CT Imaging for the PA Flashcards
What is a CT?
Computed Tomography
Used gantry with rotating xray beam and multiple detectors in various arrays along with sophisticated computer algorithms to process the data
What does a CT scan produce?
large number of 2D, slicelike images each of which are millimeters in diameter
What are the two major types of CT?
Helical
Conventional, Axial, Step and Shoot CT
What is a conventional, axial, Step and shoot CT used for?
high-resolution scanning of lungs, coronary artery calcium scoring, and prospective ECG-triggered coronary CT angiography
What are the principal components of a CT machine?
xray tube
diametrically opposed array of detectors
What does collimation determine?
thickness of a section
What is the standard chest collimation for mediastinum?
2.5 mm
What is the standard chest collimation for lung parenchyma?
1.25 mm
What is narrow collimation?
1 to 1.25 mm
What is narrow collimation used for?
CT pulmonary angiography
high resolution CT scanning of the lung parenchyma
imaging of small pulmonary nodules
What is wide collimation?
2.5 mm
What is wide collimation used for?
heavy patients to decrease the quantum noise at the cost of decreased spatial resolution
What is a CT image composed of?
matrix of thousands of tiny squares called pixels which are computer assigned a Hounsfield Number
What is the Hounsfield number?
a measure of how much of the xray beam is absorbed by the tissues at each point in the scan
What is the Hounsfield number of air?
-1000HU
What is the Hounsfield number of fat?
-40 to -100 HU
What is the Hounsfield number of water?
0 HU
What is the Hounsfield number of soft tissue?
20 to 100 HU
What is the Hounsfield number of bone?
400-600 HU
What is the Hounsfield number of metal?
> 1000 HU
What are the characteristics of Denser substances on a CT?
absorb more xrays
have high CT numbers
demonstrate increased attenuation
displayed as whiter densities on CT scans
What are the characteristics of less dense substances?
absorb fewer xrays
have low CT numbers
said to demonstrate decreased attenuation
displayed as blacker densities on CT scans
What is attenuation?
the chemical make up of the material; determined by how many xrays make it through the patient to hit the detector and what the image looks like
stopping of xrays
What can a CT image do after it has been taken?
the image can be post processed to optimize visibility
What type of imaging is a CT image?
structural; represents a moment in time
What is a CT used to rule out?
Tumors
Trauma
Infections
bleeding
What are the limitations of a CT?
Size of patient - 500 lb max Girth Claustrophobia Hold still for extended period of time hold breath for extended period of time Contrast usually needed
What is girth?
camera cannot touch patients arms or sides
What are the risks of using contrast?
acute adverse reaction
contrast-induced nephropathy (CIN)
What are the methods for contrast administration?
IV
Enteric = PO or rectal
Intraarticular
What is the base for all modern CT contrast?
iodine
What are the types of CT contrast?
Low osmolality contrast media (LOCM)
Iso-osmolarity contrast media (IOCM)
High osmolarity contrast media (HOCM)
What is positive enteric contrast?
High attenuation on CT
Consists of dilute suspension of Barium sulfate or dilute solution of an iodinated agent
Used for most indications
What is negative enteric contrast?
water attenuation on CT
used to delineate the bowel mucosa
used to improve detection of active inflammation or active GI bleeding
What can be used a negative contrast?
plain water
What resource is available to determine what type of contrast administration to use?
The American College of Radiology’s Appropriateness Criteria
When would you use IV contrast for Head CT?
neoplasm meningitis encephalitis focal neurologic deficit orbital/vision disorders pituitary imaging complicated sinonasal disease seizures Ct angiography
When would you NOT used IV contrast for Head CT?
head trauma
acute stroke
Intracranial hemorrhage
When would you use IV contrast for Cervical Spine CT?
cervical mass/lymphadenopathy
suspected tumor or infection
abnormalities of cranial nn. X, XI, and XII
brachial plexopathy
When would you NOT use IV contrast for Cervical Spine CT?
Trauma; unless possible arterial injury or mechanism of injury is penetrating
When would you use IV contrast for Cardiothoracic CT?
evaluating heart and thoracic vessels
trauma
for staging primary thoracic neoplasms
When would you NOT use IV contrast for Cardiothoracic CT?
coronary calcium scoring
pulmonary parenchymal evaluation
Lymph node evaluation
When would you use IV contrast for Abdominopelvic CT?
Gi
Hepatopancreaticobiliary
GU
GYN
When would you NOT use IV contrast for Abdominopelvic CT
CT colonography
renal stone evaluation
extraparenchymal lymphoma
When would you use IV contrast for musculoskeletal CT?
evaluation of soft tissue masses
evaluation of suspected septic arthritis
evaluation of infected prostheses
When would you NOT use IV contrast for musculoskeletal CT?
for most CT exams:
extremities
spine
When would you use IV contrast for angiography CT?
evaluating lumen of an artery, vein, or a pseudoaneurysm
assess for end-organ ischemia outside brain or lung
to eval for PE
to detect active bleeding
When would you NOT use contrast for angiography CT?
monitoring a known aneurysm for growth
for detection of a hematoma
When would you use Oral contrast for CT?
Eval gastric/small bowel perforation following trauma or surgery
Helps separate bowel loops in thin patients with minimal visceral fat
Improve delineation of bowel from mesentery and other peritoneal content
When would you NOT use oral contrast for CT?
accurate diagnosis of appendicitis and diverticulitis Eval of: liver kidneys collecting systems bladder spleen adrenal glands retroperitoneum bones vasculature
When is negative enteric contrast helpful?
detecting occult GI bleeding when paired with multiphasic CT angiography
When would you use rectal contrast for CT?
detection of rectal or distal colonic leak
When would you NOT use rectal contrast for CT?
basically never
What are the contraindications for enteric contrast?
in patients at risk of aspiration
eval patients with suspected high grade bowel obstruction
avoid barium based in patients who have/at risk for bowel perforations or leak from surgical anastomosis
What are not cross reactive?
Gadolinium for MRI and CT contrast
When will Adverse reactions to CT contrast occur?
after enteric, intracavitary or intravascular administration
When is the incidence of acute adverse reaction greatest?
when IV administration of HOCM
What is a mild reaction to CT contrast?
self limited
does not progress
rarely requires treatment
What is a moderate reaction to CT contrast?
requires treatment
may progress to a severe reaction if untreated
What is a severe reaction to CT contrast?
life threatening
can cause significant morbidity
What are allergic like reactions to CT contrast?
idiosyncratic- not related to dose
may occur in patient w/o history of prior exposure
do not predictably recur after each antigen exposure
What are the signs and symptoms of an allergic like reaction to CT contrast?
itching urticaria Erythema Stridor Hoarseness Bronchospasm Anaphylactoid shock
What are physiologic reactions to CT contrast?
dose dependent
not mitigated by corticosteroid prophylaxis
What are the signs and symptoms of physiologic reaction to CT contrast?
transient warmth or chills nausea vomiting metallic taste arrhythmia hypertension vasovagal reaction
