEMPORAL RESOLUTION
 Number of factors influence the temporal resolution
 Gantry rotation time
 Acquisition mode
 Type of image recon
 Pitch
 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
 Incresased gantry rotation, increase in stress on gantry
structure because of higher G forces. (mechanical stress
caused by heavy equipments)
22

23.  Acquisition mode:
 Prospective ECG triggering:
 Similar to conventional step and shoot method
 Pt.s cardiac functions are monitored continuously
through ECG signals
 Protocols so built to start exposure at a desired distance
from R-R peak.
 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).
 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
23

24. 24

25.  Additional technical effect- As the pt table has to
move by the distance covered by multiple slices in
between the scans, a technical delay time (so-
called scan-cycle time) has to be taken into
consideration.
 The minimum scan-cycle time between two
consecutive ECG-triggered scans depends on the
table-feed between the two scans and on the
acquisition time of one scan.
 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.
25

26.  Filters:
 Two filter techniques used for prospective
estimation of position of the following R-wave.
 mean filtering (e.g., 3 previous RR-intervals) and
 median filtering (e.g., 5 previous RR-intervals).
 Median filter approach shows increased robustness
for patients with moderate arrhythmia due to the
fact that single extra beats are eliminated.
26

27. 27
 With MDCT,
increasing
number of
detector rows in
z-axis allows
larger volume
coverage.
 Higher number
of slices helpful
in minimizing
motion artifacts.

28.  One of the advantages of the prospective triggering
approach is reduced radiation exposure, because the
projection data are acquired for short periods and not
throughout the heart cycle.
 Temporal resolution with this type of acquisition can range
from 200 to 250 msec.
 Prospective triggering is the mode of data acquisition used
for calcium scoring studies, since calcium scoring analysis
is typically performed in axial scan mode.
 The scan technique such as tube current (milliamperes) for
a calcium scoring protocol can be quite low, yielding low
radiation dose, since calcium has a high CT number and is
easily visible even with a noisier background.
 Also, each data set is obtained during the most optimal
ECG signal to reduce motion artifacts. 28

29.  Retrospective ECG gating:
 Main choice of data acquisition in MDCT
 The pt.s 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 pt.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
data corresponding to partial scan data or with
segmented reconstruction. 29

30.  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. Can range from 80 to 250 msec.
 The disadvantage is the increased radiation dose,
because the data are acquired throughout the heart
cycle, even though partial data are actually used in
the final image reconstruction.
 Also, since this scan is performed helically, the
tissue overlap specified by the pitch factor is quite
low, indicating excessive tissue overlap during
scanning, which also increases radiation dose to
the patients.
30

31. 31

32. Retrospective ECG gating Prospective ECG triggering
continuous volume coverage and
better spatial resolution in the
patients' longitudinal direction, as
images can be reconstructed with
arbitrary, overlapping slice
increments
is usually restricted to scanning with
nonoverlapping adjacent slices or
slice increments with only small
overlap.
in less sensitivity to heart-rate
changes during the scan
estimation of the next RR-interval
may be incorrect when heart-rate
changes are present (e.g.,
arrhythmia, Vasalva maneuver) and
scans may be placed in inconsistent
heart phases.
Faster volume coverage Scan cycle time limits the duration of
scan
acquisition allows for imaging in a
complete cardiac cycle using the
same scan data set, thus providing
information on cardiac function.
targets only one specific phase of
the cardiac cycle and requires
additional examinations with new
breath-hold levels and additional
contrast agent to cover more phases
of the cardiac cycle.
32

33. RECONSTRUCTION METHOD
 Partial scan:
 most practical solution is the partial scan
 Partial scan reconstruction 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.
33

34.  The equation αP = π + βf states that a minimum data segment
of 180° has to be available for every fan angle β.
 Thus, a partial rotation usually covers about two-thirds of a
rotation (≈240°).
 Conventional partial scan reconstruction techniques based on
fan-beam geometry make use of all acquired data even if
more data than the minimum angle of 180° are available for a
fan angle β.
 These techniques produce a temporal resolution equal to the
partial-scan acquisition time (≈two-thirds of the rotation time).
 Eg. for a gantry rotation of 500 msec, 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. The G
force generated due to rapid gantry motion is growing
exponentially and is reaching a limit for the existing
technology. 34

35.  Better temporal resolution can be achieved with
special reconstruction algorithms that use the
minimum required amount of scan data.
 These algorithms, 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.
 Half scan is the method of choice for reconstruction
in cardiac applications.
35

36. 36

37.  Multisegment 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. The CT projection data are acquired
continuously throughout many sequential heart cycles.
 For eg; if one chooses to select half of the data set required for
partial scan reconstruction from one heart cycle and the rest
from another heart cycle, this results in temporal resolution that
is about one-fourth of the gantry rotation time.
 This is done by using projection data from two separate
segments of the heartbeat cycle for image reconstruction.
 Improvement in temporal resolution can be achieved by
cleverly selecting projection data from three or four different
heart cycles, resulting in temporal resolution as low as 80
msec.
37

38.  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.
38

39. 39

40.  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.
41

41. 42

42. 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.
 Following phase selection strategies can be used.
 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.

43.  Relative delay: A temporal
delay relative to the onset of the
previous R-wave is used for
determining the start point of the
ECG-triggered acquisition or the
start point of the reconstruction
data interval
 Absolute reverse: Fixed times
prior to the onset of the next R-
wave define the start point of the
ECG-triggered acquisition or the
start point of the reconstruction
data interval. 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 or the start
point of the reconstruction data
interval

44. 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 and reduced patient
dose.
 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, and
certain reconstruction algorithms work best at
certain pitch values, which are lower than 0.5 in
cardiac imaging.
 Typical pitch values used for cardiac imaging range
from 0.2 to 0.4.
45

45.  At higher pitch, there are substantial data gaps.
 As a result, most cardiac CT protocols require
injecting beta-blockers to lower the subject’s
heartbeat within the desirable range of less than 70
beats per minute.
 When the subject’s heart rates are rapid and
difficult to control, the diastolic ranges are smaller,
so images are reconstructed using multiple-
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.
46

46. 47

47. 48

48. CORONATY CT ANGIOGRAPHY
PROCEDURE
 Main purpose: morphology
 Detection and analysis of CAD
 Depict anatomy of coronary vasculature
 Possible to obtain functional info
 Contractility of myocardium
 Valve morphology and function
 Viability of myocardium
49

49. PATIENT PREPARATION
 Clinical history (symptoms such as chest pain and dyspnea)
 History of allergies (e.g., iodinated contrast material and
medications)
 History of asthma or hyperthyroidism
 History of renal disease or multiple myeloma (recent
creatinine level)
 Previous diagnostic examinations (stress test,
electrocardiogram [ECG], and echocardiogram).
 Intravenous access via a intravenous line is ensured

50. HEART RATE CONTROL
 A stable, low heart rate is required at the time of the
procedure,
 The highest image quality for current CCT scans is achieved
at heart rates of less than 65 beats per minute (bpm)
 Oral or intravenous b-blockers should be administered before
the study, b-Blockers help reduce heart rate variability during
the scan, and for that reason,their administration is
recommended almost routinely unless they are
contraindicated (e.g., patients with asthma).
 In such situations, diltiazem or verapamil may be used as an
alternative agent, although these drugs are not as effective as
b-blockers

51. BREATH HOLDING
 During the test, a breath hold of 15–20 s will need to be
performed
 Before the scan, practicing breath holding helps.
ECG GATING
 First, the skin is cleaned
 Up to 12 self-adhesive electrodes will be attached to select
locations of the skin on the arms, legs and chest
 Three ECG leads are attached to obtain an adequate ECG
tracing for CT
 A noise-free ECG signal is important to synchronize the ECG
signal to the raw image data

52. SIEMENS SOMATOM DEFINITION AS+ 128
SLICE CORONARY CTA PROTOCOL
 Calcium scoring (low dose tech), 40mAS, 120 kvp
 Method :- Retrospective ECG triggered
 Head first, supine
 Kvp – 120, mAs – 160
 Detector conf.: 64 x 0.625 mm
 Pitch :- 0.2
 GR :- 0.33 sec
 Delay :- 5 sec
 Fov :- 200 mm
 Matrix:- 256
 Slice thickness :- 3 mm, recon :- 0.6 mm ,
increment :- 0.3 mm

53. 54

54. DISPLAY OF IMAGES
 LAD artery is generally best visualized at 60%–70% of
the cardiac cycle.
 RCA is most consistently visualized early in diastole, at
approximately 40% of the R-R interval.
 LCX artery is best visualized at 50% of the cardiac cycle.
 Both axial images and multiplanar reformatted
(MPR)images, which permit the visualization of coronary
arteries in multiple orientations orthogonal and
perpendicular to the long axis of the vessel.
 Once a suggestive location with luminal narrowing in the
presence of calcified or noncalcified plaques has been
identified on axial images,2 long axis views of the
respective location be created with 3- to 5-mm thin-slab
maximum-intensity-projection (MIP) images.
 Enables the reconstruction of a true cross-sectional
image of the vessel orthogonal to the long-axis view,
which is similar to an intravascular ultrasound view. 55

55.  An interactive evaluation with oblique MPR images
and sliding thin-slab MIP (STS-MIP) images also
has been proven helpful.
 Oblique MPR images are ideal for the confirmation
of pathologic findings in the long and short axes of
the vessel.
 STS-MIP images enhance the visualization of
coronary artery stenosis in a long-axis view of the
vessel if narrowing is caused by noncalcified
atherosclerotic plaque.
 STS-MIP images may not be used to evaluate the
presence of stenosis in the short axis of the vessel
and can cause false-positive readings in the
presence of calcium.
56

56. 57

57. 58

58. 59

59. CALCIUM SCORING
 Intended to detect calcified atherosclerotic
plaque burden as a surrogate marker for
coronary atherosclerosis.
 Based on the principle that–
 Obstructive atherosclerotic plaques are
calcified – so called “Hard Plaque”
 Calcium is not present within the wall of a
normal coronary artery
60

60. INDICATION
 Women over the age of 55 and men over the age of
45 should consider the coronary calcium scan, if
they have coronary artery disease risk factors
 Family history of heart disease
 High cholesterol level (hypercholesteremia)
 High blood pressure
 Smoking, Obesity
 Diabetes
 High-stress lifestyle
61

61. HOW THE PROCEDURE IS DONE
 Preparation:
 No special preparation is necessary
 Avoid caffeine and smoking four hours before the
exam.
 Heart rate ＞ 90/min → β- blocker
 Protocol :
 No contrast used
 2.5 mm to 3 mm slice thickness
 Prospective ECG –gated acquisition for calcium
scoring. 62

62.  The threshold for calcification is set at an attenuation value of ≥
130 HU, for an area of > 1mm2 along the course of the coronary
arteries.
 For MDCT the threshold value for calcification is 90 HU ( because
of high signal to noise ratio )
 Automated measurement of the lesion area in mm2 and maximum
CT No. (HU) of each lesions are recorded.
 Density score of the lesions are determined as
 The total as well as individual coronary artery calcium score is
calculated using special software at the workstation
63

63. METHODS
 Quantitative calcium scores are calculated
according to the method described by Agatston et
al .
Calcium score= density score x volume
 CAC scores are typically reported for each major
coronary artery (left main, left anterior descending,
circumflex, right coronary artery) separately
 The total score is achieved by adding up each of
the scores for all the slices
64

64. WHAT DOES THE CALCIUM SCORE
REPRESENT
 Detection of any degree of coronary calcium on CT
indicates that CAD is present
 It provides a quantitative estimation of plaque
burden. Higher the score the larger the plaque
burden & higher the subsequent cardiac events.
 Score of zero indicates unlikely chance of CAD,
does not eliminate the possibility.
 ≥80 has a sensitivity for prediction of coronary
events of 0.85.
 >160 is related to a 20–35-fold increased risk for a
cardiac event
65

66. ARTIFACTS IN CARDIAC CT
 Due to factors such as tachycardia, arrhythmia, or
inappropriate scanning delay with retrospective
electrocardiographic gating.
 Motion artifacts
 Misalignment and slab artifacts
 Blooming artifacts
 Respiratory artifacts
67

67. 68
 Motion Artifacts:
 Completely motion-free imaging of the coronary arteries
requires a temporal resolution of 50 ms, motion artifacts occur
at high rates and most often in the midsegment of the right
coronary artery.
 RCA has highest-velocity movement and greatest positional
change in the x and y planes, followed (in order of decreasing
movement) by the LCX, LMCA, and LAD.
 Easiest way of reducing -lower the heart rate.
 Patient-specific factors such as rapid heart rate or arrhythmia
may be controlled with administration of beta -blocker.

68.  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) and can be best recognized in a sagittal or
coronal view.
 Often limit the diagnostic assessment of coronary artery
segments in patients with atrial fibrillation and frequent PVCs.
 One option is to reconstruct the dataset at different phases of
the cardiac cycle.
69

69.  Blooming Artifacts:
 High-attenuation structures, such as calcified plaques or
stents, 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, they have little effect on calcified
plaques.
 A noncontrast calcium screening scan before coronary CTA
could be performed to decide whether to perform subsequent
coronary CTA.
70

70.  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.
71

71. POST 64 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 equipped with two 64 detector row units,
each with an alternating focal spot.
 The 360° gantry rotation time is 280 ms, translating to a temporal resolution
of approximately 75 ms when the scanner operates with both x-ray tubes
collecting data at the same energy.
 For coronary CT angiography, the typical phase window required for a
diagnostic quality examination regarding motion artifact is 10% of the R-R
interval .
 The pitch required for multiphase acquisition ranges from 0.2 to 0.5
(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.
 Slow and regular heart rates are the prerequisites for this acquisition that is
prospectively triggered by ECG and is anticipated to scan the entire heart
(12 cm) in 270 ms, with a pitch of 3.2.
72

72.  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.
 The iCT has 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.
 During the diastole of the first heartbeat, the upper half
of the heart is imaged.
 During the second heartbeat, the patient table translates
62.4 mm. Subsequently, the lower half of the heart is
acquired during the diastole of the third heartbeat.
73

73.  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.
 Entire heart is imaged with temporal uniformity (i.e, at
the same time point without temporal delay from the
base to apex).
 Temporal resolution of approximately 175 ms, one half
the gantry rotation time.
74

74. 75

75. CONE BEAM RECONSTRUCTION
 Cardiac cone-beam reconstruction algorithms gain importance for CT
scanners with a higher number of detectors.
 Generate double-oblique image stacks (so-called booklets) that are
individually adapted and optimally fitted to the spiral path which are
reformatted to a set of overlapping trans-axial images.
 For single-segment reconstruction, one booklet is used for
reconstruction and reformation in each heart cycle. Data segments
that cover 180° of parallel beam projections are employed for the
generation of these booklets, leading to a temporal resolution of the
images within the booklet of half of the gantry rotation time.
 Segmented cardiac cone-beam reconstruction algorithms permit
reconstruction of image booklets from data segments covering less
than 180°.
 The incomplete CT images from individual heart cycles are
separately reformatted and subsequently combined to complete axial
CT images containing contributions from multiple consecutive heart
cycles that add up to a total parallel projection interval of 180°. 76

76. 77

77. DUAL SOURCE CCTA
 Principles: benefit of improved temp resolution
 MSCT- temp resolution : ½ gantry rot.
 DSCT- temp resolution : ¼ gantry rot
 Independent of heart rate and without the need of
multisegment reconstruction
 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, which are off set by 90.
 As both quarter scan segments are acquired
simultaeously within a quarter of a rotation, the total
acquistion time and temoral resolution of the resulting
half scan data set are a quarter of the rotation time.
78

78. 79

79.  The two quarter scan segments are appended with
a smooth transition function.
 Since the second detector doesnot cover the entire
SFOV, its projections are truncated in objects that
extend beyond the 26 cm FOV and have to be
extrapolated by using data acquired with the first
detector at the same projection angle.
 Constant gantry rotation time Trot/4 is achieved in a
centered region of SFOV that is covered by both
acquisitions.
 For Trot= 0.33s, the temp. resolution is therefore 83
ms, independent of the patients heart rate and with
data used from one cardiac cycle only.
80

80. 81

81.  The basic mode of operation corresponds to single
segment reconstruction.
 Nevertheless, multisegment approaches can also be
applied to DSCT to improve temp resolution even further.
 In a two segment reconstruction, the quarter scan
segments achieved by each of the two detectors are
independently divided into smaller subsegments
acquired in subsequent cardiac cycles of the patient-
similar to two segment recon in conventional MSCT.
 Temp resolution again varies as a function of the pt.s
heart rate and a best temp. resolution of 42 ms can be
established based on Trot of 0.33s at certain heart rates.
82

82. 83

83. 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.
84

84. 85

85.  ECG gated dose modulation:
 If reconstruction in different cardiac phases is not needed
and instead only a very limited interval (i.e., diastolic phase)
in the cardiac cycle is targeted during reconstruction, a
significant portion of the acquired data and radiation
exposure is reduced.
 The nominal tube output is only required during those
phases of the cardiac cycle that will be reconstructed.
 During ECG-gated tube-current modulation, the tube output
is modulated on-line with prospective ECG control and
commonly used ECG gated reconstruction algorithms can be
further used unchanged.
 Within every cardiac cycle, tube output is raised to the
nominal level during a limited interval in a pre-selected
phase (usually the diastolic phase) in which the data are
most likely to be reconstructed with thin slices and a high
signal-to-noise ratio needs to be maintained.
 During the remaining part of the cardiac cycle, the tube
output can be reduced by about 80% by a corresponding
decrease of the tube current.
86

86.  Thus, continuous volume reconstruction is still
possible in all phases of the cardiac cycle.
 In particular, functional imaging is still feasible, as it
does not require thin slice reconstruction.
 For imaging during phases of reduced tube output,
an appropriate signal-to-noise ratio can be
maintained by primary or secondary reconstruction
of thicker slices.
 The width of the time interval ΔTN with nominal
tube output during diastole has to be selected such
that patient-individual shifting of the ECG gating
interval is still possible to obtain the best possible
image quality.
 For normal heart rates between 50 and 90 bpm, the
exposure is reduced by 35–50% for ΔTN = 400 ms
and by 45–60% for ΔTN = 300 ms. 87

87.  Radiation exposure savings are maximized for low
heart rates, as the total time with low tube output
during the scan is high.
 For increasing heart rates, the relative reduction
decreases, as the time intervals of low tube output
are shorter.
 However, for very high heart rates, the selection of
a higher spiral pitch can decrease exposure via
shorter scan time as compared to low heart rates.
 Where available, this technique should be used for
all patients with reasonably steady heart rates.
88

88. 89

89. 90

90. LIMITATIONS OF CARDIAC CT
 Extensive calcifications
 Stents : spatial resolution
 Heart rate: temporal resolution
 Radiation risk
 Small branches/ septal branches
91

91. NEW GENERATION SCANNERS APPLICATIONS
BEYOND CCTA
 Volumetric Myocardial Imaging for Function,
Perfusion, and Viability
 Evaluation for Acute Chest Pain and “Triple
Rule-Out” Test- for evaluation of thoracic aorta,
coronary and pulmonary arterial trees in pt.s with
suspected PE, aortic dissection and acute coronary
syndrome.
 Endothelial Shear Stress and Coronary Vascular
 Cine Volumetric Imaging and Four-Dimensional
Subtraction Angiography Profiling
 Coronary Artery Opacification Gradients
92

92. CONCLUSION
 Cardiac imaging is a highly demanding application of
multiple-row detector CT and is possible only due to
recent technological advances.
 Understanding the trade-offs between various scan
parameters that affect image quality is key in optimizing
protocols that can reduce patient dose.
 Benefits from an optimized cardiac CT protocol can
minimize the radiation risks associated with these
cardiac scans.
 Cardiac CT has the potential to become a reliable tool
for noninvasive diagnosis and prevention of cardiac and
coronary artery disease.
93

93. FIND MORE AT:
 Principles of Multislice Cardiac CT imaging; Bernd
Ohnesorge and Thomas Fhlor; Multislice and Dual
Source Cardiac CT
 Physics of Cardiac Imaging with Multiple Row detector
CT; Mahadevappa Mahesh and Diana D. Cody;
Radiographics, 2007
 Coronary CT angiography; Udo Hoffmann et.al; Journal
of Nuclear Medicine, 2006
 Pitfalls, artifacts and remedies in multi-detector row CT
coronary angiography; Hyun Seok Choi et.al;
Radiographics, 2004
 CT coronary angiography: 256 slice and 320 row
detector scanners; Edward M. Hsiao et.al; Curr. Cardio
Rep; 2010 94

94. 95