CT Flashcards
name 3 types of contrast media used
- oral
- intravenous
- air or CO2
what is the use of contrast media?
to assist in providing visual representations of information from within the body
aid in providing additional information
why is iodinated contrast media used?
iodine has high atomic number –> ability to absorb X-ray
what are the 4 basic principles of contrast media
- iodinated CM
- osmolarity and viscosity of CM
- ionicity of CM
- properties of an ideal CM
describe osmolality of CM
measurement assessing the number of particles dissolved in 1kg of water
osmolality of CM should be as close as possible to human plasma (300 mOsm/kg H2O)
what are the 3 classifications of CM osmolality
- high osmolar (1000- over 2400 mOsm/kg H2O; 5-8x)
- low-osmolar (900 mOsm/kg H20; 3x)
- iso-osmolar (290 mOsm/kg H2O)
explain osmolar toxicity
when too high osmolar CM is given, fluids in patient’s cell leave the intracellular compartment and enter the extracellular space –> causing cell shrinkage –> crenation of RBC –> impaired cell functioning
describe viscosity of CM
a measure of friction or resistance of liquid to flow
factors affecting viscosity of CM
size and shape of solute particle
temperature
describe ionicity of CM
property of a molecule to break into positively charged cations and negatively charged anions
presence of ionicity: ionic
absence of ionicity: non-ionic
ionic contrast can disturb the normal dynamic ionic equilibrium of the body
ionic toxicity can affect physiological processes such as heart rate, BP and neurotransmission
therefore, non-ionic CM are preferred
how many iodine atoms are in a benzene molecule
3 (tri-iodinated benzenes ring)
how to differentiate between ionic and non-ionic CM molecular structures
ionic molecular structures will contain ions at the R-group, showing positive and negative charges
describe the distribution of CM
- ICM does not cross cellular membrane easily, and is primarily distributed in the bloodstream
- pathological tissue contains membranes that are less obstructive, thus more permeable to ICM (allow CM to enter the cell)
describe neurotoxicity of ionic CM
- may cause seizures in susceptible patients
- high risk of neurotoxicity in patients with disrupted BBB
properties of an ideal CM
- water soluble
- biologically inert
- low viscosity
- non-ionic
- low or iso-osmolar
- increase sensitivity and enhance differentiation of different soft tissues
4 types of risk associated with IV contrast scan
- adverse reactions
- contrast induced nephropathy (CIN) or contrast induced acute kidney injury (CIAKI)
- metformin induced lactic acidosis
- extraversation
probability of acute adverse reaction event occurring with high osmolar CM
5-15%
probability of acute adverse reaction event occurring with low osmolar CM
0.2-0.7%
probability of severe adverse reactions occurring with low osmolar CM
0.04%
how long does it take for the reactions to take place
manifest <1hour
can start to manifest within 5 mins after contrast injection
majority of the life-threatening events <20mins
what are some of the mild reactions signs and symptoms
- vomiting
- warm feeling
- urtcaria
- nausea
- headache
- sweat
- cough
- itching
what are some of the moderate reaction signs and symptoms
- tachycardia
- bradycardia
- hypertension
- hypotension
- dyspnea
what are some severe reaction signs and symptoms
- convulsions
- cardiac arrest
- unresponsiveness
- profound hypotension
risk factors for adverse reactions
- previous history of adverse reaction to CM
- hx of allergic rxn that required medical intervention
- hx of asthma
- hx of significant cardiac diseases
- multiple drug allergy
what is the usual premedication for adverse reaction
prednisolone 30mg 12hours and 2hours before CT scan
what is given for premedication for adverse reaction in the event of emergency
- if scan can wait, prepare IV hydrocortisone 100/200mg every 4 hours until time of scan; with/without IV diphenhydramine 50mg 1hr prior to scan
risk factor for CIN
eGFR + :
- age >60
- hx of kidney disease as an adult, including tumour and transplant
- family hx of kidney failure
- diabetes treated with insulin or other prescribed med
- hypertension
- paraproteinemia syndromes or disease (eg myeloma)
- current use of nephrotoxic medications
metformin use for ICM
- eGFR >45 - continue metformin
- eGFR >30 - stop metformin 48hrs before CM injection and resume after 48hrs in the absence of deteriorating renal function
- eGFR<30 - metformin is contraindicated and should be avoided
how does body weight affect iodine dose
body weight increase = iodine concentration decrease
how does cardiac output affect the arterial peak enhancement
poorer CO = longer time for heart to pump = peak gets pushed back
how does contrast volume affect the iodine concentration
contrast volume increase = iodine concentration increase
contrast volume increase = longer time to inject CM = peak gets pushed back
how does injection flowrate affect iodine concentration
injection FR increase = iodine flux increase = iodine peak is higher
characteristic of saline chaser
- pushes contrast in tubing and peripheral veins into central veins
- allows reduction in contrast vol
- increases peak attenuation
- reduced streak artifacts from veins and right heart
- simpler to implement with dual head injectors
what are some patient preparation for IV contrast scan
- risk of physiologic and allergic-like adverse reactions
- premedication for adverse reactions
- risk of CIN
- fasting
- IV cannula
- metformin induced lactic acidosis
- explained risk of extravasation
what is included in a CT protocol
- region/scan range
- any use of contrast
- phases/sequences
- scan details
- recon details/phases
difference between epidural hematoma and subdural hematoma
epidural hematoma:
- occurs between the skull and dura mater
- convex shape
subdural hematoma:
- occurs between the dura mater and the arachnoid
- concave shape
how does infarction appear on CT
- appears hypodense
- due to disruption of blood supply to the brain
- causing necrosis
- brain tissue replaced by fluids
how does bleed appear on CT
- acute bleeding appears white and overtime becomes less dense, eventually shrinking
describe the patient positioning for a brain CT scan
- remove any metals from head
- supine
- head first
- chin angled down slightly to bring orbits away from radiation
- arms by patient side
- velcro straps or pads used to immobilise head
what is a localiser radiograph
- needs to be performed first
- to plan scan range and FOV
what needs to be included in a brain CT
brain stem
- medulla, pons, midbrain
limbic system
- diencephalon (thalamus and hypothalamus)
cerebellar
cerebrum
how does mA, kV, pitch and rotation time affect radiation dose
- increase mA and kV = increase radiation dose
- increase pitch = table move faster = decrease scan time = decrease radiation dose
- increase rotation time = increase radiation dose
what are the differences between thin and thick slices
thin:
- acquired when reformatted images required
- provide excellent definition of the temporal bones, pituitary fossa, focial bones and foramina
- improve spatial resolution
- decreased SNR –> require increase in mAs and kV = increase radiation dose
thick:
- increase the definition of gray and white matter in the brain
- used to improve visualization of herniated lumbar disk in larger patients
what is the window width and window level for bone and brain window
brain window:
WW80-150/WL40
bone window:
WW2000-4000/WL300-400
what is the formula to calculate the upper and lower grey level
WL +/- (WW/2)
what are some indication for CT orbits
plain:
- foreign body
- trauma
contrast:
- tumour
what are some indications for CT temporal bone
plain:
- loss of hearing
- otalgia
- acute otitis media
- cholesteatoma
- trauma
- tinnitus (non pulsating)
contrast:
- glomus tumour
- malignant otitis externa
- tinnitus (pulsating)
what should be included in the scan range in CT orbits
roof of orbit, floor of orbit and zygoma
what are the advantages of orbital CT imaging
- radiopaque FB accurately localised
- improved characterizing of orbital mass lesions
- allows imaging in multiple planes
what are some indications for CT neck/C-spine
plain:
- trauma/ degenerative changes
- ? FB (contrast if necessary)
contrast:
- infection
- tumours/masses
- abscess
- nodule
- vocal cord paralysis
what is the scan range for neck/C-spine
base of skull to level of T4
why is the scan range up to the level of T4
to include nasophraynx, oropharynx and laryngopharynx + lymphatic system of the neck
what are the 4 types of scan for a CT chest/thorax
- non-contrast
- arterial
- venous
- delayed
patient postioning for CT chest/thorax
- remove any metal from the thorax
- supine
- head/feet first
- arms above head
- velcro straps or pads can be used to immobilise patients or stop from falling off the table
- breathing instructions
what is the scan range for a CT chest/thorax
apex of lungs to adrenal gland or mid kidney
what are some indications for a CT chest/thorax
plain:
- ?FB
contrast (venous):
- persistent cough
- hemoptysis
- dyspnea
- chest pain
- pneumonia
- mediastinum indications
- metastasis
what should be included in a CT thorax
- thoracic cage
- sternum
- lungs
- hilum of lungs
- bronchus
- mediastinal compartments
- thymus glands
- lymph nodes
- pulmonary vessels
- SVC tributaries
- adrenals
what indication will require the use of a high resolution CT
interstitial lung disease (ILD)
- to detect and characterise disease affecting the pulmonary parenchyma and airways
why HRCT
- ability to detect lung disease in symptomatic patients with a normal CXR
- provide more specific diagnosis or exclude certain disease in patients with non-diagnostic findings on CXR
- ability to detect or evaluate specific problems or diagnoses
what are the 4 common types of ILD
- tree in bud
- ground glass opacity
- consolidation
- brochectasis
what is tree in bud
it represents dilated and impacted (mucus or pus filled) bronchioles
what is ground glass opacity
- filling of the alveolar spaces with pus, edema, hemorrhage, inflammation or tumour cells
- thickening of the interstitium or alveolar walls
what is consolidation
when the air that usuallly fills the small airways in the lungs is replaced with something else
what is bronchiectasis
localised bronchial dilatation
- signet-ring sign
- bronchial wall thickening
- visibility of airways in the peripheral lung
- mucus retention in the bronchial lumen
why are some CT chest done in prone instead of supine
essential in the context of distinguishing dependent density (atelectasis) from pulmonary inflammation or fibrosis-atelectasis detected on supine scan
what are some indications for CT abdomen
plain:
- ?FB
contrast:
- ?metastasis
- abdo pain / IO
- inflammation/ infection/ abscess
- appendicitis
- LOW/LOA
- sepsis
patient positioning for CT Abdomen
- remove any metals from the abdo pelvis
- supine
- head/feet first
- arms above head
- velcro straps or pads can be used to immobilise patients or stop from falling off the table
- breathing instructions
what is oral CM used for
to provide adequate distention of bowel loops
why need to administer OCM
- absence of OCM can result in under-distended bowel loops which can give false impression of a mass lesion or mask lesion
- optimal differentiation of lymph nodes from bowel loops becomes difficult without bowel distention
what are the 3 different types of OCM
neutral/gray - water
white - barium or iodine
black - air
benefit of neutral/gray OCM
- allow assessment of luminal pathology (within) and bowel wall
- excellent for upper GIT if scanned at appropriate time
limitations of neutral OCM
- transits rapidly, absorbed distally
- suboptimal distention can mask or mimic lesion
benefit of positive OCM
- extra luminal findings are more confidently identified
- peritoneal metastatic lesions cannot be optimally differentiated from bowel loops or ascetic fluid with the use of neutral OCM
limitation of positive OCM
- obscuration of bowel wall enhancement
- ischemic bowel or inflammatory small bowel disease
- radiologists need to assess bowel wall enhancement. NOCM should be served instead
- when POCM not properly diluted, can result in streaking artifact to the liver
what is rectal contrast
given in case of suspected bowel perforation or anastomosis leakage (150-250ml)
when to avoid rectal contrast
- oncologic patients on chemotherapy –> develop neutropenia from chemo
- rectal cancer cases - already diagnosed
- anal fissure and perianal fistulas –> very painful
scan range for CT abdo pelvis
above diaphragm to below SP
what is included in a abdomen pelvis scan
- abdominal viscera
- peritoneum and retroperitoneal
- adrenals
- vasculature
- lymph nodes
what is included in the abdominal viscera
stomach
intestine
liver and biliary system
pancreas
spleen
kidneys
ureters
suprarenal glands
what is CT angiography
radiographic technique using CT to investigate the vessels and organs after the introduction of ICM
CT Angiography techniques
- high flowrate
- 18 or 20G cannula
- test bolus or bolus track
- 3D recon; peak arterial enhancement
- lower kV
- high speed table, wider detector coverage
factors affecting arterial phase of contrast bolus
- time-to-peak enhancement (differs for different target arteries)
- distance from venous access site
- individual cardiac output
- diseases or disorders with the blood vessels
- high speed table, wider detector coverage
4 steps that create 3D images
- data acquisition
- image post processing
- 3D rendering
- image display and analysis
patient positioning for CTA COW and carotid
- remove any metals from head
- supine
- head first
- chin angled down slightly
- arms by patient’s side
- velcro straps or pads can be used to immobilise patient’s head
- follow CTA carotid, instruct patient to stop swallowing during the scan
indications for a CT COW
- ? aneurysm
- acute stroke
- trauma
- arterial venous malformation (AVM)
- arterial venous fistula (AVF)
patient positioning for CT aortogram
- remove any metal from thorax and abdo pelvis
- supine
- head/feet first
- arms above head
- velcro straps or pads can be used to immobilise patients or stop from falling off the table
indications of CT aorta
- ? aneurysm
- dissection
- surgical planning
patient positioning for CTA UL
- remove any metal from the affected UL
- supine
- head first
- affected arms above head
- velcro straps or pads can be used to immobilise patients or stop from falling off the table
patient positioning for CTA LL
- remove any metal from the affected LL
- supine
- feet first
- arms above head
- velcro straps or pads can be used to immobilise patients or stop from falling off the table
scan range for CTA LL
- CTA may be acquired in 2 blocks:
1. above diaphragm to below knee
2. above knee and clear both feet
