CAT SCAN  [ back to Neuroimaging ]
CAT SCAN or computerized axial tomography is an imaging technique in use since the 1970s used to detect soft tissue abnormalities inside the body. The scanning device transmits x ray beams in a fan pattern through a pancake thin "slice" of the human body at many different angles, between 700-1,500. As the beams pass through the body they are scattered or absorbed by the structures within such as bone, cartilage, muscle, fat and blood. This thins out or "attenuates" the beams of x-ray photons, which are detected and measured as they leave the opposite side of the body. Different organs and structures within the body attenuate the beams to different extents related to their tissue composition and density. The beam attenuation data is collected, digitized and analyzed by a computer which reconstructs the location and appearance of 3 dimensional bodily organs and structures in the form of a 2 dimensional slice which cuts through the body on a transverse plane. The scanner measures and reconstructs x-ray attenuation in the form of an attenuation map of the body across a spatial grid made up of tiny blocks of tissue called "voxels" equivalent to 1x1 millimeter. The 2 dimensional picture of computer reconstructed organs is made up of visual units called "pixels" which come in hundreds of different shades of gray between white and black.

The clarity or sharpness of a CAT scan is less than that of an x-ray, but the scan is far superior to standard x-ray in terms of its capacity to detect and visually represent abnormalities of soft tissues, such as tumor growth, rupture, bleeding or swelling. Whereas an x-ray can only tell us whether the skull has been fractured, a CAT scan can tell us, through visual contrast of dark and light, whether the brain is actively bleeding, is swollen or is compressed because of a blood clot or enlarged ventricle. CAT scan is much cheaper than an MRI because it does not require expensive equipment to generate a steady magnetic field. It is also much quicker to use than MRI. For those reasons it is the neuro-imaging technique of choice in emergency rooms when the doctor needs immediate answers to questions such as why is this patient stuporous or comatose and does he require neurosurgical intervention to drain a hematoma. CAT scan is slightly more hazardous than MRI, because it uses x-rays. Whereas, MRI is harmless because it uses radiofrequency magnetism only, it can injure patients with metallic objects in their bodies such as shrapnel, aneurism clips or pace makers.

For non-emergency uses, MRI is the neuroimaging technique of choice, because it has much higher resolution (clarity of fine detail) than CAT scan and can be done not only in the transverse (horizontal) plane, but also in the sagittal (side) and coronal (front to back). CT can be misleading, and inferior to MRI, in two situations. If taken right after head trauma, CT is likely to miss a slow leak of blood from a partially torn blood vessel, which may take days or weeks to grow a clot large enough to show up. CT can also miss delayed onset brain swelling, which is why emergency room doctors pass out a Closed Head Injury sheet telling family members to bring the patient back if she vomits, becomes lethargic or develops slurred speech or visual problems. CT is not useful if taken weeks or months after head trauma, because once active bleeding has stopped and the clot begins to undergo hardening and resorption it is "isodense" with the brain surface, and visually cannot be distinguished from it. Thus CT works best soon after head trauma with a large, actively bleeding lesion site, but not in other situations.  

There seems to be no authoritative and uniform guidelines as to the circumstances under which a CT should always be done following closed head injury. Clients of our office with similar injuries get treated differently in different emergency rooms. Often the key determinant is whether a family members insists on a CT scan being done. A Canadian study of 3,121 patients with "minor head injury" (defined as Glasgow Coma Scale of 13-15 with witnessed loss of consciousness, amnesia or disorientation) that addresses this issue was published May 5, 2001 in the Lancet (Vol. 357). In that study the authors found the following criteria highly predictive of which patients had "clinically important brain injury requiring neurosurgical or neurological intervention": GCS score lower than 15 two hours after injury; suspected open or depressed skull fracture; basal skull fracture; vomiting more than twice; and age over 65. These criteria existed for just 32.2% of the group. Two moderate risk criteria judged to be helpful were retrograde amnesia to 30 minutes before the trauma and "dangerous mechanism of trauma" such as ejection from a vehicle or a fall of 3 feet or more onto one's head.