How To Read a CT Scan of the Brain (Part 1)

Identification of the CT scan anomalies was not part of the curricula when i graduated, yet very often you come across CT scan interpretation both at under-grad and post-grad levels.

This technique is replacing conventional x-rays in most parts of the world, but what you also need to remember is that a CT scan usually puts about 500 times more radiation to the body as compared to an X-ray, yet there are specific indications for both, that you'll learn as you progress.

There is no short book for CT scans, no one teaches you in college too, so thats how I learnt the basics of CT interpretation. This post is for head CT, abdominal and chest CT details will follow soon Inshallah.

Lets start then:

How To Read a CT Scan of the Brain

A) Identification

Identify the subject of the exam and some technical details.

• Who & When Name, Age, & Date: This crucial information is printed on each CT image. Be especially careful to check dates!

• Orientation: Right - Left, Anterior - Posterior, & Scale

R = Right

P = Posterior

Scale shows Centimeters

• Region and Planes: Head CT's are usually transaxial and extend from the foramen magnum to vertex. Other planes used are coronal and sagittal.

• Brain windows are standard. Bone windows and Subdural windows can make a big difference in resolution for these parts of the examination.

BRAIN WINDOW

Shows brain best

WL = 35

WW = 70

SUBDURAL WINDOW

May show subdural fluid (not seen)

in other windows

WL = 70

WW = 300

BONE WINDOW

Shows bone detail best

WL = 600

WW = 3000

The image below shows the magnified bottom left corner of the brain window.

With practice, the difference between these 3 windows will become obvious to your eye. Until then, the WL and WW measures will enable you to distinguish each window type. Just to make things interesting, the WL and WW measures can appear in different places on the CT images depending on the machine, the technician and interaction between them.

WL = Window Level WW = Window Width

The yellow arrows point to the WL and WW labels on these scans...much easier to read in real life!

• Slice Thickness can vary from 1 mm to 10 mm. Thin cuts can help clarify imaging of small areas such as the internal auditory canals.

In the top left corner of each of the 2 scans above, you can see the slice number in line 2 (13 on the image above, 14 on the image below to it) Just below that are numbers (a little hard to read) in mm: 10.5mm for the scan on the left and 17.5 mm for that on the right above. Subtract one from the other to determine the slice thickness for the scans. In this case, 7 mm "cuts" were used.

• Contrast Enhancement

Plain scans are done without infusion of any medication or agents. IV infusion for contrast enhancementmakes some lesions (esp. tumours) much more visible.

Several different agents can be used. Omnipaque is used most often at our institution. It is given IV. Be sure renal function is OK (via serum creatinine test) before administration and be cautious if there is a tendency to congestive heart failure. Scans after contrast infusion are usually labeled "enhanced" or "omnipaque" or "contrast." You can also recognize enhanced scans through the base of the brain because they show the circle of Willis which does not normally appear on un-enhanced scans.

In the following example, the non-enhanced scan is shown above followed by the same view after contrast enhancement, the vascular nature of the lesion has led to a better view:

• Axis = a line from the bottom of the feet to top of the head.

Transaxial plane is perpendicular to the axis and shows left and right, anterior and posterior as seen from below. It is the plane used most often for CT scans of the brain and for all of the images shown in this web site.

Coronal plane shows left and right, superior and inferior as seen from in front. It is often useful for CT scans of the sella and pituitary gland and paranasal sinus areas.

Sagittal plane shows anterior to posterior, superior and inferior as seen from the side. Rarely used for CT scans of the brain but is often used for spinal imaging and for MRI of the brain.

To be continued Inshallah......

Dr. AK Malik

Part 2 !