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Cross Sectional Imaging Nuclear 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

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Cross sectional imaging nuclear medicine l.jpg

Cross Sectional ImagingNuclear Medicine

Dr. LeeAnn Pack

Dipl. ACVR

Cross sectional imaging l.jpg
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

Ct generations l.jpg





min scan time




pencil thin

2.5 min




narrow fan

10 sec




wide fan

0.5 sec




wide fan

1 sec


electron beam

1284 detectors

wide fan electron beam

33 ms

CT Generations

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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  • 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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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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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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  • 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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  • 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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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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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

  • Thyroid scintigraphy l.jpg
    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

    Portosystemic shunts l.jpg
    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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    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 l.jpg
    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 l.jpg
    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