Computed Tomography / Cross Sectional

indexing

how we move pt through the gantry

host computer

in CT it has limited storage capacity, so image data can be stored only temporarily

equilibrium phase

not arterial or venous (for contrast)

osmolality

number of particles dissolved in a solution - more concentrated **LOW OSMOLALITY IS PREFERRED 300mOs/dl of human blood

partial volume effect

obscures any object that is smaller than the voxel width (section thickness) when that object is scanned by two sections

ring artifacts (annular or ring artifact)

occurs when a single detector goes out of calibration and does not properly record incoming attenuation data, the projection (or view) includes a detector error

**most scanners used today are:

3rd and 4th generation

negative contrast media

**appears black/gray on x-ray - air - carbon dioxide - methylcellulose - nitrogen

positive contrast media

**appears white on x-ray - barium (non- sterile) - iodine (sterile) - (ionic vs non-ionic) - gastrografin (non-sterile)

non-ionic contrast

- STABLE - chemical goes in and comes out the same way **PREFERRED - costly - safer; less reactions ** LESS CONCENTRATED / LOW OSMOLALITY (600mOs/dl)

contrast

- administered intravenously, orally, or rectally - generally same contrast as excretory urograms - non-ionic because of low incidence of reaction and known safety facturs - useful for showing tumors within the head

gantry

- circular device that houses x-ray tube, DAS, and detector array **helical CT units houses the slip-ring and high-voltage generator in gantry - gantry can be tilted forward or backward up to 30-degrees

Circle of Willis

- comprised of 5 arteries (paired anterior cerebral arteries, a single anterior communicating artery, paired internal carotid arteries, paired posterior cerebral arteries and paired posterior communicating arteries) - has its own back-up system - collateralization: body develops new vessels that can bypass an obstruction (not as efficient, but can restore some of the lost blood supply to the organ) -

spatial resolution

- detail - looks at the degree of blur or the ability to see the difference between two objects that are close together

6th generation

- dual energy source (2 x-ray tubes) - 2 sets of detectors that are offset by 90-degrees - improved imaging structures of heart (or other moving structures) **decreased radiation dose)

Hounsfield units or CT numbers

- each pixel within the matrix is assigned a number that is related to the linear attenuation coefficient of the tissue within each voxel ** water= 0 ** air= -1000 ** bone= +150 to +1000

detectors

- function as image receptors - measures the amount of radiation transmitted through the body and converts the measurement into an electrical signal proportional to the radiation intensity - 2 types: scintillation (solid state) and ionization (xenon gas) - smaller detectors = better image quality (due to more detectors)

5th generation

- high-speed CT due to millisecond acquisition - EBCT (electron beam scanners): x-rays are produced from an electron beam in a fan beam configuration that strikes stationary Tungsten rings (+/- 210 degree arc) - primarily used for cardiac studies

gastrografin

- hypertonic in body - water soluble iodinated contrast - contrast is taken orally **adding iodine into gastrografin speeds up bowel movement

CT tables

- made of wood or low-density carbon composite - automated device linked to the computer and gantry - designed to move in increments (INDEX) according to the scan program

CT diagnostics

- most common: head, chest, & abdomen - best for head trauma- shows skull fractures and subdural hematomas - excels at showing soft tissue structures within chest, abdomen, and pelvis -head evauls for stroke or CVA (rule out hemorrhage) - imaging of CNS shows infarctions, hemorrhage, disk herniations, craniofacial and spinal fractures, and tumors/cancers - CT used for interventional procedures: abscess drainage, tissue biopsy, cyst aspiration, radiofrequency ablations and cryoablations of tumors

exposure factors

- performed at a set kVp - time is not a factor, must be controlled by the scanning program to provide sufficient exposure to the detectors - radiographer can vary the mA

computed tomography

- process of creating a cross-sectional tomographic plane of any body part of the body ** preselected algorithm, it assembles the date in a matrix to form an axial image (each slice is displayed in a cross-sectional format --- during CT exam, a tightly collimated x-ray beam is directed through the patient from many different angles, resulting in an image that represents a cross section of the area scanned

pressure injectors

- rapidly injects large amounts of contrast into body (16-18G needle) - about 100cc in 10 seconds **used in angio, cardiac, and abodmen - must get a big vein to do it - make all of the air in the needle is pushed out

CSF

- regulates brain temp - cushioning - fights off infection

4th generation

- rotate only movement (tube rotates around pt, detectors are fixed) - stationary detectors required greater # of detectors - high pt dose (more than previous scanners)

3rd generation

- rotate/rotate (tube and detector both move simultaneously) - number of detectors increased (>750) - curved array - scan time: 0.35-10 seconds - reduced reconstruction time ****Helical CT: slip-ring technology, 360-degree rotation ****Multislice: mult. slices taken in one rotation ****Volume: mult. slice scanner with 64 detector rows or more ****Flat-panel: tube and detector travel a full 360-degrees. functions like fluoro. (increase) spatial resolution, (decrease) contrast resolution

streak artifacts/ metal or star

- starbursts - metallic objects (dental fillings, pacemakerss, and artificial joints - metal objects attenuate nearly 100% of the primary beam (incomplete projection)

1st generation scanners

- translate/ rotate - finely collimated pencil beam - 1-3 detectors - scan time: 3-5 minutes per scan (pt holds still for long periods of time) - limited to neurologic exams

2nd generation scanners

- translate/rotate - tube emitted a fan-shaped beam - approx. 30 detectors - gantry rotated 10-degrees between each translation - improved image quality - decreased scan time to 20-sec. for a single slice

ionic contrast

- unstable - breaks molecules into charged particles - can recombine into something else in the body and create toxins or harm the body - dangerous - OK to use when its not systemic ** HIGH OSMOLALITY (1200mOs/dl)

history of CT

- used clinically only for scanning of the brain - originally called CAT (computed axial tomography) - first performed in 1970 in England by Hounsfield

methylcellulose

- used in double contrast studies - used in ENTEROCLYSIS

MRI (magnetic resonance imaging)

- used magnets and FM radio-frequency (non-ionizing radiation)

gadolinium

- very toxic to liver - only used when its not systemic (like stomach) - used in MRI studies (given in small doses)

hypertinicity

- water filling - causes body to fill rapidly to maintain balance - pulls fluid from other organs (not a good thing)

ROI (region of interest)

- will give Hounsfield number - after # is given, radiologist can determine what it is

basillar artery

.

falx cerebri

.

images are viewed from the bottom-up

...

cradle

helps hold the head still - used for pt's for direct coronal images

temporal resolution

ability of the CT system to freeze any motion of a scanned object (heart studies)

contrast resolution

adjacent gray tones - difference in densities

corpus callosum

allows communication between hemispheres

lateral ventricles

divided in 3 horns

FOV

field of view - how much data (body part) is displayed on the monitor

helical interpolation

helical scanning occurs in a pattern that is at an angle to the perpendicular plane that is desired for imaging. - "straighten up" the sectional image (after reconstruction from computer software) - slinky effect

translation

linear tube movement

choroid plexus

makes CSF fluid

gyri

mountains of the brain

motion

produces streak artifacts - produces blank pixels, which appear as streaks

misregistration

pt unable to hold breath at the same time because missed slices can occur while scanning tissue - 4th generation scanners allows to scan whole thing in about 10 seconds

viscosity

resistance to flow

beam hardening

result of the attenuation of the beam as it passes through the pt - broad dark bands or streaks known as cupping artifcats

meniscus

sharp line between air and fluid levels on an x-ray image

voxel

slice thickness

cistern

something that holds fluid

pitch

term that is used to define this extension or contraction of the helix (overscan/overlap) ***SLINKY effect*** - simply the ratio of the distance, the table moves during one 360-degree tube rotation to the total beam collimation

ventrical

tissue that secrets CSF

nitrogen

used in vascular system

sulci

valleys of the brain

AZICUS

vein that drains the posterior chest wall