Lecture 7 Cross sectional imaging nuclear med.ppt
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Transcript of Lecture 7 Cross sectional imaging nuclear med.ppt
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Cross Sectional ImagingNuclear Medicine
Dr. LeeAnn Pack
Dipl. ACVR
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Computed Tomography (CT)
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Cross Sectional Imaging
• No superimposition of structures• Excellent contrast resolution – can see
the difference between 2 similar tissues• For CT – scan can be performed in one
plane (usually transverse) and reformatted in the others (sag, dorsal)
• CT – good for bone and soft tissue• MRI – better for soft tissue
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Computed Tomography
• Uses X-rays, X-ray tube, detectors, collimators – very similar to radiography in how it works.
• Patient placed in gantry
• Multiple samples are taken from around the patient and then reconstruction can occur to make a slice
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CT GenerationsGeneratio
nconfiguration detectors beam min scan time
Firsttranslate-rotate
1~2 pencil thin 2.5 min
Secondtranslate-rotate
3~52 narrow fan 10 sec
Third Rotate-rotate 256~1000 wide fan 0.5 sec
Fourth Rotate-fixed 600~4800 wide fan 1 sec
Fifth electron beam1284 detectors
wide fan electron beam
33 ms
Helical and Multislice CT’s are used now
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How It Works
• Scout image is made first to pick the area to scan
• Parameters set on the computer
• Scan begins
• Linear attenuation coefficient of tissues
• Houndsfield units calculated
• Shade of grey assigned to a CT number
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CT Principles
• The image is divided into small areas called pixels– Each pixel has a location– Each pixel has an attenuation value
• Using this information and very complex math formulas, the computer constructs the image
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CT numbers
• High CT number = white because of increased attenuation
• Low CT number = black because of decreased attenuation
• Houndsfield scale– Water is zero, air is –1,000 and bone is
1,000
• 256 shades of grey
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Windowing
• Level– Center portion of the Houndsfield scale
that is being used• Should be near the tissue of interest
• Width– How much of the Houndsfield scale is used
• Values within the window will be various shades of grey - rest black or white
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Level and Width
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Windowing - Use• Narrow window – enhance contrast of the
tissues– Brain
• Wide window – area with high inherent contrast– Lungs
• Soft tissue window• Bone window• Reformatting – can not be better than original
slice – decreased spatial resolution
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CT Terminology
• Density– Hypodense– Isodense– Hyperdense
• IV Contrast can also be administered – then contrast enhancing, ring enhancement etc can be used
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Soft Tissue Bone
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Choroid Plexus Tumor
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Fibrosarcoma Cat Back
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Multilobular Osteochondrosarcoma
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CT images
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Bone Lysis Nasal Tumor
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Nasal Adenocarcinoma
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Retrobulbar Mass
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Pituitary Tumor
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Magnetic Resonance Imaging (MRI)
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Magnetic Resonance Imaging
• Does not involve ionizing radiation
• Uses magnetic field and radiofrequency pulses
• Hydrogen proton on tissues (water)
• Water = like tiny magnets
• When placed into magnetic field H protons line up along field
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MRI
• Radiofrequency pulse passed through patient
• Protons flip and spin
• Pulse turned off and H protons return to normal state = relaxation
• T1
• T2
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Meningioma Hydrocephalus
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MRI
• Tissues that have little H protons have little signal and are black– Air, bone, moving blood
• Good for soft tissue imaging though• Paramagnetic contrast agent – Gad• No reformat – must scan all planes
– Thus much longer scan than CT
• Transverse, sagittal, dorsal
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T1 vs. T2
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MRI Machines
• Can vary from .3 Tesla to 3 Tesla for routine working machines
• Many are superconducting – use helium
• Magnet is always on and must be contained in a Faraday cage (blocks stray radiofrequency signals)
• Open and closed magnets
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MRI Terminology
• Intensity– Hyperintense– Isointense– Hypointense
• Contrast enhancing with Gadolinium
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MRI Safety• Augment T waves on EKG• Light flashes – Mild skin tingling• Involuntary muscle twitching• Increased body temperature• Projectile effects• Effects on surgical implants – ferrous• Magnetic foreign bodies • Life support devices
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MRI Contraindications
• Pacemaker
• Intra-cranial implants, clips
• Metallic foreign bodies
• Implanted electrical pumps, mechanical devices
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Nuclear Scintigraphy (Nuc Med)
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The Basics
• Radionuclides (radioisotopes) are used– Injected, oral, per rectal etc. administration– They undergo decay over time– Linked to a radiopharmaceutical
• Determines the area of distribution
• Gamma rays come from the patient– Radioactive – ionizing radiation is involved
• Gamma camera detects the radiation• Good for physiologic function stuff• Does not provide a good anatomical info
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The Ideal Radionuclide
• Technetium 99m
• Short half life = 6 hours
• Binds to radiopharmaceuticals
• Cheap to purchase
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The Gamma Camera
• The gamma rays produce scintillations
• They are converted to electrical signals and multiplied by photomultiplier tubes
• The computer records the strength and location of the scintillation events
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Types of Scanning
• Static– Images are acquired os structures at a single point
in time
• Dynamic– Images are acquired of a structure over a period of
time• Provides functional activity• Time activity curves
– Activity in a region is followed over time and a graph made
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Bone Scans
• One of the most common scans we do– Equine
• 3 phases:
• Vascular phase
• Soft tissue phase
• Bone phase
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Items to Consider
• Age of the animal– Young animals – physis– Older animal – longer time to distribution of
radiopharmaceutical
• Must scan both limbs etc even if only one is suspected of being abnormal
• Symmetry is your friend
• Animals are radioactive for a time after the scan
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Normal Equine Bone Scan
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Bone Scans
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Equine Head
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Thyroid Scintigraphy
• Technetium99m Pertechnetate• Uptake in thyroid glands is compared to
uptake in salivary glands – should be equal
• Hyperthyroid – Benign adenoma– Thyroid glands exceed salivary glands
• Functional thyroid tumors– Patchy irregular inconsistent pattern
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Thyroid Scintigraphy Scans
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Portosystemic Shunts
• Technetium 99m is placed in the rectum and dynamic images every 4 seconds are acquired over 2-3 minutes
• Non invasive, quick, accurate, quantitative
• Liver then heart = normal• Heart then liver = abnormal (shunt)• Time Activity Curves - important
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0
200
400
600
800
1000
0 20 40 60
Heart
Heart
Liver
Time Activity Curve – Portosystemic Shunt
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Shunt vs. No Shunt
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Other Scan Types
• Renal Scans– To determine GFR and ERPF
• Cardiac Scans• Hepatobiliary Scans
– Hepatocyte function, function of the reticuloendothelial system, biliary function
• Gastrointestinal scans• Lung Scans• Infection and tumor imaging
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Nuc Med Safety
• Higher energy radiation– Especially before injection
• Urine from horses• Bedding• Isolation• Lead for workers – not work
– Wear plastic gloves to keep off hands
• Wear monitoring badges, rings
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Release Protocol
• Isolation of the animals is necessary
• Limited contact with the animal– Very sick animals may not be best to inject
• Bedding must be monitored
• Animal must be released after scanning with Geiger counter