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