Medical Physics Imaging PET CT SPECT CT Lecture

Lecture 14: Emission Tomography III
Shahid Younas
NUCLEAR IMAGING
Emission Tomography III
Single Photon Emission Computed Tomography (SPECT)

Attenuation correction
X- or gamma rays that must traverse long paths through the patient produce
fewer counts, due to attenuation, than those from activity closer to the near
surface of the patient.

Introduction-Attenuation correction
 Images acquired with SPECT has,
 Poor spatial resolution
 Apparent decrease in activity

 Transverse image slices of a phantom with a
uniform activity distribution will show a
gradual decrease in activity toward the center.

 The primary mechanism for attenuation in tissue is Compton
Scattering.
 This changes photon direction with loss of energy.
 The change of direction results in missed count.

The effects of attenuation
are more intense at lower
energies but are still
significant at the highest
energy value.

Summing two planar
projection images
separated by 180.

The magnitude of
attenuation effect depends
on the tissue type.

Attenuation correction
Thus, to accurately represent the activity distribution measured with SPECT,
it is necessary to accurately correct for the effects of attenuation.

Attenuation correction Techniques
 Approximate methods are available for attenuation correction.
 Change Method, assumes a constant attenuation coefficient
throughout the patient.
 Over-undercompensate-as attenuation is not uniform

Constant Attenuation Coefficient
A1 A1 A1
A1 A1 A1
A1 A1 A1

 Some SPECT cameras have radioactive sources to measure the
attenuation through the patient,
 After acquisition, the transmission projection data are reconstructed
to provide maps of tissue attenuation characteristics across transverse
sections of the patient, similar to x-ray CT images.

 Some SPECT cameras have radioactive sources to measure the
attenuation through the patient,
 Finally these attenuation maps are used during SPECT image
reconstruction to provide attenuation-corrected SPECT images.

 Transmission sources are available in several configurations,
 Scanning Collimated Line Sources
 Fixed Line Sources

 Transmission data usually acquired simultaneously with the
acquisition of the emission projection data,
 Performing the two separately poses significant problems in
the spatial alignment of the two data sets.

 Radionuclide used for transmission measurements is chosen to have
primary gamma-ray emissions that differ significantly in energy from
those of the radiopharmaceuticals.
 Separate energy windows are used

 Scattering of the higher energy photons in the patient and in the
detector causes some cross-talk in the lower energy window.
 AC using transmission sources is used Myocardial perfusion
imaging.
 AC using transmission sources is promising but it is still under
development.

SPECT Collimator
Most commonly used is the high-resolution parallel-hole collimator
 Fan-beam collimators mainly used for brain SPECT
 FOV decreases with distance from collimator

Multihead SPECT Cameras
 Two or three scintillation camera heads reduce limitations imposed
by collimation and limited time per view.
 Y-offsets and X- and Y-magnification factors of all heads must be
precisely matched throughout rotation.

SPECT Performance
 Spatial resolution
 X- and Y-magnification factors and multi-energy spatial registration
 Alignment of projection images to axis-of-rotation
 Uniformity
 Camera head tilt

SPECT Spatial resolution
 Can be measured by acquiring a SPECT study of a line source
(capillary tube filled with a solution of Tc-99m, placed parallel to
axis of rotation).
 FWHM of the line sources are determined from the reconstructed
transverse images (ramp filter).

 National Electrical
Manufacturers
Association (NEMA)
specifies a cylindrical
plastic water-filled
phantom, 22 cm in
diameter, containing 3
line sources

 NEMA spatial resolution measurements are primarily determined by
the collimator used.
 Tangential resolution 7 to 8 mm FWHM for LEHR
 central resolution 9.5 to 12 mm
 radial resolution 9.4 to 12 mm

 NEMA measurements not necessarily representative of clinical
performance
 Studies can be acquired using longer imaging times and closer orbits
than would be possible in a patient.

 NEMA measurements not necessarily representative of clinical
performance
 Studies can be acquired using longer imaging times and closer orbits
than would be possible in a patient.
 Filters used for clinical studies have lower spatial frequency cutoffs
than the ramp filters used in NEMA measurements.

Comparison with conventional planar scintillation camera imaging
 In theory, SPECT should produce spatial resolution similar to that of
planar scintillation camera imaging.
 In clinical imaging, its resolution is usually slightly worse.
 Camera head is closer to patient in conventional planar imaging than
in SPECT.

 Short time per view of SPECT may mandate use of lower resolution
collimator to obtain adequate number of counts.
 In planar imaging, radioactivity in tissues in front of and behind an
organ of interest causes a reduction in contrast.

 Main advantage of SPECT is markedly improved contrast and
reduced structural noise produced by eliminating the activity in
overlapping structures.
 SPECT also offers promise of partial correction for effects of
attenuation and scattering of photons in the patient

Magnification factors
The X- and Y-magnification factors, often called X and Y gains,
related distances in the object being imaged, in the x and y directions, to
the numbers of pixels between the corresponding points in the resultant
image.

Magnification factors
 Magnification factors determined from a digital image of two point
sources placed against the camera’s collimator
If X- and Y-magnification factors are unequal, the projection images will
be distorted in shape, as will coronal, sagittal, and oblique images.

COR calibration
 The axis of rotation (AOR) is an imaginary reference line about
which the head or heads of a SPECT camera rotate.
 If a radioactive line source were placed on the AOR, each projection
image would depict a vertical straight line near the center of the
image.

COR calibration
 This projection of the AOR into the image is called the center of
rotation (COR).
 Ideally, the COR is aligned with the center, in the x-direction, of each
projection image.

COR calibration
 Misalignment may be mechanical or electronic.
 Camera head may not be exactly centered in the gantry.

COR calibration
COR Degradation and Sinogram

COR calibration
 COR misalignment causes a loss of spatial resolution in the resultant
transverse images.
 Large misalignment cause a point source to appear as “doughnut”.
 Doughnut are not centered in the image so can be distinguished from
“ring” artifacts produced by non-uniformities.

COR calibration
 COR alignment is assessed by placing a point source or line source in
the camera field of view.
 Projected imaged and or sinogram is analyzed by the camera’s
computer.

COR calibration
 Misalignment may be corrected by shifting each image in the x-
direction by the proper number of pixels prior to filtered back-
projection
 If COR misalignment varies with camera head angle, it can only be
corrected if computer permits angle-by-angle corrections.

Uniformity
 Nonuniformities that are not apparent in low-count daily uniformity
studies can cause significant artifacts in SPECT.
 Artifact appears in transverse images as a ring centered about the
AOR.

Uniformity
Cylinder filled with a uniform
radionuclide solution showing
a ring artifact due to non-
uniformity.

Uniformity
 Primary intrinsic causes of non-uniformity are,
a. Spatial non-linearities
 stretch the image in some areas
 reducing the local count density
 compress other areas of the images
 Increasing the count density
a. Local variation in the light collection efficiency

Uniformity
 Lookup table can not correct,
 Local variations in detection efficiency such as dents or
manufacturing defects in the collimators.

Uniformity
 High-count uniformity images used to determine pixel correction
factors,
 At least 30 million counts for 64 x 64 images
 At least 120 million counts for 128 x 128 images
 Collected every 1 or 2 weeks; separate images for each camera head

Camera head tilt
 Camera head or heads must be exactly parallel to the AOR.

Medical Physics Imaging PET CT SPECT CT Lecture

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