Felis ISSN 2398-2950

Computed tomography: brain

Contributor(s): Fraser J McConnell, Federica Morandi, Tobias Schwartz

Introduction

  • In recent years computed tomography (CT) Computed tomography (CT) has become more readily available in the veterinary field.
  • CT uses x-ray to generate cross-sectional images of the body that avoid the superimposition of neighboring structures, which makes it particularly useful when imaging areas of complex anatomy, such as the skull. Furthermore, CT provides excellent contrast resolution, many times superior to that of conventional radiography, and is recognized as superior to all other imaging modalities for imaging bone. In general, however, MRI Magnetic resonance imaging: brain is preferred over CT for brain imaging due to its superior soft tissue contrast.
  • CT has been traditionally carried out with the patient under general anesthesia, since the animal must remain immobile for the duration of the scan, usually 3-10 minutes depending on the area imaged. With the implementation of helical CT in the 80's, and more so with the newly developed multiple row detectors CT scanners in the late 90's, the time necessary for image acquisition has dramatically decreased. A state-of the-art multislice scanner can acquire images of the whole body of an adult man in <12 seconds, of the entire thorax in approximately 3 seconds. Even if the cost of such equipment makes it prohibitive in a veterinary practice, single slice helical scanners allow for shorter acquisition times for which patient immobilization may be achieved using heavy sedation alone, rather than general anesthesia. This has the potential to increase the number of animals that can be scanned as well as allowing scanning of animals that are not good candidates for anesthesia.

Uses

Indications

Advantages

  • The major advantage of CT over conventional radiographs is the higher contrast resolution: CT can discriminate density differences in tissues of 0.25-0.5%,  whereas radiography can only differentiate differences of only about 10%.
  • Cross-sectional images avoid superimposition of structures in areas of complex anatomy, which is a great advantage for the skull and nose.
  • Pre and postprocessing manipulation of the raw CT data is possible, allowing tissues of varying density to be better evaluated.
  • Multiplanar image reconstruction and 3D renderings can be configured.
  • Initial cost and maintenance of equipment is often lower compared to MRI Magnetic resonance imaging: basic principles Magnetic resonance imaging: brain.
  • Imaging times are significantly reduced as compared to MRI, even when axial (conventional) CT is used, and more so with single slice helical (and multiple-row detectors) scanners.
  • Refurbished CT equipment can be purchased at affordable prices.
  • CT is superior to all other imaging modalities when examining bone.
  • Patients with metallic implants/foreign material in the head can usually still be imaged (this is generally not possible with MRI).
  • With single-slice helical, and more so with the newest multiple-row detectors scanners, sedation alone could be sufficient for restraint.

Disadvantages

  • General anesthesia is typically required for older, single-slice scanners General anesthesia: overview.
  • Area of interest must fit within the CT gantry (usually not a problem when imaging cats).
  • Study interpretation may be lengthy due to the large number of images acquired with most CT examinations.
  • Ionizing radiation is utilized.
  • Low sensitivity for vascular and inflammatory lesions compared to MRI.
    For most intracranial disease MRI is more informative due to better soft tissue contrast.
  • High density streak artifacts from metal implants/foreign material may reduce image quality.
  • Caudal fossa imaging (cerebellum and brainstem) can be problematic due to beam hardening artifact.

Requirements

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Preparation

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Procedure

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Aftercare

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Outcomes

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Prognosis

Image interpretation

  • Normal anatomy:
    • Structures that normally show contrast enhancement include the pituitary gland, choroid plexus and falx cerebri.
    • In animals with congenital hydrocephalus, there is often incomplete closure of the fontanelle and other suture lines.
    • The lateral ventricles can also be asymmetrical; in the absence of a mass effect and/or deviation of the septum, this is most likely a variation of normal.
  • Pathology:
    • Tumors Brain: transverse post-contrast - bone window CT   Brain: transverse post-contrast - soft tissue window CT :
    • Meningiomas Meningioma  Meningioma: transverse - bone window CT  Meningioma: transverse - brain window CT :
      • Extra-axial (outside the brain).
      • Usually iso- to slightly hyper-attenuating on pre-contrast images.
      • May show mineralizations.
      • Cystic meningiomas have been reported.
      • May be associated with hyperostosis of the overlying calvarium.
      • Mass effect is variable depending on the size.
      • May be multiple.
      • After IV contrast administration show uniform, intense enhancement, due to the abundant vascularity and location outside the blood-brain barrier.
      • A "dural tail" sign is difficult to see compared to MRI images.
    • Pituitary tumors Pituitary gland: neoplasia  Pituitary: dorsal post-contrast - brain window CT  Pituitary: transverse post-contrast - brain window CT  Pituitary: transverse pre-contrast - brain window CT :
      • Extra-axial (outside the brain).
      • Usually iso- to mildly hyper-attenuating on pre-contrast images.
      • Well defined mass located dorsal to the sella pituitaria.
      • After IV contrast administration show uniform, intense enhancement, similar to meningiomas.
    • Choroid plexus tumors:
      • Extra-axial, well defined mass associated with the ventricles.
      • Often associated with hydrocephalus due to CSF production by the tumor.
      • After IV contrast administration show uniform, intense enhancement, similar to meningiomas.
    • Gliomas:
      • Intra-axial (inside the brain).
      • Often cause marked mass effect depending on location, size and presence of associated peritumoral edema.
      • The tumor itself is virtually never seen on pre-contrast images.
      • Contrast enhancement is very variable but usually not very intense: it can range from no discernible enhancement, to ring (peripheral) enhancement to mild and patchy enhancement.
      • There can be significant peritumoral edema, seen as surrounding patchy areas of hypoattenuation.
      • In general MRI is more sensitive than CT in detecting gliomas.
    • Metastasis:
      • Should be considered when more than one lesion is seen
      • Enhancement is variable; in humans, there can be significant associated peritumoral edema.
    • Inflammatory lesions:
      • Multifocal lesions of variable size, often ill-defined.
      • Patchy enhancement of the brain parenchyma.
      • Meningitis alone is difficult to diagnose due compared to MRI.
      • Brain abscesses are rare in the cat; they appear as hypoattenuating masses with thick, enhancing wall.
      • In some cases the brain can appear completely normal.
      • Cause: fungal (variable geographical distribution), bacterial, viral, parasitic, unknown etiology.
      • FIP Feline infectious peritonitis commonly causes obstructive hydrocephalus with abnormal contrast enhancement of the choroid plexi, ependymal and meninges.
    • Trauma:
      • Ideally performed with sedation alone if single slice helical (or multislice helical scanner) is available.
      • May be performed without any sedation in the comatose patient as long as the animal is immobile (for instance, strapped to a board).
      • Skull fractures, brain edema, intracranial hematomas and parenchymal hemorrhages can be identified:
      • CT is very sensitive for the detection of intracranial and parenchymal hemorrhage and hematomas in the acute phase.
      • CT is very sensitive in detecting intracranial gas.
    • Congenital lesions:
      • Hydrocephalus:
        • Readily available with dilation of the ventricles and thinning of the overlying cerebral cortices.
    • Vascular lesions:
      • MRI is superior for the diagnosis of brain infarcts.
    • Metabolic diseases:
      • CT of limited value, MRI is superior for the diagnosis of metabolic disease.

Further Reading

Publications

Refereed papers


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