pptx..

1. BASIC PRINCIPLE OF SPECT
AND PET

2. SPECT
• Tomographic imaging technique using gamma rays
• Able to provide true 3D information
• Image is presented as cross-sectional slices through the
patient, but can be reformatted or manipulated as required.
• Requires injection of a gamma-emitting radioisotope (called
radionuclide). (The radioactive isotope decays, resulting in
the emission of gamma rays).
• Used for visualization of functional information

3. • Standard planar projection images are acquired from an arc of
180 degrees (most cardiac SPECT) or 360 degrees (most
noncardiac SPECT) about the patient.
• Most SPECT systems use one or more scintillation camera heads
that revolve about the patient
• Transverse images are reconstructed using either filtered back
projection (as in CT) or iterative reconstruction methods.
• Multi-headed gamma cameras can provide accelerated
acquisition (dual-headed camera can be used with heads spaced
180 degrees apart, allowing 2 projections to be acquired.
simultaneously Triple-head cameras with 120-degree spacing )

5. PRINCIPLES
• Multiple 2-D images are taken from multiple angle. (Projections)
• A computer is then used to apply a tomographic reconstruction
algorithm to the multiple projections,(Projections are acquired at
defined points during the rotation, typically every 3–6 degrees)
• This dataset may then be manipulated to show thin slices along
any chosen axis of the body.
• Tracer used in SPECT emits gamma radiation that is measured
directly.

6. IMAGE ACQUISITION
• Camera head or heads revolve about the patient, acquiring
projection images from evenly spaced angles
• May acquire images while moving (continuous acquisition)
or may stop at predefined angles to acquire images (“step
and shoot” acquisition)
• Each projection image is acquired in frame mode

7. • If camera heads produced ideal projection images (i.e., no
attenuation by patient and no degradation of spatial
resolution with distance from camera), projection images
from opposite sides of patient would be mirror images then
180 degree only sufficient .
• Attenuation greatly reduces number of photons from activity
in the half of patient opposite camera head; this information
is blurred by distance

8. Schematic diagram of SPECT data
acquisition
• For each projection view, the
computer
• sends a message to the gamma
camera to step to the next
viewing angle and, after the
camera sends a message back to
the computer
• that it is ready to acquire, the
computer acquires projection
image acquisition time.
• The total SPECT study acquisition
time is T=mt, where m is the
number of views acquired

9. TRANSVERSE IMAGE RECONSTRUCTION
• After projection images are acquired, they are usually
corrected for axis-of-rotation misalignments and for non
uniformities
• Following these corrections, transverse image reconstruction
is performed using either filtered back projection or iterative
methods

10. FILTER KERNELS
• Projection images of better spatial resolution and less
quantum mottle require a filter with higher spatial frequency
cutoff to avoid loss of spatial resolution in the reconstruction
transverse images
• Projection images of poorer spatial resolution and greater
quantum mottle require a filter with lower spatial frequency
cutoff to avoid excessive quantum mottle in the
reconstructed transverse images

11. ITERATIVE RECONSTRUCTION
• An initial activity distribution in the patient is assumed
• Projection images are calculated from the assumed activity
distribution, using the known characteristics of the
scintillation camera
• Calculated projection images are compared with actual
projection images and, based on this comparison, the
assumed activity distribution is adjusted
• Process repeated several times until calculated projection
images approximate the actual ones

12. • Calculation of projection images takes into account the
decreasing spatial resolution with distance from the camera
face
• If a map of the attenuation characteristics of the patient is
available, the calculation of the projection images can
include the effects of attenuation
• Iterative methods can partially compensate for effects of
decreasing spatial resolution with distance, photon
scattering in the patient, and attenuation in the patient

13. ATTENUATION AND 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
• Transverse image slices of a phantom with a uniform activity
distribution will show a gradual decrease in activity toward
the center
• Attenuation effects are more severe in body SPECT than in
brain SPECT

14. • A common correction method assumes a constant attenuation
coefficient throughout the patient
• 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 image
• These attenuation maps are used during SPECT image
reconstruction to provide attenuation-corrected SPECT images.

16. SPECT COLLIMATORS
• Most commonly used is the high-resolution parallel-hole
collimator
• Fan-beam collimators mainly used for brain SPECT
• FOV decreases with distance from collimator
• If used for body SPECT, portions of the body are excluded
from the FOV, creating artifacts in the reconstructed images

18. MULTIHEAD SPECT CAMERAS
• Two or three scintillation camera heads reduce limitations
imposed by collimation and limited time per view.
• Permits use of higher resolution collimators for a given level
of quantum mottle
• Requirement for electrical and mechanical stability of the
camera heads

19. SR-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

20. • 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

21. ADVANTAGES OF SPECT
• Improved image contrast.
• Improved quantification of cardiac function, tumor/organ
volume determination, the quantification of radioisotope
uptake.
• Problems of gamma-ray attenuation and scatter may be
better handled by SPECT (although, as yet, not completely),
over planar projection imaging.
• SPECT acquires 2-D projection of a 3-D volume.

22. SPECT applications
1.Brain:
• Perfusion (stroke, epilepsy schizophrenia,
dementia[Alzheimer])Tumors
2.Heart:
• Coronary artery disease,Myocardial infarcts.
3.Bone scan
4.Tumor and tumor staging.

23. IMPROVEMENTS IN SPECT TECHNOLOGY
• The application of multiple gamma camera heads.
• Noncircular orbits.
• The application of non-uniform attenuation correction
methods.
• Gated SPECT perfusion scans with 99mTc agents and 201-Tl,
also gated SPECT blood pool.
• SPECT systems lack anatomical resolution-Hybrid technique
SPECT/CT .

24. SPECT/CT
• SPECT/CT system was only introduced in 1999.
• The advantage of using CT data for attenuation correction of
emission data.
• low-power X ray tube with separate gamma and X ray detectors
mounted on the same slip ring gantry.
• X-ray system operated at 140 kV with a tube current of only 2.5
mA (low dose)
• It provides a high photon flux that significantly reduces the
statistical noise associated with the correction in comparison to
other techniques (i.e., radionuclide's used as transmission

25. • The total imaging time is significantly reduced because of
fast acquisition speed of CT scanners.
• The anatomic images acquired with CT can be fused with the
emission images to provide functional anatomic maps for
accurate localization of radiopharmaceutical uptake.
• CT images are acquired as transmission maps with a high
photon flux and are high-quality representations of tissue
attenuation and thus can provide the basis for attenuation
correction.

26. POSITRON EMISSION TOMOGRAPHY

27. POSITRON DECAY
• Also known as Beta Plus decay. A proton changes to a
neutron, a positron (positive electron), and a neutrino
• Mass number A does not change, proton number Z reduces.
• The positron may later annihilate a free electron, generate
two gamma photons in opposite directions.
• These gamma rays are used for medical imaging (Positron
Emission Tomography)

28. INTRODUCTION
• Is a NM imaging technique that produces a three-dimensional
image or picture of functional processes in the body.
• The system detects pairs of gamma rays emitted by a positron
emitting radionuclide (tracer).
• Three-dimensional images of tracer concentration within the
body are then constructed by computer analysis.
• If biologically active molecule chosen for PET is FDG an analogue
of glucose, the concentrations of tracer imaged then give tissue
metabolic activity, in terms of regional glucose uptake.

29. • short-lived radioactive tracer isotope is injected into the
living subject (usually into blood circulation).
• The tracer is chemically incorporated into a biologically
active molecule.
• There is a waiting period while the active molecule becomes
concentrated in tissues of interest; then the subject is placed
in the imaging scanner.
• The molecule most commonly used for this purpose is
fluorodeoxyglucose (FDG).

30. • As the radioisotope undergoes positron emission ,it emits a
positron, an antiparticle of the electron with opposite charge
• The emitted positron travels in tissue for a short distance
(typically less than 1 mm)
• The encounter annihilates both electron and positron, producing
a pair of annihilation (gamma) photons moving in approximately
opposite directions.
• These are detected when they reach a scintillator in the
scanning device, creating a burst of light which is detected by
photomultiplier tubes.

31. • Simultaneous or coincident detection of the pair of photons
moving in approximately opposite direction. (accepted
within a timing-window of a few nanoseconds)
• Most significant fraction of electron-positron decays result in
two 511 keV gamma photons being emitted at almost 180
degrees to each other.
• It is possible to localize their source along a straight line of
coincidence.(also called the line of response)

33. ANNIHILATION COINCIDENCE DETECTION
• Positrons emitted in matter lose most of their kinetic energy
by causing ionization and excitation
• When a positron has lost most of its kinetic energy, it
interacts with an electron by annihilation
• The entire mass of the electron-positron pair is converted
into two 511-keV photons, which are emitted in nearly
opposite directions

34. ANNIHILATION COINCIDENCE DETECTION
• Detect two events in opposite directions occurring
“simultaneously "Time window is 2-20 ns, typically 12 ns
• No detector collimation is required(Higher sensitivity) less
wasteful of photon.

35. True, random, and scatter coincidences
• A true coincidence is the simultaneous interaction of emissions
resulting from a single nuclear transformation
• A random coincidence, which mimics a true coincidence, occurs
when emissions from different nuclear transformations interact
simultaneously with the detectors
• A scatter coincidence occurs when one or both of the photons
from a single annihilation are scattered, but both are detected

36. DESIGN OF A PET SCANNER
• Scintillation crystals coupled to PMTs are used as detectors in
PET
• Signals from PMTs are processed in pulse mode to create
signals identifying the position, deposited energy, and time
of each interaction
• Energy signal is used for energy discrimination to reduce
mispositioned events due to scatter and the time signal is
used for coincidence detection

37. • Early PET scanners coupled each scintillation crystal to a
single PMT. Size of individual crystal largely determined
spatial resolution of the system
• Modern designs couple larger crystals to more than one PMT
• Relative magnitudes of the signals from the PMTs coupled to
a single crystal used to determine the position of the
interaction in the crystal

39. SCINTILLATION MATERIALS
• Material must emit light very promptly to permit true
coincident interactions to be distinguished from random
coincidences and to minimize dead-time count losses at high
interaction rates
• Must have high linear attenuation coefficient for 511-keV
photons in order to maximize counting efficiency

40. • Most PET systems use crystals of bismuth germanate
(Bi4Ge3O12, abbreviated BGO)
• BGO light output is less and light is emitted slowly.Inorganic
crystal like lutetium oxyorthosilicate (Lu2Sio4O)LSO and
gadolinium oxyorthosilicate(Gd2SiO4O) GSO both activated
with cerium are newer crystal of choice
• They have Faster light emission than BGO produces better
performance at high interaction rates

41. 2D DATA ACQUISITION
• In 2D (slice) data acquisition, coincidences are detected and
recorded within each detector ring or small groups of adjacent
detector rings
• PET scanners designed for 2D data acquisition have thin annular
collimators (typically tungsten) to prevent most radiation emitted
by activity outside a transaxial slice from reaching the detector
ring for that slice
• Fraction of scatter coincidences reduced because of the geometry

42. • Coincidences within one or more pairs
of adjacent detector rings may be
added to improve sensitivity
• Data from each pair of detector rings
are added to that of the slice midway
between the two rings
• Increasing the number of adjacent
rings used in 2D acquisition reduces
the axial spatial resolution

43. 3D DATA ACQUISITION
• In 3D (volume) data acquisition,axial
collimators are not used and
coincidences are detected between
many or all detector rings
• Greatly increases the number of true
coincidences detected; may permit
smaller activities to be administered to
patients

44. COMPARISON OF PET WITH SPECT
• PET scanner more efficient than scintillation camera due to
use of annihilation coincidence detection instead of
collimation; also yields superior spatial resolution
• Spatial resolution in SPECT deteriorates from edge toward
center; PET is relatively constant across transaxial image,
best at center
• Attenuation less severe in SPECT; accurate attenuation
correction possible in PET (with transmission source)
• Cost: SPECT ~US$500,000; PET ~US$1M - $2M

45. FACTORS AFFECTING AVAILABILITY
• Main factors limiting availability of PET are the relatively high
cost of a dedicated PET scanner and, in many areas, the lack
of local sources of F-18 FDG.
• Positron emitter -short half life require nearby cyclotron
facility.
• Multi head SPECT cameras with coincidence circuitry and
SPECT cameras with high-energy collimators provide less
expensive, although less accurate, alternatives for imaging
FDG

46. COMBINED PET CT SYSTEM
• CT -High resolution anatomical image
• PET-Low resolution functional image
• Combining both we can achieve physiological status within
anatomical region.

47. • If examinations are performed on separate machines
• The process is time-consuming, expensive, and logistically
demanding for patients and staff.
• Patient repositioning causes inaccurate anatomic matching,
and side-by-side interpretation of images results in
diagnostic inaccuracy.
• Software fusion of images is hampered by varying image
properties such as spatial resolution, shifting, tilting,
rotation, distortion, partial-volume effects, and non rigid
organ deformation.
• Manipulating the vast amount of imaging -High demands on
computer and software technology

48. Clinical application of PET
• Characterizing brain disorders such as Alzheimer disease and
epilepsy and cardiac disorders such as coronary artery disease
and myocardial viability
• Diagnosis and staging of non small cell lung cancer
• Diagnosis and staging of colorectal cancer
• Malignant melanoma.(to see distant mets)
• Hodgkin and Non-Hodgkin Lymphoma.
• Esophageal Carcinoma
• Head and Neck Cancer
• Breast Carcinoma

49. • Refractory epilepsy-(preoperative evaluation of abnormal
focus interictal SPECT less reliable but PET more sensitive
and will localize a high proportion of abnormal foci during
interictal phase
• Prognosis and Monitoring Therapeutic Effect
• Treatment Planning in Radiation Therapy

50. REFERENCES
• Essential physics of medical imaging ( Bushberg)
• Varoius internet sources..

51. • THANK YOU..