This document discusses fusion imaging, which combines images from different modalities to create a hybrid image. It describes fusion imaging techniques like PET-CT and SPECT-CT that merge functional imaging data with anatomical images. The primary advantage of fusion imaging is that it allows correlation of findings from two concurrent imaging modalities, providing both anatomical and functional/metabolic information in a single exam. Specifically, PET-CT fusion improves diagnostic accuracy and lesion localization by overcoming the limitations of each individual modality. In conclusion, combined PET-CT exams are more effective than PET alone for localizing lesions and differentiating normal variants from tumors.

1. F S OU I N
I A I GM G N
Dr. Vibhuti
Kaul

2. Fusion: “The union of different things
by, or as if by, melting……”
Imaging: “Making a representation or
imitation of an object.”

3. Fusion Imaging: Melting together images
from different modalities to create a new
(hybrid) image.
TRANSMISSION
IMAGING
EMISSION
IMAGING
H
Y
B
R
I
D

4. • “It is the exact superimposition of physiological
information from a nuclear medicine study with the
anatomic information from CT, MRI using fiduciary
markers.”

6. NUCLEAR MEDICINE
• SCINTIGRAPHY
• SINGLE PHOTON EMISSION
TOMOGRAPHY
• POSITRON EMMISION TOMOGRAPHY

7. PLANAR SCINTIGRAPHY
• – Use radiotracers that generate gamma decay, which
generates one
photon in random direction at a time
• – Capture photons in one direction only, similar to X-ray,
but uses emitted gamma rays from patient
• – Use an Anger scintillation camera

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Radionuclides Half-life Uses
Technetium-99m 6 hrs Skeleton and heart muscle imaging, brain, thyroid, lungs (perfusion and
ventilation), liver, spleen, kidney (structure and filtration rate), gall bladder,
bone marrow, salivary and lacrimal glands, heart blood pool, infection
Xenon-133 5 days Used for pulmonary (lung) ventilation studies.
Ytterbium-169 32 days Used for cerebrospinal fluid studies in the brain.
Carbon-11
Nitrogen-13
Oxygen-15
Fluorine-18
They are positron emitters used in PET for studying brain physiology and
pathology, cardiology, detection of cancers and the monitoring of progress in
their treatment.
Iodine-131 8 days Imaging of thyroid
Gallium-67 78 hrs Used for tumour imaging and localization of inflammatory lesions (infections).
Indium-111 2.8 days Used for brain studies, infection and colon transit studies
Rubidium-82 65 hrs PET agent in myocardial perfusion imaging
Thallium-201 73 hrs Used for diagnosis of coronary artery disease other heart conditions and for
location of low-grade lymphomas.

9. 9

10. SINGLE PHOTON EMISSION COMPUTED TOMOGRAPHY
(SPECT)
• SPECT was developed as an enhancement of planar
imaging.
• It detects the emitted gamma photons (one at a time) in
multiple directions.
• Uses one or more rotating cameras to obtain projection data
from multiple angles.
• SPECT displays traces of radioactivity in only the selected
plane: Axial, coronal and sagittal
• Computer manipulation of the detector radiation is also
possible.

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12. POSITRON EMISSION TOMOGRAPHY
(PET)
• Positron emission tomography (PET) is a nuclear medicine
imaging technique which produces a three-dimensional
image or picture of functional processes in the body.
• The system detects pairs of gamma rays emitted indirectly
by a positron-emitting radionuclide (tracer).
• The most widely used tracer, 18F-fluorodeoxyglucose
(18FDG), acts as a glucose analogue allowing imaging of
glucose utilization, a process that is known to be enhanced
in many malignant tumours

13. • The flexible chemistry afforded by a number of PET
radionuclides, including 18F-fluorine,11C-carbon, 13N-
nitrogen and 15O-oxygen, provides the potential to
investigate multiple aspects of tumour biology in addition to
glucose utilization, including cellular proliferation, hypoxia,
neo-angiogenesis and apoptosis.
• One of the disadvantages of 18FDG PET is that glucose
utilization is not entirely specific to malignant tissue, some
benign processes being associated with enhanced
glycolysis.

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16. 3 types of coincidence detection
16

17. Advantages
i. The FDG-PET scan has shown better sensitivity and
specificity than CT/MRI in staging, detecting recurrences
(particularly in whom anatomic imaging is inconclusive
due to locoregional distortions rendered by surgery and
radiotherapy).
ii. Detection of unknown and second primary
malignancies and in monitoring treatment.
iii. Provide functional detail.

18. Disadvantages
i. High negative predictive value.
ii. Lack of anatomic detail because anatomic
contrast and resolution is inherently little and
there are various normal physiological
uptakes which can be confused with
pathology.

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20. COMPUTED TOMOGRAPHY
PRINCIPLE OF COMPUTER TOMOGRAPHY
In conventional CT, the X-ray tube and detector
rotate around the patient with the table stationary.
The X-ray beam is attenuated by absorption and
scatter as it passes through the patient with the
detector measuring transmission
Multiple measurements are taken from different
directions as the tube and detector rotate.
A computer reconstructs the image for this single
“slice.” The patient and table are then moved to
the next slice position and the next image is
obtained.

23. IMAGE RECONSTRUCTION:
In CT a cross- sectional layer of the body is divided in to many tiny blocks.
Each block is assigned a number proportional to the degree the block attenuated
the x-ray beam.
The individual blocks are called “Voxels and Pixels”.
Their composition & thickness along with the quality of the beam determines the
attenuation coefficient .

24. IMAGE DISPLAY
A CT image is usually displaced on a television monitor
for immediate viewing and recorded on a film for
interpretation.
The display matrix was on average 512 x 512 & the pixel
size was a average size of 0.1mm.
The center CT number is called Window .
The range of CT number above and below are called
window width.

25. Advantages
i. CT is well tolerated.
ii. Readily available.
iii. Allows for fast examination of the
whole body.
iv. Provide the anatomical
landmarks needed to precisely
localize lesions.

26. Disadvantages
i. They require ionizing radiation.
ii. Exposure to metallic artifacts from dental
restorations.
iii. Intravenous contrast medium is contraindicated
in known or suspected hyperthyroidism (without
appropriate patient preparation) and renal
insufficiency.
iv. Less sensitive because they do not provide
functional detail.
v. Physiologic information is not readily generated.
vi. Less effective when normal anatomy is variable,
as in postsurgical patients.

27. FUSION PET / CT
27

28. FUSION PET / CT
• CT achieves excellent anatomical resolution, tissue
differentiation and high imaging speed but offers little
functional information, largely depending on size and
morphology to differentiate tumour from normal structures.
• In view of the different but complementary advantages and
short-comings of PET and CT it makes eminent theoretical
sense to combine both techniques within one scanning
gantry.
• Combining PET and CT has the potential to improve
lesion localization, increase specificity, reduce
interpretative pitfalls and to allow fast, low-noise
attenuation correction, significantly increasing throughput

30. • Patient preparation is the same as for standard PET
protocols with PET emission imaging usually starting
at approximately 1 h after injection of 18FDG.
• Images are routinely acquired from head to thighs
over a period of 20–30 min.

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32. Potential advantages of PET/CT:
• Throughput
• The use of CT for correcting attenuation effects on the
PET data has been estimated to increase patient
throughput by as much as 40%.
• There is therefore potential to reduce waiting lists for
PET imaging and to minimize patient discomfort and
reduce artefacts due to movement.

33. Localization
• The most obvious potential advantage of PET/CT compared
with PET is the ability to accurately anatomically locate an
18FDG-positive lesion with regard to relationships to
neighbouring structures and the presence of local invasion
• The complex anatomy of the head and neck region poses
difficulties in the interpretation of PET images without
anatomical registration.
• Image software co-registration with CT or magnetic
resonance imaging (MRI) has been shown to increase the
specificity of lesion localization in head and neck cancers.

34. • Sensitivity
• There is substantial evidence that FDG
PET improves sensitivity in the
detection of malignant tissue when
compared with conventional
anatomical imaging, frequently
upstaging patients and leading to
changes in subsequent management.

35. • Specificity
• Probably the most important reason for fewer
equivocal scans and an improvement in
diagnostic accuracy is an improvement in
specificity that results from combined PET/CT
compared with PET alone.

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37. Artefacts and pitfalls in PET/CT
• Differences in breathing patterns between CT and PET
images may lead to misregistration of pulmonary nodules,
particularly in the peripheral and basal lung regions where
the differences may approach 15 mm.
• Voluntary motion between CT and PET acquisitions is
minimized by ensuring patient comfort on the scanning
couch. This is particularly important in head and neck
studies where adequate immobilization is essential.
• High-density contrast agents, e.g. oral contrast mediums,
or metallic objects can lead to an overestimation of PET
activity if CT data are used for attenuation correction,
leading to development of artefactual “hot spots”.

38. 38
Computer
Correlated Image
CT
X-Ray CT Radionuclide
Emission
SPECT

39. Physiologic parotid uptake. Axial CT (a) and fused PET-
CT (b) scans show moderate symmetric FDG uptake
in the parotid glands

40. • Coregistered fused images obtained by 99mTc-MDP
SPECT/CT in the transverse, coronal, and sagittal planes,
showing focal hyperconcentration of the radiopharmaceutical in
the left TMJ (SPECT/CT and left TMJ).

41. • Coregistered fused images obtained by 99mTc-MDP
SPECT/CT in the transverse plane of a patient, showing
intense focal hyperconcentration of the radiopharmaceutical
in the left inner ear and mastoid (SPECT/CT and extra TMJ).

42. PET/ MRI SCAN
42

43. • Main goal of fusion is to provide information on
anatomical landmarks
• Anatomical information provided by MRI is superior to CT
due to greater inherent contrast resulting from differences
in proton density and magnetic relaxation properties of
tissue versus differences in tissue density.

44. ADVANTAGES:
MRI offers range of relevant, quantitative
information on tumor body related to blood flow,
vascular and tissue spaces, hypoxia, cellularity
and metabolic concentrations without exposing to
radiation

45. Advantages
i. As compared to CT, has higher soft tissue contrast
resolution, the lack of iodine-based contrast agents,
and high sensitivity for perineural and intracranial
disease.
ii. It is superior in depicting the replacement of
marrow fat and the extraosseous extension of
neoplasm and offers a higher sensitivity in the
evaluation of small tumors.
iii. Dense bone or dental fillings like, gold and
amalgam cause no artifacts, which permits the
depiction of small tumors in the oral cavity.

46. Disadvantages
i. Higher incidence of motion artifacts due to the longer
duration of the image acquisition compared to CT, and lower
patient tolerance.
ii. Contraindications include claustrophobia, pacemakers
and certain other implanted metallic devices.
iii. Less sensitive because they do not provide functional
detail.
iv. Physiologic information is not readily generated.
v. Less effective when normal anatomy is variable, as in post
surgical patients.
vi. In both, MRI and CT, detection of small occult lymph node
metastases is limited as only gross criteria, such as size,
number, contour and enhancement pattern exist.

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48. ADVANTAGES OF FUSION IMAGING
i. More accurately identifying the margins of a
tumor/metastasis
ii. There is better identifying small recurrent tumors obscured
by scar tissue at site of incipient radiation or postoperative
necrosis.
iii. Detecting large tumors that lay in clinically inaccessible
areas, such as the hypopharynx or maxilla.
iv. Locating the primary lesion in unknown primary
tumors.
v. Combined imaging is superior in staging of tumors.
vi. Guiding of treatment including planning, guidance of
biopsy, surgery or radiation therapy.

49. ADVANTAGES OF FUSION IMAGING
vii. Providing structural and functional information in the same
image.
viii. Improving reading efficiency.
ix. Improving confidence in diagnosis when one modality
alone is not definitive.
x. Quantification of the difference between scans.
xi. Radiation therapy planning

50. FDG-PET

51. MRI

52. Image fusion

62. CONCLUSION

63. • The primary advantage of PET-CT fusion technology is the
ability to correlate findings from two concurrent imaging
modalities in a comprehensive examination that combines
anatomic data with functional and metabolic information.
• CT demonstrates exquisite anatomic detail but does not
provide functional information, whereas FDG PET lacks
anatomic landmarks but reveals aspects of tumor function
and allows metabolic measurements.
• Physiologic FDG uptake in nonmalignant conditions limits
the specificity of PET, particularly in the post therapy
setting.

64. • Hybrid PET-CT scanners allow PET and CT image fusion
for differentiation of physiologic variants from juxtaposed or
mimetic neoplastic lesions and more accurate tumor
localization.
• Software based fusion of separately acquired PET and CT
scans is more likely to lead to misregistration due to
independent parameters and differences in patient
positioning.

65. • In summary, combined PET-CT scans are more effective
than PET scans alone for precise localization of neoplastic
lesions and differentiation of normal variants from
juxtaposed neoplastic lesions.
• Hence, PET-CT may significantly affect patient treatment by
improving diagnostic specificity more than sensitivity.

66. REFERENCES
• Oral Radiology- White and Pharoah
• Radiography and Radiology for Dental Care Professionals- Whaites &
Drage
• T.B. of Oral & Maxillofacial Radiology- Freny R. Karjodkar
• T.B. of Oral Radiology- Ghom
• Kostakoglu L, Hardoff R, Mirtcheva R, Goldsmith SJ. PET-CT fusion
imaging in differentiating physiologic from pathologic FDG uptake.
RadioGraphics 2004; 24:1411–1431.
• Ghom S, Ghom A, Debta FM, Deoghare A, Diwan R, Sikdar SD, et al.
Fusion Imaging: The double impact. JIAOMR. 2011;23(3):225-228.
• Zaidi H, Montandon ML, Alavi A. The clinical role of fusion imaging using
PET, CT and MR imaging. Magn Reson Imaging Clin N Am 18 (2010)
133–149.

67. THANK YOU