fusion imaging

F S O

U I NI A I GM G N

Dr. Vibhuti Kaul

Fusion: “The union of different things by, or as if by, melting……”

Imaging: “Making a representation or imitation of an object.”

Fusion Imaging: Melting together images from different modalities to create a new (hybrid) image.

TRANSMISSIONIMAGING

EMISSION IMAGING

HYBRID

• “It is the exact superimposition of physiological

information from a nuclear medicine study with the

anatomic information from CT, MRI using fiduciary

markers.”

NUCLEAR MEDICINE• SCINTIGRAPHY

• SINGLE PHOTON EMISSION

TOMOGRAPHY

• POSITRON EMMISION TOMOGRAPHY

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.

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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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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

• 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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3 types of coincidence detection

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Advantages

i. The FDG-PET scan has shown better sensitivity andspecificity 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.

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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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.

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 .

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.

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 preciselylocalize lesions.

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.

FUSION PET / CT

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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

• 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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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.

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.

• 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.

• 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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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”.

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Computer

Correlated Image

CT

X-Ray CT RadionuclideEmission

SPECT

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

• 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).

• 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).

PET/ MRI SCAN

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• 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.

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

Advantages

i. As compared to CT, has higher soft tissue contrastresolution, 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.

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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ADVANTAGES OF FUSION IMAGING

i. More accurately identifying the margins of a tumor/metastasis

ii. There is better identifying small recurrent tumors obscuredby scar tissue at site of incipient radiation or postoperativenecrosis.

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.

ADVANTAGES OF FUSION IMAGING

vii. Providing structural and functional information in the sameimage.

viii. Improving reading efficiency.

ix. Improving confidence in diagnosis when one modalityalone is not definitive.

x. Quantification of the difference between scans.

xi. Radiation therapy planning

FDG-PET

MRI

Image fusion

CONCLUSION

• 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.

• 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.

• 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.

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.

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