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.

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