Download Cross Sectional Imaging Nuclear Medicine

Cross Sectional Imaging
Nuclear Medicine
Dr. LeeAnn Pack
Dipl. ACVR
www.upei.ca/~vetrad
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 Generations
Generation
configuration
detectors
beam
min scan time
First
translate-rotate
1~2
pencil thin
2.5 min
Second
translate-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 beam
1284 detectors
wide fan electron beam
33 ms
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Helical and Multislice CT’s are used now
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
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Bone
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
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Hydrocephalus
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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Time Activity Curve – Portosystemic Shunt
1000
800
600
Heart
400
Liver
200
Heart
0
0
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20
40
60
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
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