Technical Aspects of MSCT and ECG Gating S. Edyvean Imaging - - PowerPoint PPT Presentation

▶
technical aspects of msct and ecg gating
SMART_READER_LITE
LIVE PREVIEW

Technical Aspects of MSCT and ECG Gating S. Edyvean Imaging - - PowerPoint PPT Presentation

Jan 21, 2024 517 likes •1.75k views

Technical Aspects of MSCT and ECG Gating S. Edyvean Imaging Performance Assessment of CT Scanners St. Georges Hospital www.impactscan.org ImPACT ImPACT Harefield BSCR 2006 Technical Aspects of MSCT and ECG Gating MSCT scanning

slide-1
SLIDE 1

Harefield BSCR 2006

  • S. Edyvean

Imaging Performance Assessment

  • f CT Scanners
  • St. Georges Hospital

www.impactscan.org

Technical Aspects of MSCT and ECG Gating

ImPACT ImPACT

slide-2
SLIDE 2

Harefield BSCR 2006 2

  • MSCT scanning

– Principles – Current technology

  • Particular challenges of imaging the heart
  • ECG gating techniques
  • Practical approaches to optimisation
  • Dose
  • The future

Technical Aspects of MSCT and ECG Gating

slide-3
SLIDE 3

Harefield BSCR 2006 3

Cardiac CT

  • Godfrey Hounsfield, inventor of clinical CT, 1972

– 1979 Nobel prize – died Aug 12th 2004

  • James Ambrose – Neuroradiologist AMH

– Standing ovation at RSNA 1972, died March 12th 2006

slide-4
SLIDE 4

Harefield BSCR 2006 4

Godfrey Hounsfield – Nobel Speech 1979

WHAT IMPROVEMENTS SHOULD WE EXPECT TO SEE IN THE FUTURE? Various attempts have been made to achieve useful pictures of the heart. The time available for taking a picture of the heart is obviously longer than one heart beat. Some experiments were conducted some time ago using conventional CT machines but in which the traverse of the detectors was synchronised to the heart beat via an electro-cargiograph, passing over the heart in diastole (when the heart movement is at a minimum).

slide-5
SLIDE 5

Harefield BSCR 2006 5

Godfrey Hounsfield – Nobel Speech 1979

  • Fig. 14 shows a picture from the experiment. The

heart chambers can be discerned by a little intravenous injected contrast media. A further promising field may be the detection of the coronary arteries. It may be possible to detect these under special conditions of scanning.

slide-6
SLIDE 6

Harefield BSCR 2006 6

Cardiac CT: before multislice

  • Single slice limited to gross morphology – tumour

depiction, large vessels

  • EBCT – Electron Beam CT scanner used

extensively for calcium scoring

slide-7
SLIDE 7

Harefield BSCR 2006 7

Electron Beam CT

  • Temporal resolution 50 ms
  • Slice thickness 3 mm
  • Use primarily for calcium scoring
slide-8
SLIDE 8

Harefield BSCR 2006 8

Development of Cardiac CT

  • Number of advances in last 8-10 years
  • Increase in scan speeds (0.5, 0.4, 0.33 sec / rot)
  • Multi-slice technology

– Up to 64 thin slices in one shot enabling multi-sector reconstruction and/or heart to be covered in shorter time

  • Software

– Developments in ECG gating techniques – Specialised methods of image reconstruction for cardiac

slide-9
SLIDE 9

Harefield BSCR 2006 9

  • Tube and detectors rotate continuously around the patient

– Slip rings transfer power and data to and from the gantry – Current rotation times down to 0.33, 0.4 sec

Power Data

X Y

Multi-slice CT Scanners

slide-10
SLIDE 10

Harefield BSCR 2006 10

Multi-slice CT Scanners

  • Beam widths (20 – 40 mm)
  • 4, 16, 64 slice (+others) data acquisition

Z

slide-11
SLIDE 11

Harefield BSCR 2006 11

Multi-slice CT Scanners

  • Axial acquisition

– In multi-slice limited number of slices due to diverging beam (not reccomended above ~ 10 slices)

Z

slide-12
SLIDE 12

Harefield BSCR 2006 12

Multi-slice CT Scanners

  • Helical Acquisition
  • Acquiring up to 64 slices of data simultaneously

– 64 x 0.625 mm, 64 x 0.6 mm, 64 x 0.5 mm

Z

slide-13
SLIDE 13

Harefield BSCR 2006 13

Multi-slice CT Scanners

  • Attenuation data taken at different angles through the patient
  • Images are reconstructed from the helical data set by

interpolating projection data to the required reconstructed image position

Z X Y

slide-14
SLIDE 14

Harefield BSCR 2006 14

Multi-slice CT Scanners

  • Attenuation data taken at different angles through the patient
  • In helical acquisition, projection data is interpolated to the

required reconstructed image position

X Y

slide-15
SLIDE 15

Harefield BSCR 2006 15

Pitchx = table travel in one rotation beam width

  • Normal helical scanning performed at pitch ~ 1
  • Cardiac scanning generally performed in helical

mode at very low pitches (pitchx = 0.2)

Pitchx = 1.25 x

Drot

Multi-sector reconstruction and pitch

Pitchx = Drot X

slide-16
SLIDE 16

Harefield BSCR 2006 16

  • Cardiac scanning generally performed at very low

pitches (pitchx = 0.2)

Multi-sector reconstruction and pitch

x

Drot

Pitchx = table travel in one rotation beam width Pitchx = 0.25 Pitchx = Drot X

slide-17
SLIDE 17

Harefield BSCR 2006 17

Detectors -16 Slice Scanners

GE Philips / Siemens Toshiba

16 x 0.625 4 x 1.25 4 x 1.25 16 x 0.75 4 x 1.5 4 x 1.5 16 x 0.5 12 x 1 12 x 1

20 mm 24 mm 32 mm 16 x 0.63 mm 16 x 0.75 mm 16 x 0.5 mm 16 x 1.25 mm 16 x 1.5 mm 16 x 1 mm 16 x 2 mm

Z

slide-18
SLIDE 18

Harefield BSCR 2006 18

28.8 mm 32 mm

IGE, Philips Siemens Toshiba

  • GE LightSpeed 32, 64, Philips Brilliance 40, 64

– 64 x 0.625mm, length = 40 mm

  • Siemens Sensation 64

– 32 x 0.6 (double sampled in z-axis to give 64) and 8 x 1.2, length = 28.8 mm, length for 0.6 mm elements = 19.2

  • Toshiba Aquilion 32, 64

– 64 x 0.5mm, length = 32 mm

Detectors – 64 Slice Scanners

Z

40 mm

slide-19
SLIDE 19

Harefield BSCR 2006 19 Oversampling 0,6 mm

Z

32 Slice Detector 64 Slice DAS

0,6 mm

  • 64-Slice CT: double z-Sampling: Overlap doubles

information

Sampling distance 0.3 mm

Siemens

Courtesy Siemens

slide-20
SLIDE 20

Harefield BSCR 2006 20

  • MSCT scanning

– Principles – Current technology

  • Particular challenges of imaging the heart
  • ECG Gating techniques
  • Practical approaches to optimisation
  • Dose
  • The future

Technical Aspects of MSCT and ECG Gating

slide-21
SLIDE 21

Harefield BSCR 2006 21

Imaging the heart

  • ‘Shutter speed’
  • Coverage

– Mis-registration

slide-22
SLIDE 22

Harefield BSCR 2006 22

What are the challenges ?

  • Moving object (60 -120 beat/min ie 1 -2 beat /sec)

– Scanners ~ 1 – 2 rotation /sec – Vessels move at different times/rates

slide-23
SLIDE 23

Harefield BSCR 2006 23

Image window

  • Need a snap shot of ~100 ms, at heart rates ~ 60 bpm

–For less than 1 mm movement, in 3-D, of a coronary artery at diastole –More strict criteria

  • reconstruction at more than one phase
  • small distal parts of CA
  • quantifying coronary stenoses
  • assessment of plaque

^ Roos et al BIR Jan 2006 1 sec 0.1 sec

slide-24
SLIDE 24

Harefield BSCR 2006 24

What are the challenges ?

  • Tortuous vessels, narrowing to < 1 mm

– Good isotropic spatial resolution

0.8 3.5

LAD

1.8 3.7

RCA

1.3 3.2

LCX

  • 4.3

LM

Distal segment mm Proximal segment mm

Lumen diameter of normal coronary artery segments J.T. Dodge et al., Circulation, 1992, 86:232-246

slide-25
SLIDE 25

Harefield BSCR 2006 25

Spatial Resolution

slice width (z-axis) pixel voxel

  • 3-D resolution of image

– In-plane – Z-axis (slice thickness)

  • Isotropic resolution

– Voxel equal sided z-axis

X Y

slide-26
SLIDE 26

Harefield BSCR 2006 26

  • In-plane (X-Y) determined by detector size,

sampling, convolution kernel (+ many other factors)

  • Z-axis determined by detector size and sampling
  • Actual perceived resolution depends on heart motion

In plane

Standard resolution ~ 0.6 mm High resolution ~ 0.4 mm

Z-Axis

Down to ~ 0.4 mm

Spatial Resolution

slide-27
SLIDE 27

Harefield BSCR 2006 27

Spatial Resolution

slide-28
SLIDE 28

Harefield BSCR 2006 28

Spatial Resolution

slide-29
SLIDE 29

Harefield BSCR 2006 29

  • ~ 12 cm in length

– To image heart in one breath-hold

  • ~ Varying and irregular heart rates

– Few beats

  • What are the challenges ?
slide-30
SLIDE 30

Harefield BSCR 2006 30

Heart Rate and Breath-hold

Nieman, Heart ’02

HR (t) / HR (t0)

slide-31
SLIDE 31

Harefield BSCR 2006 31

Stability with Time

  • 100 Cardiac patients

– evaluated on LightSpeed 16 and Pro 16 scanners – Average scan time 20 sec, heart rates ranged from 40 to 110 bpm

  • % of Case with Stable Heart Rate

– 4 beat 97% – 5 beat 92% – 8 beat 39% – 10 beat 10%

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 110% 1 2 3 4 5 6 7 8 9 10 11 12 13 14 time (sec) % patients scanable in first t-sec

Courtesy GE

slide-32
SLIDE 32

Harefield BSCR 2006 32

  • Good low contrast resolution (plaque)

– require noise levels equivalent to current CT imaging

What are the challenges ?

L – Vessel Lumen N – Non-calcified plaque C – Calcified plaque

slide-33
SLIDE 33

Harefield BSCR 2006 33

What are the challenges ?

  • Requirements for imaging the heart

– Image for <100ms – Isotropic resolution < ~ 1 mm – Low contrast differentiation – One breath hold and few beats

slide-34
SLIDE 34

Harefield BSCR 2006 34

Applications of cardiac CT

  • Calcium scoring
  • Coronary angiography
  • Follow-up of interventional work
  • Coronary plaque imaging
  • Functional imaging
slide-35
SLIDE 35

Harefield BSCR 2006 35

What are the challenges ?

  • Requirements for imaging the heart

– Image for <100ms ? – Isotropic resolution < ~ 1 mm – Low contrast differentiation – One breath hold and few beats

slide-36
SLIDE 36

Harefield BSCR 2006 36

  • MSCT scanning

– Principles – Current technology

  • Particular challenges of imaging the heart
  • ECG gating techniques
  • Practical approaches to optimisation
  • Dose
  • The future

Technical Aspects of MSCT and ECG Gating

slide-37
SLIDE 37

Harefield BSCR 2006 37

CT imaging requirements

  • To reconstruct images

– 180° of scan data is required (180 ° + fan angle)

  • Effective image acquisition time is ~ 0.5 x rotation time

– ~ 250 ms (for .5s)

Power Data

X Y

slide-38
SLIDE 38

Harefield BSCR 2006 38

CT Imaging Requirements

  • ECG

– ECG is monitored before and throughout scan

  • Contrast

– Uniform distribution of contrast media throughout study

  • Beta – blockers

– Sometimes required to lower heart rate

slide-39
SLIDE 39

Harefield BSCR 2006 39

  • Imaging window during period of least cardiac motion

– ~ 100 ms at 60 bpm ie ~ 10% of cardiac cycle

  • Position defined in terms of percentage of phase relative to

R-R interval (+/- %)

Principles of Data Acquisition

Cardiac motion (ventricular volume) ECG Window = ~ 100 ms t phase R R

100 % +70%

slide-40
SLIDE 40

Harefield BSCR 2006 40

Choosing the best phase for reconstruction

Optimal Recon

LV Volume ECG Imaging window

ED ES

slide-41
SLIDE 41

Harefield BSCR 2006 41

Phase of reconstruction

  • Actual phase depends on particular area of interest

– ~70% of the R-R interval for LCA – sometimes 40% for the RCA

  • Many phases can be reconstructed

– eg can be reconstructed at 5, or 10% intervals for functional imaging

ECG

slide-42
SLIDE 42

Harefield BSCR 2006 42

  • Image window may be too wide for higher heart rates

Principles of Data Acquisition

  • At higher heart rate 100 ms window covers region of

greater movement ⇒ Need smaller temporal window higher heart rate

slide-43
SLIDE 43

Harefield BSCR 2006 43

Image acquisition

  • Cardiac CT data are acquired in two main modes

Sequential, ‘stop and shoot’ Helical Prospective ECG gating Retrospective ECG gating (ECG triggering)

Z

slide-44
SLIDE 44

Harefield BSCR 2006 44

Prospectively gated cardiac CT

  • Axial - ‘step and shoot’
slide-45
SLIDE 45

Harefield BSCR 2006 45

Prospective gating – ECG Triggering

  • X-rays on only for data collection
  • Coverage limited by breath hold considerations on 4

slice scanners

  • Tends to be used for calcification scoring

– Can’t acquire 64 thin slice, slice widths of ~ 3mm

  • Increasing heart rate leads to poorer images

– More heart motion included in 180 degree window

slide-46
SLIDE 46

Harefield BSCR 2006 46

Retrospective gating of image data

  • Continuous irradiation and data collection in helical

acquisition

  • Single or multi-sector reconstruction

Power Data

X Y Z

slide-47
SLIDE 47

Harefield BSCR 2006 47

Multi-sector reconstruction

  • Single sector

– 180° sector of data – Sector time window = ½ rotation time

  • eg 0.5 sec rotation (500 ms), sector = 0.25 s (250 ms)

– Data from one heart beat

~0.25 s 0.25s

acquisition

‘temporal resolution’ Constant irradiation

slide-48
SLIDE 48

Harefield BSCR 2006 48

Multi-sector reconstruction

  • Two sector

– Two sectors each of 90 ° – Sector time = ¼ rotation, eg = 125 ms (with 0.5 s rot) – Data from 1 ¼ rotations, two heart beats

Time for (1 rotation + 1 sector) sector length = ‘temporal resolution’

slide-49
SLIDE 49

Harefield BSCR 2006 49

Multi-sector reconstruction

  • 3-sector
  • 4-sector

360° plus sector θ 360° plus sector θ θ θ

sector length = ‘temporal resolution’

slide-50
SLIDE 50

Harefield BSCR 2006 50

Multi-sector reconstruction

  • Temporal resolution

– No. of sectors – Heart rate – Rotation time

  • Pitch

– No. of sectors

  • Time to cover heart

pitch heart rate speed of coverage

  • f heart

temporal resolution registration number of sectors

slide-51
SLIDE 51

Harefield BSCR 2006 51

  • Two sector - optimum timing (rotation and heart rate)

Temporal resolution

0.96 s 63 2 + 1/4 105 420 1+1/4 No rotations 96 Heart rate (bpm) 0.625 s Beat to beat time 125 Sector time (ms) 500 Rotation time (ms) rotation time + time for sector

θ

rotation time + ¼ rotation Heart cycle time

slide-52
SLIDE 52

Harefield BSCR 2006 52

  • Two sector – synchronisation

Temporal resolution

rotation time 2 Heart rate (bps) 0.5 s Each beat 120 Heart rate (bpm) 0.5 Rotation time (s) Heart cycle time

slide-53
SLIDE 53

Harefield BSCR 2006 53

ECG-HR

  • ptimum

timing Synchronized

Coronary arteries

slide-54
SLIDE 54

Harefield BSCR 2006 54

  • Two sector – midway

Temporal resolution

slide-55
SLIDE 55

Harefield BSCR 2006 55

  • Two sector – midway
  • Unequal sectors can be used
  • Temporal resolution determined by largest sector

Temporal resolution

slide-56
SLIDE 56

Harefield BSCR 2006 56

  • Can use 180° opposite projections

– More options of data for next sector – Perfect matching at (1/2 rotation +1/4)

  • 2-sector

Temporal resolution

slide-57
SLIDE 57

Harefield BSCR 2006 57

Temporal resolution graph – 2 sector

  • Maximum two sector reconstruction, 0.33s rotation

40 60 80 100 120 140 160 180 50 60 70 80 90 100 Heart rate (bpm) temporal resolution (ms)

slide-58
SLIDE 58

Harefield BSCR 2006 58

  • Same principles apply for many sectors

– eg 0.5 sec rotation, each sector = ~ 68 ms with perfect matching

Temporal resolution

Sector length ‘temporal resolution’ = (500/2)/4 = 68 ms

slide-59
SLIDE 59

Harefield BSCR 2006 59

  • Peaks at single sector, troughs at increasing

number of multi-sectors

Temporal resolution graph – multi-sector

Courtesy Toshiba

Temporal resolution (ms) Heart rate (bpm) 400 ms /rotation

slide-60
SLIDE 60

Harefield BSCR 2006 61

Temporal resolution

slide-61
SLIDE 61

Harefield BSCR 2006 62

2 Sector 3 Sector

Courtesy Philips

Temporal resolution

slide-62
SLIDE 62

Harefield BSCR 2006 63

Temporal Resolution and Rotation Time

  • Optimum temporal resolution depends on

asynchrony of heart rate and rotation time

Courtesy Toshiba Pitch 0.33

Temporal resolution (ms) Heart rate (bpm)

Pitch 0.33

500 ms /rotation 400 ms /rotation

slide-63
SLIDE 63

Harefield BSCR 2006 64

Temporal resolution

slide-64
SLIDE 64

Harefield BSCR 2006 65

Temporal Resolution

  • ‘Temporal resolution’ = sector length

– Fastest rotation time gives shorter sector lengths – Multi-sector gives shorter lengths - avoid synchronisation

  • More sectors require more beats
  • Require steady heart beat for good registration
slide-65
SLIDE 65

Harefield BSCR 2006 66

  • Dual tube imaging

– Siemens Definition launched RSNA '05

  • Two tubes at 90°

– 2 x 1/4 sectors simultaneously in 83 ms

Temporal resolution

slide-66
SLIDE 66

Harefield BSCR 2006 67

Siemens Dual Source CT

= 83 ms Rotation Time 4 Temporal Resolution =

Temporal resolution of 83 ms

Courtesy Siemens

slide-67
SLIDE 67

Harefield BSCR 2006 68

Multi-sector reconstruction - pitch

temporal resolution detector transit time

  • To reconstruct from a number of sectors, the detectors need

to image the given slice of heart for the equivalent number of heart beats

time distance

slide-68
SLIDE 68

Harefield BSCR 2006 69

Multi-sector reconstruction - pitch

  • Different detector banks contribute to each sector

– Overlapping pitch

slide-69
SLIDE 69

Harefield BSCR 2006 70

  • eg 2 segments

transit time

low pitch

Multi-sector reconstruction - pitch

slide-70
SLIDE 70

Harefield BSCR 2006 71

  • eg 2 segments

transit time

high pitch

Multi-sector reconstruction - pitch

– But if pitch too high there will be gaps in the reconstruction data

slide-71
SLIDE 71

Harefield BSCR 2006 72

  • Four segments – lower pitch (slower table speed)

transit time

Multi-sector reconstruction - pitch

slide-72
SLIDE 72

Harefield BSCR 2006 73

Temporal resolution (ms) Heart rate (bpm) 400 ms /rotation

400 ms/rotation

Pitch 0.25 Pitch 0.33

Courtesy Toshiba, see also Manzke et al, Med. Phys. 30 (12) December 2003

Temporal Resolution and Pitch

  • Pitch does not affect optimum matching of rotation

time and heart rate

slide-73
SLIDE 73

Harefield BSCR 2006 74

  • Increased heart rate

– Same number of sectors

  • Avoid synchronisation - change rotation time?
  • Pitch can increase ⇒ lower dose, faster coverage of heart

– More sectors may be used

  • Pitch must decrease

Heart rate

transit time transit time

Same no.

  • f sectors

Increased

  • no. of

sectors

transit time

slide-74
SLIDE 74

Harefield BSCR 2006 75

Time to cover heart

  • Depends on

– pitch, rotation time, detector acquisition length

Power Data

slide-75
SLIDE 75

Harefield BSCR 2006 76

  • Depends on

– detector acquisition length

Heart Length 120 mm

4 slice (10 mm) 16 x slice (20 mm) 64 slice (40 mm) 64 sec 32 sec 16 sec Assumes 0.5 sec, 0.375 pitch

Time to cover heart

slide-76
SLIDE 76

Harefield BSCR 2006 77

  • Depends on

– pitch, rotation time, detector acquisition length

0.5 0.6 0.6 0.625 0.625 Acquisition width 32 19.2 19.2 40 40 Detector length (mm) 7.5 5.1 10.3 6.3 5.3 Time to cover 120 mm ^ (s) 0.33 Siemens (1 tube) 0.4 0.33 0.42 0.35 Min rotation times (s) Toshiba Siemens (2 tube) Philips IGE 64 slice scanners

^assume pitch 0.2

Time to cover heart

slide-77
SLIDE 77

Harefield BSCR 2006 78

Multi-sector reconstruction

  • Temporal resolution

– No. of sectors – Heart rate – Rotation time

  • Pitch

– No. of sectors

  • Time to cover heart

pitch heart rate speed of coverage

  • f heart

temporal resolution registration number of sectors

slide-78
SLIDE 78

Harefield BSCR 2006 79

  • MSCT scanning

– Principles – Current technology

  • Particular challenges of imaging the heart
  • ECG Gating techniques
  • Practical approaches to optimisation
  • Dose
  • The future

Technical Aspects of MSCT and ECG Gating

slide-79
SLIDE 79

Harefield BSCR 2006 80

Practical approaches to optimisation

  • Monitor pre-scan heart rate

– To determine best combination of pitch, rotation time, number of sectors

  • Finding the best phase

– Motion maps

  • Responding to change in heart rate

– ECG editing

slide-80
SLIDE 80

Harefield BSCR 2006 81

Pre-scan resting heart rate

Heart rate during breath hold is monitored

Courtesy Toshiba

slide-81
SLIDE 81

Harefield BSCR 2006 82

Pre-scan resting heart beat

  • Optimum combination of parameters – rotation

time, pitch, no of sectors

  • Selection - automatic using complex algorithms,

semi-automatic, or guided by protocols

0.42 0.33 0.33 0.42 0.35 Minimum scan time 1 or 2 Siemens (1 tube) Up to 5 1 or 2 Up to ?5 1, 2, 4 No of sectors Toshiba Siemens (2 tube) Philips IGE

slide-82
SLIDE 82

Harefield BSCR 2006 83

Pitch 0.25

Automatic selection of rot. time, pitch & sectors

Courtesy Toshiba

slide-83
SLIDE 83

Harefield BSCR 2006 84

Automatic selection of rot. time, pitch & sectors

Pitch 0.25

Courtesy Toshiba

slide-84
SLIDE 84

Harefield BSCR 2006 85

Phase of reconstruction

  • Actual phase depends on particular area of interest

– ~70% of the R-R interval for LCA – sometimes 40% for the RCA

  • Exact matching of phase to heart motion
slide-85
SLIDE 85

Harefield BSCR 2006 86

  • Motion Maps

Using a raw data motion map movement in the cardiac cycle is determined For reference only

Phase

Raw data motion map

Z- Axis Position

Finding the optimum phase

slide-86
SLIDE 86

Harefield BSCR 2006 87

Raw data motion map

Phase Z- Axis Position

The raw data motion map is converted into a motion graph Motion graph

Finding the optimum phase

  • Motion Maps
slide-87
SLIDE 87

Harefield BSCR 2006 88

The troughs indicate the least motion phases used for reconstruction

Systole Diastole

Motion graph Raw data motion map

Phase Z- Axis Position

  • eg optimum phase may be 72% not 70%

Finding the optimum phase

slide-88
SLIDE 88

Harefield BSCR 2006 89

Cardiac Phase Slice Number

Example: Min:56, Max:67, Avg 60

Courtesy: Philips, R. Manzke, M. Grass, Philips Research Labs, Hamburg, Germany

Finding the optimum phase

slide-89
SLIDE 89

Harefield BSCR 2006 90

Responding to change in heart rate

Courtesy Siemens

  • Retrospective ECG Editing of reconstruction data
slide-90
SLIDE 90

Harefield BSCR 2006 91

ECG Editing

16 slice scanner 64 slice scanner 32 slice scanner 24 beat scan 6 beat scan 12 beat scan

Courtesy Toshiba

  • Important in 64 slice scanners where fewer

beats are used to cover heart

1 beats recorded incorrectly

slide-91
SLIDE 91

Harefield BSCR 2006 92

2 beats recorded incorrectly 24 beat scan 6 beat scan 12 beat scan

Courtesy Toshiba

16 slice scanner 64 slice scanner 32 slice scanner

ECG Editing

  • Important in 64 slice scanners where fewer

beats are used to cover heart

slide-92
SLIDE 92

Harefield BSCR 2006 93

3 beats recorded incorrectly 24 beat scan 6 beat scan 12 beat scan

Courtesy Toshiba

16 slice scanner 64 slice scanner 32 slice scanner

ECG Editing

  • Important in 64 slice scanners where fewer

beats are used to cover heart

slide-93
SLIDE 93

Harefield BSCR 2006 94

During registration T-peak exceeds R-peak

Enhanced T-peak

Example - ECG Editing

Courtesy Toshiba

slide-94
SLIDE 94

Harefield BSCR 2006 95

R-peaks are incorrectly recognized and time markers are incorrectly set

Example - ECG Editing

Courtesy Toshiba

Enhanced T-peak

slide-95
SLIDE 95

Harefield BSCR 2006 96

Raw data from wrong phase is used prior and after the T-peak

Example - ECG Editing

Enhanced T-peak

Courtesy Toshiba

slide-96
SLIDE 96

Harefield BSCR 2006 97

Correct phase, specific raw data is used for reconstruction

Example - ECG Editing

Enhanced T-peak

Courtesy Toshiba

slide-97
SLIDE 97

Harefield BSCR 2006 98

Sub optimal ECG

64 slice, one T instead of R-peak

Example - ECG Editing

Courtesy Toshiba

slide-98
SLIDE 98

Harefield BSCR 2006 99

ECG Editor

64 slice, move T-peak to R-peak

Example - ECG Editing

Courtesy Toshiba

slide-99
SLIDE 99

Harefield BSCR 2006 100

Track the R-to R in real time

Variable Beat-to-Beat Fixed Offset

Courtesy: Philips (Dr. Martin Hoffmann, Uni-Ulm, Germany)

ECG tracking to deal with irregular beat

slide-100
SLIDE 100

Harefield BSCR 2006 101

  • MSCT scanning

– Principles – Current technology

  • Particular challenges of imaging the heart
  • ECG Gating techniques
  • Practical approaches to optimisation
  • Dose
  • The future

Technical Aspects of MSCT and ECG Gating

slide-101
SLIDE 101

Harefield BSCR 2006 102

Dosimetry

  • Overlapping pitch

– High dose – Where possible increase pitch to reduce dose

  • Might expect dose to increase

– only reconstruct part of the data set for any one image – 10% of data ⇒ ~ 3 x dose to achieve similar iq

  • Typical dose values

– Comparison with other examinations and modalities

  • Dose saving techniques

– ECG dose modulation – Special beam shaping filters

slide-102
SLIDE 102

Harefield BSCR 2006 103 54 55 55 56 52 3 3 3

13 18 13 7

15 9 16 17 19

10 20 30 40 50 60

A B C D mean

CTDI vol (mGy)

CTA Ca Score (ax) Ca score (hel) Abdo

Organ Doses for Cardiac Scanning

2003 RDL

slide-103
SLIDE 103

Harefield BSCR 2006 104

Effective Doses

16.6 0.8 4.5 5.5

0.0 5.0 10.0 15.0 20.0 25.0

CTA Ca Score (ax) Ca score (hel) Abdo Effective dose (mSv)

EBCT CTA

1.2

EBCT Ca scoring

0.8

slide-104
SLIDE 104

Harefield BSCR 2006 105

Dosimetry

  • Cardiac CT radiation doses are relatively high.
  • Ball park figures (dependent on technique etc)

mSv

~ 10 – 15 CT angiography ~ 5 planar coronary angiography ~ 5 PET 82Rb ~ 2 PET 13NH3 ~ 10 SPECT

slide-105
SLIDE 105

Harefield BSCR 2006 106

ECG Tube Current Modulation

  • ECG current modulation is used

– mA reduced outside of required reconstruction phase down to ~ 20% – Net dose savings ~ 50%

  • Can be automatically suspended if ECG changes

Tube current I maging window ECG signal 100% mA 20% mA

slide-106
SLIDE 106

Harefield BSCR 2006 107

ECG Tube Current Modulation

ECG signal Tube current I maging window 100% mA 20% mA

  • ECG current modulation is used

– mA reduced outside of required reconstruction phase down to ~ 20% – Net dose savings ~ 50%

  • Can be automatically suspended if ECG changes
slide-107
SLIDE 107

Harefield BSCR 2006 108

0 % 12.5 % 25 % 37.5 % 50 % 62.5 % 75 % 87.5 %

Noise α 1/√mAs

Estimated dose Saving ~ 45%

ECG Tube Current Modulation

  • Image noise affected in phases of lower mA

0.2 0.4 0.6 0.8 1 1.2 50 100

Calculated relative mAs from noise values

slide-108
SLIDE 108

Harefield BSCR 2006 109

Beam Shaping Filters

  • Beam shaping filters more appropriate for small fov

reconstruction within a larger fov

slide-109
SLIDE 109

Harefield BSCR 2006 110

  • MSCT scanning

– Principles – Current technology

  • Particular challenges of imaging the heart
  • ECG Gating techniques
  • Practical approaches to optimisation
  • Dose
  • The future

Technical Aspects of MSCT and ECG Gating

slide-110
SLIDE 110

Harefield BSCR 2006 111

What are the challenges ?

  • Requirements for imaging the heart

– Image for <100ms - multi-sector, or two tube – Isotropic resolution < ~ 1 mm – Low contrast differentiation at the expense of high dose – One breath hold and few beats

slide-111
SLIDE 111

Harefield BSCR 2006 112

Future of cardiac CT

  • Extending current developments:

– Faster gantry rotation

  • <0.2s/rot, need mechanical forces >75G

– Bigger tubes and generators – Advanced reconstruction methods – EBCT?

  • Larger detector arrays
  • Flat panel detectors
  • Dual energy imaging
slide-112
SLIDE 112

Harefield BSCR 2006 113

Faster gantry rotation ?

slide-113
SLIDE 113

Harefield BSCR 2006 114

Larger detector arrays

  • 128/256 row scanners in next couple of years

– Allow whole organ coverage in single rotation

Detector mock-ups courtesy of Toshiba

256 row detector (256 x 0.5) = 128 mm

Aquilion 64 detector

Z - axis

slide-114
SLIDE 114

Harefield BSCR 2006 115

Large matrix detector array

  • 256 x 0.5 mm = 128 mm coverage

Courtesy of Toshiba

slide-115
SLIDE 115

Harefield BSCR 2006 116

Total Organ Scanning

Wide Area Detector

Philips MDCT (WIP)

O ne rotation

RSNA 2005

Nano-Panel

Large matrix detector array

Courtesy of Philips

slide-116
SLIDE 116

Harefield BSCR 2006 117

Flat panel detectors

From Nikolaou

slide-117
SLIDE 117

Harefield BSCR 2006 118

Dual energy imaging

  • Performed with two tubes (Siemens)

– Each tube operates at different voltage

  • Detector discrimination (Philips, GE)

– Dual layer detector, sensitive to different energies

  • Contrast resolution can be improved

– Plaque discrimination?

Courtesy Siemens

slide-118
SLIDE 118

Harefield BSCR 2006 119

  • MSCT scanning

– Principles – Current technology

  • Particular challenges of imaging the heart
  • ECG Gating techniques
  • Practical approaches to optimisation
  • Dose
  • The future

Technical Aspects of MSCT and ECG Gating

slide-119
SLIDE 119

Harefield BSCR 2006 120

References

  • Kalender. Computed Tomography (ISBN 3-89578-216-5)
  • Hseih Computed Tomography, Design, Principles, Artifacts and Recent

Advances (ISBN 0-8194-4425-1)

  • Roos et al, BJR Jan 2006, Cardiac Applications of Multislice Computed

Tomography

  • Flohr et al European Radiology, 2005, First Performance of a dual

source CT (DSCT) system

  • Manzke et al, Med Phys Dec 2004 pp. 3345–3362. Automatic phase

determination for retrospectively gated cardiac CT

  • Bruder et al, Design Considerations in Cardiac CT Medical Imaging

2006 Proceedings of SPIE

  • McCollough, Patient Dose in Cardiac Computed Tomography Herz 2003
slide-120
SLIDE 120

Harefield BSCR 2006 121

Acknowledgements

  • Maria Lewis, James Weston – ImPACT
  • Nicholas Keat – GSK
  • Koos Gelijns – Leiden Medical Centre
  • Manufacturers

– Jackie Bye, Sandie Jewell – GE – Derek Tarrant, Mike Hayden – Philips – Susie Guthrie - Siemens – Henk DeVries - Toshiba

slide-121
SLIDE 121

Harefield BSCR 2006

  • S. Edyvean

Imaging Performance Assessment

  • f CT Scanners
  • St. Georges Hospital

www.impactscan.org

Technical Aspects of MSCT and ECG Gating

ImPACT ImPACT