Single Photon Emission Computed Tomography: What it is, how it works, advantages and disadvantages in Nuclear Medicine.

1. S P E C TSINGLE PHOTON EMISSION COMPUTED TOMOGRAPHY
By
Victor EKPO, Nusirat ADEDEWE, Oluwafemi AJIBADE, Dorathy DAVID,
Dare ADEWA, Aderonke ADEDOKUN.
MSc. Medical Physics. College of Medicine. University of Lagos. 2017.

2. CONTENTSCONTENTS
 What is SPECT?
 How SPECT works
 Radionuclides, e.g. 99m
Tc
 Collimators, (e.g. fan beam collimator)
 Algorithms (e.g. FBP vs Iterative)
 Advantages and Disadvantages
 Artefacts
 Advanced Modalities
 Conclusion

3. WHAT IS SPECT?WHAT IS SPECT?
SINGLE
PHOTON
EMISSION emission imaging
(NOT Transmission imaging like X-ray
or Reflection-based imaging like Ultrasound)
COMPUTED  uses algorithms (NOT Geometric Tomography)
TOMOGRAPHY  produces 3D imaging (NOT planar)
Unlike PET, which uses dual annihilation
photons for image creation,
SPECT uses single photons.

4. INTRODUCTIONINTRODUCTION
Single Photon Emission Computed Tomography is a:
Nuclear Medicine imaging modality,
which involves the use of radionuclides injected
intravenously into the body,
to produce a 3D distribution of the gamma rays emitted by
the radionuclide,
giving physiological information about the organ of interest.

5.  Whereas other medical imaging modalities depend on
external photon’s physical interactions (e.g. absorption,
attenuation, scattering) with imaged organs for
anatomical images, Nuclear Medicine imaging uses
metabolic or physiologic interactions of radionuclides
to produce functional images.
 Nuclear Medicine imaging is also called Scintigraphy.
Nuclear Medicine ImagingNuclear Medicine Imaging

6. SPECT as Gamma Camera + CTSPECT as Gamma Camera + CT
 SPECT is the 3D version of the 2D (planar imaging) gamma
camera technology.
 It uses 1 or more gamma camera heads rotated round the patient.
 SPECT combines conventional scintigraphic and computed
tomographic methods.
 So gives 3D functional information about the patient in more
detail and higher contrast than found in planar imaging.

7. SPECT as Gamma Camera + CTSPECT as Gamma Camera + CT
 SPECT avoids the superposition of active and
non-active layers, which restricts the accurate
measurement of organ functions found in the
planar gamma camera.

8. Picture of SPECT system
It looks like a CT or MRI system
but hardware is different.

9. DESIGN AND INSTRUMENTATION
Same design as Gamma Camera.
Same radionuclides as Gamma Camera.

10. HOW SPECT WORKS?
 A radiopharmaceutical is injected into the patient’s body.
 It travels into the blood stream, and concentrates in the Region
of Interest.
 There, it decays, emitting gamma rays.
 The gamma rays travel out of the patient’s body, and are
detected by the gamma camera head of the SPECT machine.
 The gamma ray is collimated by the collimators to minimize
scatter, and improve image quality.

11. HOW SPECT WORKS?
 The collimated gamma rays hit the crystal detector, usually
Sodium Iodide crystals doped with Thallium [NaI (Tl)], which
converts the energy of the gamma rays to visible light.
 As visible light travel through the Photo Multiplier Tubes
(PMT), they absorb the light and emit electrons.
 The electrons emitted are used for image formation.
 They are detected by a Positioning and Summing Circuit,
which decode the body position of the original photon.

12. Fig: How PMT Works

13. HOW SPECT WORKS?
 A Pulse Height Analyzer (PHA) decodes the energy of the
emitted photon.
 The information is passed on to a digital circuit on a computer,
where algorithms are used to reconstruct the image.
 The resultant image gives a physiological state of the organ.
 Hot spots (areas of increased uptake) and cold/dark spots or
photopenia (areas of decreased intake) may indicate
pathology, such as arthritis, infections, fractures, tumours.

14. HOT SPOTS AND COLD SPOTSHOT SPOTS AND COLD SPOTS

15. RADIONUCLIDES COMMONLY USED IN
GAMMA CAMERA AND SPECT
RADIONUCLIDE
Technetium-99m
Thallium-201
Gallium-67
Iodine-131
Iodine-123
Xenon-127
Xenon-133
HALF-LIFE
6h
73h
3.3d
8.04d
13.2h
36.4
5.25d
ENERGIES OF
PRIMARY
PHOTONS(keV)
140
72
88,185,300
365
159
172, 203, 375
81, 161

16. PROPERTIES OF A RADIONUCLIDE
 A physical half life of few hours
 Decay to a stable daughter
 Emit gamma rays but no alpha or beta or very low energy photons
 Emit gamma rays of energy 50-300KeV
 Ideally emit monoenergetic gamma rays so that scatter can be
eliminated by PHA
 Be easily and firmly attached to the pharmaceutical at room temp
 Have a very high Specific activity
 Have a very Low toxicity

17. Imaging done as close as possible to the body, to minimize scatter.

18. SPECT Image Reconstruction
 Gamma camera head(s) rotates around patient taking images.
 Images may be taken in 2 modes:
 Continuous Acquisition: while camera heads are in
motion
 Step and Shoot: camera heads stop at defined angles to
acquire image.
 Rotation may be full 360o
around the body, or 180o
with
projection algorithms used to construct for the other half.
 180o
is most common generally.
360o
is common for brain SPECT.

19. SPECT Image Reconstruction (contd.)
 After acquiring the 2D projection images, the first step is to reformat
the data acquired in terms of axial position.
 Each reformatted projection data is represented as a 3D function:
p’(x’, z, θ) ,
where x is same as x location in raw projection image
z is the axial location
θ is the projection angle
 The image p’(x’, θ) is called a sinogram. It forms a sine wave.
 The sinogram is useful in detecting artefacts, as there will be a
discontinuity in the curve, if for instance, there is patient movement.

20. SINOGRAMSINOGRAM

21. SPECT Image Reconstruction (contd.)
 After series of planar images (projections) are created, there might
be overlap of anatomy.
 In conventional tomography, structures out of a focal plane are not
removed from the resultant image; instead, they are blurred by an
amount proportional to their distance from the focal plane
(i.e. 1/r blurring effect).
 However, computed tomography, such as SPECT, use computer
algorithms to remove the overlying structures completely, and solves
blurring effect.

28. DIAGRAM SHOWING SUMMING UP OF 2-D IMAGES

29. SPECT Image Reconstruction (contd.)
 Images are usually in transverse/axial plane, but pixels can be
reordered to produce coronal and sagittal images.
 Fourier transform and mathematical filtering of data are done
to remove inherent noise in data.
 End result: noise-free final image.

30. DESIGN AND OPERATION OF SPECT
The components of the SPECT/gamma camera are:
o Collimators
o Scintillation crystal / Detector
o Photomultiplier tube (PMT)
o Position Logic circuits
o Pulse Height Analyzer (PHA)
o Data Analysis Computer

31.  COLLIMATORS - immediately in
front of the detectors - essential to
provide positional information, and
minimize scatter / false events.
 They select ray orientation.
 Types
 Parallel,
 Pinhole,
 Convergent, and
 Divergent

32. COLLIMATORSCOLLIMATORS
 High Sensitivity Parallel Hole collimators are the most
common.
 Converging collimators are rarely used. Though its imaging
characteristics are superior in theory to the parallel-hole
collimator, but its decreasing FOV with distance and varying
magnification with distance discourages its use.
 However, a hybrid of the parallel-hole and converging
collimator, called a Fan-Beam Collimator can be used to take
advantage of the superior spatial resolution-efficiency of the
converging collimator.

33. FAN-BEAM COLLIMATORFAN-BEAM COLLIMATOR
 The fan-beam collimator is a parallel hole collimator in the
y-axis, and a converging collimator in the x-axis.
Fig: Fan-Beam Collimator

34. SCINTILLATION CRYSTAL / DETECTOR
Usually NaI (Tl) crystal
Converts gamma rays to light photons
PHOTOMULTIPLIER TUBE
Converts energy from visible light photons to electrons
Magnitude of signal proportional to photon energy
COMPUTER
Data analysis (uniformity correction and linearity)
Processes data- readable image.
MONITOR
For display
OTHER COMPONENTSOTHER COMPONENTS

35. FACTORS THAT AFFECT SPECT IMAGES
The quality and accuracy of SPECT images are affected
by two factors:
1. Physical factors - due to interaction of emitted
photons with matter inside the patient.
2. Instrumentation factors- related to the SPECT
imaging system.

36. Physical Factors
1. Attenuation- Reduction of the number of primary photons
passing through a given thickness of material via
photoelectric absorption and Compton scatter.
- Photons originating from different depths in patient -
experience different levels of attenuation.
- Resolution of SPECT detectors degrades rapidly with
distance.
- Spatial resolution of SPECT systems is 10 -14 mm.
2. Scatter -Photons that have been scattered before reaching
the detector provide misplaced spatial information about
the origin of the radioactive source.
The resulting image becomes blurred having low resolution.

37. Attenuation (including scatter) results in:
 High image noise
 Poor resolution
 Low contrast, and
 Reconstruction artifacts and distortions.

38. Instrumentation Factors-
COLLIMATION -DETECTOR SYSTEM
-Most important component that determines both:
- Spatial Resolution (blurring) and
- Sensitivity (detection efficiency).
- Higher resolution collimator imply lower sensitivity, and vice
versa.
-The main challenge in SPECT is finding a balance between
resolution and sensitivity.

39. Spatial ResolutionSpatial Resolution
 Spatial resolution is a measure of a camera's ability to accurately
portray spatial variations in activity concentration and to
distinguish as separate radioactive objects in close proximity.
 Expressed in Full-Width at Half Maximum (FWHM) of a line
source.
 The overall System Spatial Resolution (RS) is a function of
collimator resolution (RC) and the intrinsic resolution (Ri), and
given by:
 Intrinsic measurements are those of crystal detector alone – with
the collimator removed.
 Resolution decreases with increasing distance of source from
collimator (SDD).

40. SensitivitySensitivity
 Sensitivity is the fraction of gamma rays emitted from the source
that produces count in the image.
 It is defined by the detection efficiency of the SPECT system (ES)
 The overall System Efficiency (ES) is a function of collimator
efficiency (EC), intrinsic resolution (Ri), and fraction (f) of
photons accepted by the energy discrimination circuit:
Es = Ec x Ei x f
Ep
where Ep is the photopeak efficiency , the fraction that produces count in the relevant photopeak.
 High sensitivity collimators are thinner, have larger and fewer

41. SPECT IMAGE RECONSTRUCTIONSPECT IMAGE RECONSTRUCTION
 The goal of image reconstruction algorithms is to
calculate accurately the 3D radioactive distribution
from the acquired projections
 The reconstruction of tomographic images is made
by two methods:
• Iterative method
• Filtered Back Projection

42. ITERATIVE METHODITERATIVE METHOD
 Iterative reconstruction starts with an initial estimate of the
image.
 Then a set of projection data is estimated from the initial
estimate using a mathematical process called forward
projection.
 The resulting projections are compared with the recorded
projections and the differences between the two are used to
update the estimated image.

43. Iterative Method (contd.)
 The iterative process is repeated until the differences between
the calculated and measured data are smaller than a specified
preselected value.
 The iterative reconstruction methods include:
algebraic methods like:
 Algebraic Reconstruction Technique (ART), and
statistical algorithms like:
 Maximum Likelihood Expectation Maximization (MLEM),
or
 Ordered-Subsets Expectation Maximization (OSEM)

44.  Iterative image reconstruction methods allow the
incorporation of more accurate imaging models rather
than the Radon model assumed in the FBP algorithm
 These include scatter and attenuation corrections, as
well as collimator and distance response, and more
realistic statistical noise models.
Iterative Method (contd.)

45. Iterative Method (contd.)

46. FILTERED BACK PROJECTION (FBP)
 It is the most widely used in clinical SPECT because of
its simplicity, speed, and computational efficiency.
 It consists of two steps:
 filtering of data, and
 back projection of the filtered data

47. FILTERED BACK PROJECTION
 Back projection technique redistributes the number of counts at
each particular point back along a line from which they were
originally detected.
 This process is repeated for all pixels and all angles. The limited
number of projection sets has as a result the creation of a star
artifact and the blurring of the image.
 To eliminate this problem, the projections are filtered before
being back projected onto the image matrix.

48. Filtered Back Projection solves the 1/r blurring effect

49. IMAGE SPECT FILTRATION
 The filters used in FBP are simply mathematical equations that
vary with frequency.
 They attempt to achieve different purposes, such as
 Star artifact reduction,
 Noise suppression,
 Signal enhancement and restoration.

50.  FBP is faster than Iterative method.
 Iterative reconstruction algorithms however produce accurate
images of radioactive distribution and seem to be more sensitive
than FBP technique.
 Further development in iterative reconstruction methods will be
very promising in improving image quality.
 FBP and Iterative-OSEM are generally both available on all
SPECT processing software developed by gamma camera
manufacturers and the nuclear medicine processing software
companies.
FBP vs Iterative Method

51. FILTERS
RAMP FILTER: The ramp filter is a high pass filter that does not
permit low frequencies that cause blurring to appear in the image.
The Ramp is a compensatory filter as it eliminates the star artifact
resulting from simple backprojection.
A severe disadvantage of high pass filtering is the amplification of
statistical noise present in the measured counts.
 In order to reduce this amplification, the ramp filter is usually
combined with a low-pass filter.

52. SMOOTHING FILTERS: The common method to reduce or
remove statistical noise in a SPECT is the application of smoothing
filters.
These filters are low-pass filters which allow the low frequencies
to be retained unaltered and block the high frequencies.
There are a number of low-pass filters that are available for SPECT
reconstruction. The most commonly used are Butterworth Filter,
Hanning Filter, Hamming Filter, Parzen Filter, and the Shepp-Logan
Filter (least smoothing, but highest resolution).
FILTERS (contd.)

53. ENHANCEMENT FILTERS: A low-pass filter may smooth
image to a high degree that does not permit discerning small
lesions, leading to contrast loss.
For this reason, a third class of filters, called enhancement or
restoration filters, is used in SPECT imaging
These filters enhance the signal with a simultaneous reduction of
noise without resolution loss.
Metz and Wiener are two types of resolution recovery filters that
have been used in nuclear medicine image processing.
FILTERS (contd.)

54. PARAMETERS DETERMINING THE
CHOICE OF SPECT FILTER TYPE
Filter choice depends on:
The energy of the isotope, the number of counts and the activity
administered.
The statistical noise and the background noise level.
The type of the organ being imaged.
The kind of information we want to obtain from the images.
The collimator that is used.

55. SPECT APPLICATIONS

56. Fig: Whole body SPECT

57. BRAIN SPECTBRAIN SPECT
 Studies the brain’s blood flow and activity pattern.
 Basically shows 3 things:
 Areas of the brain that work well
 Areas that are overactive
 Areas that are underactive
 Brain SPECT imaging can show conditions such as
tumours/cancer, Dementia – Alzheimer’s Disease, Stroke,
impairments caused by substance abuse, addictions, etc.

58. BRAIN SPECTBRAIN SPECT (contd.)(contd.)
 Brain SPECT uses the ability of the radionuclides to cross
the Blood Brain Barrier (BBB) and localize in malignant
tissues.
 There are of 2 types:
 Hydrophilic Compounds: they cross the abnormal BBB and
localize at pathological sites and not in normal brain tissue.
 Lipophilic Compounds: they cross the normal BBB and localize in
the normal brain cells.
 For brain SPECT Acquisition, a complete 360o
camera
rotation with 64 x 64 x 8 matrix is recommended.

59. RADIONUCLIDE CISTERNOGRAPHYRADIONUCLIDE CISTERNOGRAPHY
 This is the study of CSF flow (CSF: Cerebrospinal Fluid).
 It aids to detect pathophysiological changes in the pathways
of Cerebrospinal Fluid (CSF).
 The radiopharmaceutical is injected into the subarachnoid
space in the brain or spine (at level between L2-L3
vertebrae).
 Radiopharmaceuticals commonly used include
99m
Tc pertechnetate, 99m
Tc DTPA or glucoheptonate (GHP),
99m
Tc HMPAO. Newer generations SPECT use 123
I-IMP, 123
I-
HIPDM and 201
Tl DDC.
BRAIN SPECTBRAIN SPECT (contd.)

60. BRAIN SPECTBRAIN SPECT (images)

61. MYOCARDIAL PERFUSION SPECTMYOCARDIAL PERFUSION SPECT
 It evaluates the heart’s blood
supply.
 It is also called the cardiac
stress-rest test or simply MPI.
 Two sets of images showing blood
flow (perfusion) are obtained:
1. During period of rest
2. During stress period
(after exercise).
A treadmill may be provided for the second set of images, for the
exercise.

62. MYOCARDIAL PERFUSION SPECTMYOCARDIAL PERFUSION SPECT
 The patient jogs for a few minutes.
 The intensity of the exercise increases about every three
minutes to induce maximum stress.
 ECG may be attached to the chest of the patient to monitor
heart rate.
 The radiopharmaceutical is injected a second time
intravenously shortly before the exercise stops.

63. MYOCARDIAL PERFUSION SPECTMYOCARDIAL PERFUSION SPECT
 The myocardial perfusion SPECT can check for obstruction
of blood flow in heart vessels (Myocardial Ischemia) or
damage caused by heart attack (Myocardial Infarction).
 For most cardiac SPECT protocols, a 180o
camera rotation
with 64 x 64 matrix size is recommended.
 Common radionuclides are 201
Tl and 99m
Tc-Sestamibi.

64. ADVANTAGES OF SPECT
 Improved contrast and reduced structural noise, due to elimination
of overlapping structures (compared to planar imaging).
 Localization of defects is more precise and more clearly seen.
 Extent and size of defect is better defined.
 Images free of background.
 SPECT (and PET) provide the only non-invasive technique for
imaging brain neurochemicals.
 The longer SPECT half -life afford longer synthesis times and
greater flexibility in relation to administration of radiotracers.
 SPECT is available in both developing and developed countries
because of lower equipment cost and greater accessibility of
SPECT radionuclide

65. LIMITATIONS/DISADVANTAGES
 Radiation exposure
 Limited spatial and temporal resolution.
 Relatively expensive to build and maintain (compared to CT,
MRI).
 Not very effective for patient who just finished exercising
(except in MPI SPECT).

66. SPECT vs PET -
DISADVANTAGES
 SPECT imaging has lower resolution and sensitivity.
The use of collimator results in tremendous decrease in detection
efficiency as compared with PET.
 Spatial resolution in SPECT deteriorates from edge toward
center; PET is relatively constant across trans-axial image, best
at center.
 Attenuation severe in SPECT; but accurate attenuation
correction possible in PET (with external transmission source).

67. SPECT is less expensive than PET to build and maintain.
SPECT is non-invasive, while many PET scan require arterial
sampling which is invasive.
SPECT vs PET –
ADVANTAGES

68. SPECT vs PET
CONSIDERATION SPECT vs PET
Cost PET > SPECT
Availability SPECT > PET
Spatial Resolution PET > SPECT
Temporal Resolution PET > SPECT
Sensitivity PET > SPECT
Signal to Noise Ratio PET > SPECT
Variety of ligands PET > SPECT

69. QA/QC OF SPECT
 QC procedures are broken down into two levels: routine QC
procedures and acceptance testing as defined by NEMA or AAPM
standards

70. ARTEFACTSARTEFACTS
Most common artefacts are:
Star Artefacts: Caused by backprojection, and solved by Filter
Back Projection.
Motion Artefacts: Caused by movement of the patient.
Detected on a sinogram.
Edge Packing: Increased brightness at edge of crystal.
Artefacts may also be caused by damaged collimators, metal
objects worn by patient, PMT failure, cracked crystals.

71. ADVANCED MODALITIESADVANCED MODALITIES
SPECT/CT AND SPECT/MRI:
SPECT images can be co-registered with CT and
MRI images for better anatomical and
physiological imaging.

72. GATED SPECT (GSPECT)
Electrocardiographically (ECG) Gated Myocardial Perfusion
SPECT (GSPECT) is the preferred state-of-the-art technique for
the combined evaluation of myocardial perfusion and left
ventricular function (LV) within a single study.
The ECG guides the image acquisition, so that it shows the set
of SPECT images as the heart contracts and expands from
Diastole to Systole.
99m
Tc–sestamibi and 99m
Tc-tetrofosmin are preferred agents.
It can be gated either at rest or after exercise-stress.
The computer calculates for the physician the patient’s
ejection fraction (LVEF), end diastolic volume, myocardial
thickening, etc. Ideal for patients with sinus rhythm.
ADVANCED MODALITIES

73. CONCLUSIONCONCLUSION
 SPECT has become an important diagnostic tool for showing
characteristic functional information of structures and the tissues.
 One of the most important factors that greatly affect the quality of clinical
SPECT images is image filtering.
 The selection of the optimal filter and the determination of filter
parameters for any individual case remains one of the main problems of
filtering in SPECT image processing.
 Nowadays, FBP reconstruction is progressively replaced with the OSEM-
iterative reconstruction algorithm.

74. SPECT/CT for suspected bone infection on GS
A 56-y-old woman presented with fever, low back pain, and infected scar 1 month after spinal surgery and was
referred for GS for suspected vertebral osteomyelitis. (A) Planar posterior whole-body GS image (left) shows
prominent abnormal uptake in left lower back, corresponding in part to regions of increased irregular uptake
seen on planar posterior whole-body 99mTc-MDP image (right) along operated vertebrae. (B) Transaxial GS
SPECT/CT image (left) localizes abnormal uptake on GS (center) to paravertebral soft-tissue abscess seen on
corresponding CT image (right), thus defining soft-tissue infection without osteomyelitis. There was no
evidence of vertebral osteomyelitis on follow-up CT 4 wk later.

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76. THANK YOUTHANK YOU
To contact the author(s), email ekpovictortoday@gmail.com