Ct d49 Computed Tomography (CT), s. Computed Axial Tomography (cat)

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CT D49 ()

Computed Tomography (CT), s. Computed Axial Tomography (CAT)

Last updated: September 5, 2017

Physics 2

Helical (spiral) CT 3

Hounsfield scale 4

Windows 4

Contrast enhancement 5

Intravenous contrast enhancement 5

Contrast medium 5

Intrathecal contrast enhancement (CT cisternography) 7

Inadvertent intrathecal injection of ionic contrast agents* 8

Temporal bone CT 9

Sellar region CT 12

Anterior fossa CT 12

Normal variations in head CT 12

Pregnancy concerns 13

Dose considerations 13

Rate of absorption 14

Gestational age considerations 14

Counselling points 14

Iodinated contrast 15

Useful Websites 15

CTA, perfusion CT – see p. D64 >>

  • CT was developed in 1972-1974 by Sir Geoffrey Hounsfield and his colleagues in England.

Strengths of CT

  1. Fast - typical high-quality brain CT requires 10-20 seconds (it takes far more time to position patient in machine than it does to do scan) – easy to use for follow up of critically ill patients.

  2. Easy to monitor patients (vs. MRI incompatibility with ferromagnetic materials)

  3. Simplicity of interpretation - all contrast on CT images is due to differences in electron density (appearance is determined by single parameter - images are relatively easy to interpret).

If disease of interest does not change electron density, it will be invisible on CT!

  1. Can easily detect acute hemorrhage and calcification

  2. Excellent for studying bones

Weaknesses of CT

  1. Less sensitive to parenchymal lesions (than MRI) - normal intracranial structures have narrow range of electron density.

  2. Potential for significant contrast reaction

  3. Radiation exposure

  4. Artifacts related to beam hardening (limits evaluation of posterior fossa structures).

  5. Difficulty in obtaining planes other than axial (usually other planes are reconstructed from axial images)

CT is not useful in evaluation of CNS bellow upper pons and PNS (H: MRI).


  • tightly collimated X-ray beam is directed through patient.

  • patient is placed in CT gantry, and X-ray beam travels in circular path around patient.

        • for axial head CT, gantry is angled parallel to orbitomeatal line.

        • for coronal head CT, patient is either supine or prone with neck hyperextended (impossible for intubated patients, patients with cervical spine disease, airway obstruction, or obesity).

  • detectors 180° opposite beam measure X-ray attenuation.

  • X-ray beam is collimated in range from 1 to 10 mm (depending on size of structure being examined) - patient is moved by same increment as slice thickness:

        • 5 mm sections through posterior fossa.

        • 10 mm sections through supratentorial space.

        • 1-3 mm sections for detailed examination of small anatomical regions (such as temporal bone, sella, orbits, paranasal sinuses).

  • signal from radiation detector is digitized by analog-to-digital converter → image is constructed using computer (with air seen as black and bone as white).

  • typical radiation dose for head CT ≈ 30-60 mGy.

Helical (spiral) CT

  • differs from conventional CT in that patient is moved continuously through x-ray beam → dramatically shortened scanning times:

    1. traumatized or uncooperative patients

    2. enables CT angiography (rapid collection of images after contrast bolus is injected)

  • "pitch" - distance table moves in millimeters for every rotation of tube.

  • "pitch factor" (unitless term) = pitch divided by slice thickness.

  • reformatting images or 3D reconstructions are smoother and more accurate, but some loss of resolution - helical CT is not employed in routine brain imaging.

Hounsfield scale

(named after Godfrey N. Hounsfield, inventor of CT) – relative scale of electron density:


Hounsfield Unit

Gray Scale



very black







Cerebrospinal fluid



White matter



Gray matter



Extravasated blood



Contrast medium**





very white

*edema (water content↑) is seen as lucency

**in any case of suspected acute hemorrhage, contrast medium should not be administered (similar attenuation characteristics to acute extravascular blood)

N.B. acute hematoma has high attenuation owing to clot retraction with separation of high-density erythrocytes from lower density plasma;

N.B. unclotted blood (coagulopathy or hyperacute active bleeding) is seen as relative lucency!


  • brain windows (60-80 HU) accentuate minor difference in Hounsfield units between white and gray matter.

  • bone windows (2000-4000 HU): brain is not seen!

    1. bone evaluation (in trauma, bone infection, bone neoplasm)

    2. detection of intracranial air (can separate air and fat density).

  • subdural windows (150-200 HU) - to identify acute subdural hematomas (would blend with calvarium on routine brain images).

Contrast enhancement

Intravenous contrast enhancement

Basics, Indications → see p. D45 >>

Pregnancy concerns – see below >>

In general, it is preferable to perform MRI than to perform CT with contrast enhancement

CT without contrast enhancement is of little value in diagnosis of brain tumors or other mass lesions!

although hemorrhage, calcifications, hydrocephalus, shifts can be well seen on non-contrast CT, underlying causative structural abnormality can be missed.

  • routine use of nonenhanced CT scan before enhanced scan is of limited usefulness (recommended only for lesions with hemorrhagic / calcified components).

Contrast medium

- water-soluble iodinated:

    1. ionic (higher-osmolality)

    2. non-ionic (more expensive but safer! - low-osmolality - extremely low incidence of side effects)

Specific indications for nonionic contrast agents: previous adverse reaction to ionic agent, asthma, multiple allergies, cardiac problems (incl. CHF, pulmonary hypertension), severe general debilitation.

Maximum dose with normal renal function – 86-90 gm of iodine in 24 hour period.

  • given as intravenous drip or as IV bolus.

  • CTA uses ≈ 21 gm of iodine – GFR has to be > 45 (vs. > 30 for gadolinium)

Side effects - iodinated contrast agents are physiologically inert except for:

In general, both high- and low-osmolar contrast agents are extremely safe!

  1. Allergy (rare*, readily manageable); prophylaxis (e.g. in patients with allergic histories – best use MRI instead):

    1. prednisone 50 mg orally (13, 7, and 1 hour before contrast injection).

    2. diphenhydramine 50 mg orally (1 hour before contrast injection).

    3. history of laryngospasm / hypotension with previous use of contrast → anesthesiologist should be present during contrast administration.

*severe allergic reactions - 0.04% patients receiving nonionic media

  1. Oncotic load:

  1. sensation of heat, pain, nausea, vomiting (well-known side effect of ionic contrast media).

  2. potential nephrotoxicity (contrast nephropathy);

        • risk is very little in normally hydrated patients who do not have kidney disease.

        • states that predispose to kidney injury: advanced age, multiple myeloma, severe diabetes, dehydration, recent aminoglycoside exposure, preexisting chronic renal dysfunction (anuria, hepatorenal syndrome, serum creatinine > 3 mg/dL).

        • definition of contrast nephropathy: rise in serum [creatinine] of at least 1 mg/dL within 48 h of contrast administration.

        • prognosis is usually favorable.

        • prophylaxis – good hydration (­± bicarbonates, acetylcysteine) prior IV contrast administration.

Guidelines for use of IV contrast in impaired renal function:

Serum Creatinine, μmol/L (mg/dL)


< 133 (< 1.5)

Use contrast at 2 mL/kg (max. 150 mL total)

133–177 (1.5–2.0)

Use nonionic contrast; hydrate diabetics

> 177 (> 2.0)

Consider noncontrast CT or MRI

177–221 (2.0–2.5)

Nonionic contrast contraindicated in diabetics

> 265 (> 3.0)

Nonionic contrast given only to patients undergoing dialysis within 24 h

  1. Iodinated contrast (IV or intra-arterial) may delay excretion of metformin;

        • manufacturer recommends withholding metformin 48 hrs prior to and following contrast administration (or longer if there is evidence of declining renal function following use of contrast).

N.B. avoid of iodine contrast in diabetics who are getting oral antidiabetic agents like metformin - risk of lactic acidosis!!!


  1. Acute renal failure (absolute contraindication)

  2. Renal insufficiency (H: dialysis soon after study)

Normal CT following IV contrast medium: arterial and venous structures at base of brain are prominent; also normal enhancement of falx cerebri and choroid plexus of lateral ventricles:

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