Post on 02-Jun-2015
Cross Sectional ImagingNuclear Medicine
Dr. LeeAnn Pack
Dipl. ACVR
Computed Tomography (CT)
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
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
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
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
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
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
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
Level and Width
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
CT Terminology
• Density– Hypodense– Isodense– Hyperdense
• IV Contrast can also be administered – then contrast enhancing, ring enhancement etc can be used
Soft Tissue Bone
Choroid Plexus Tumor
Fibrosarcoma Cat Back
Multilobular Osteochondrosarcoma
CT images
Bone Lysis Nasal Tumor
Nasal Adenocarcinoma
Retrobulbar Mass
Pituitary Tumor
Magnetic Resonance Imaging (MRI)
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
MRI
• Radiofrequency pulse passed through patient
• Protons flip and spin
• Pulse turned off and H protons return to normal state = relaxation
• T1
• T2
Meningioma 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
T1 vs. T2
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
MRI Terminology
• Intensity– Hyperintense– Isointense– Hypointense
• Contrast enhancing with Gadolinium
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
MRI Contraindications
• Pacemaker
• Intra-cranial implants, clips
• Metallic foreign bodies
• Implanted electrical pumps, mechanical devices
Nuclear Scintigraphy (Nuc Med)
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
The Ideal Radionuclide
• Technetium 99m
• Short half life = 6 hours
• Binds to radiopharmaceuticals
• Cheap to purchase
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
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
Bone Scans
• One of the most common scans we do– Equine
• 3 phases:
• Vascular phase
• Soft tissue phase
• Bone phase
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
Normal Equine Bone Scan
Bone Scans
Equine Head
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
Thyroid Scintigraphy Scans
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
0
200
400
600
800
1000
0 20 40 60
Heart
Heart
Liver
Time Activity Curve – Portosystemic Shunt
Shunt vs. No Shunt
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
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
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