digital techniques for myocardial perfusion spect

Digital Techniques for Myocardial Perfusion SPECT

Dr. Muhammad AyubDiplomate Certification Board of Nuclear CardiologyDiplomate Certification Board of Cardiovascular CT

Punjab Institute of Cardiology, Lahore

SINGLEPHOTONEMISSIONCOMPUTEDTOMOGRAPHY

A Modern SPECT Gamma Camera

SPECT Equipment

• Camera– Single Detector– Multi detector

• Analog to digital converter• Computer interface

– Data storage– Data processing– Display

Gamma Camera• Gantry• Camera Heads

– Collimator– Scintillation

Crystal– Photo multiplier

tubes– PHA– ADC Crystal

PMTs

Lead Shield

Pulse Processing Electronics

Collimator

PHAx y

Display

Cardiac SPECT Acquisition

• Gamma camera with 1 or 2 heads (90 ° angle)

• 180° rotation• 32 steps (16 per head

if 2)• Planar projection

acquisition at each step

Step and shoot acquisition

Workstation

• Types– Unix Workstations– PCs, Macs

• Storage• Memory• Graphics• Software

PHA

Tomographic Acquisition

• Matrix• Angular Range• Angular Steps• Time per Step• Gated Vs non gated acquisition• Continuous Vs Step and Shoot

acquisition

Tomographic Acquisition

• Matrix– 64 x 64 Standard– 128 x 128

Tomographic Acquisition

• Angular Range– 180 degrees– 360 degrees

215 LPO

RAO

135

Tomographic Acquisition

Angular Steps – 3 degrees Recommended for Tc Studies– 6 degrees Recommended for Tl Studies– 12 degrees Not Recommended

Image Acquisition

• Orbit– Circular Standard– Non-circular Optional

Tomographic AcquisitionGated SPECT

Technical Considerations

• QC– Energy– Uniformity– Linearity– COR

Problems

• Motion Artifacts• COR defects• Soft tissue attenuation• Foreign Objects

Image Processing

• Filtered Back projection• Filtering• Iterative Reconstruction• Corrections

– Attenuation Correction– Scatter Correction

Theory of Back Projection 1

Imaging a point source• Rotating camera head• Step and shoot at 8

angles over 180°• Only photons to

camera face are detected

• Planar ‘projection’ acquisition at each step

0°22.5 °

45°

67.5°

90°

180°

Point source

45°

67.5°

22.5°0°

Back Projection of 4 Ray Sums

Camera anglex

Depth in transaxial slice

Transaxial Plane

Filtering

• Pre filtering• 3D Post Filtering• Types

– Low pass– Metz– Wiener– Band

Types of Filters Used

• Smoothing filter–E.g. Hanning or Butterworth–Smooths image (removes noise

generated by ramp filter)

1.0

0.0

Frequency

Magnitude• Ramp filter –Always applied–Removes star-burst effect (good!)–Amplifies high frequency noise (bad!)

Pre filtration

3 D Post FilterLow pass Filter

Metz Filter

Wiener Filter

Band Pass Filter

Filter Example: Butterworth

Filtered Raw image

Butterworth [Cutoff = 0.3 Order =5]

Filter Reconstructed slice

Too smooth!

Filter Example: Butterworth

Butterworth [Cutoff = 0.5 Order =5]

Filter Filtered Raw image

Reconstructed slice

Good quality

Image Reconstruction

• Short Axis• Vertical Long Slices• Horizontal Long Slices• Slice Thickness

Transverse Reconstruction

• Initial estimate – e.g. uniform object• Compare forward projection to real projections• Perform several iterations:

– Each time, alter original transverse estimate until pseudo-projections converge with real ones.

The Iterative Loop

Flow chart of a loop in iterative process

Copyright ©2008 American College of Cardiology Foundation. Restrictions may apply.

Sharir, T. et al. J Am Coll Cardiol Img 2008;1:156-163

New MethodsWIDE BEAM RECONSTUCTION

Oblique Reformatting

Image Display

• Cardiac Stress Rest Display Format• Colored and Black & White Display• 3D display• Cine Display• Animation

The use of different colors (as opposed to intensities) in a translation table may produce pseudocontouring: the false perception of an edge when none, in fact, is present. An image thatshows a linear transition from maximal to minimal counts is shown using 3 different color tables. The steady transition of the image is shown most faithfully by the gray scale.

Visual scoring1

0

4

2

4

4

1

2

2

3

4

4

100%

70

50

30

10

0

4 normal 100 - 70%

3 mild 70 - 50%

2 moderate 50 - 30%

1 severe 30 - 10%

0 absent 10 - 0%

Score

Normal Scan

3D Display

Quantitative Display

• Bulls Eye Reconstruction– Cequal– PTQ map

• Quantitative Gated SPECT Display

PTQ Map

3 : no uptake ; defect3 : no uptake ; defect2 : severely decreased uptake2 : severely decreased uptake1 : slightly decreased uptake1 : slightly decreased uptake0 : normal uptake0 : normal uptake

Segmental analysisSegmental analysis

A four-point grading system

A four-point grading system

apicalapical basalbasal

V-long axisV-long axisshort-axisshort-axis

1717

1818

1919

2020

22

3344

11

5566

7788

1111

99

1010

12121313

1414

1515

1616

Quantitative Gated SPECT

QGS

Thank You