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