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