CT Scans - Interpretation - Principles - Basics (2024)

Computed tomography (CT) scanning is an extremely common imaging modality in modern medicine. With advancements in technology, it is rapidly replacing many diagnostic radiographic procedures.

In this article, we will outline the basic science behind CT scans, describe the principles of interpretation, and highlight their advantages and drawbacks compared to other imaging techniques.

Basic Principles

CT scans are created using a series ofx-rays, which are a form of radiation on the electromagnetic spectrum. The scanner emits x-rays towards the patient from a variety of angles – and the detectors in the scanner measure the difference between the x-rays that are absorbed by the body, and x-rays that are transmitted through the body. This is called attenuation.

The amount of attenuation is determined by the density of the imaged tissue, and they are individually assigned a Hounsfield Unit or CT Number.

• High densitytissue (such as bone) absorbs the radiation to a greater degree, and a reduced amount is detected by the scanner on the opposite side of the body
• Low density tissue (such as the lungs), absorbs the radiation to a lesser degree, and there is a greater signal detected by the scanner.

Conventional x-rays provide the radiographer with a two-dimensional image, and require the patient to be moved manually to image the same region from a different angle. In contrast, because of the advanced mathematical algorithms involved with CT, the three-dimensional planes of the human body can be imaged and displayed on a monitor as stacked images, detailing the entirety of the field of interest.

This is accomplished by acquiring projections from different angles and through a process known as reconstruction, the three-dimensional data is viewable on a two-dimensional monitor. The data collected can theoretically never be a perfect replica of what is being scanned, but is a close enough representation to be used for medical diagnostic purposes.

Contrast Imaging

Depending on the structure being imaged, CT scans can be used with and/or withoutcontrast. The introduction of an intravenous radiofluorescent contrast into the bloodstream can be used for a variety of diagnostic purposes, for example:

• Used to visualise the cardiovascular system (e.g. investigating for suspected aneurysms, dissections, or atherosclerotic diseases).
• Used to identify whether a tumour is malignant.

After approximately 7 minutes after an intravenous injection with iodinated CT contrast, the contrast begins to expel from the body via the urinary system. The contrast can be seen in the ureters going into the bladder creating a CT Urogram; a procedure that is commonly replacing the traditional intravenous pyelogram seen in radiography.

Oral contrast can also be administered if investigation is required of the digestive system. (Crohn’s disease, bowel obstruction, diverticulitis, appendicits).

Interpreting a CT scan

Orientation

When interpreting at CT scan, it is important to determine the orientation. Images are most commonly presented in the transverse plane, and are orientated so that we arelooking up the body from the patient’s toes.

A helpful way to get your bearings is the acronymRALP. Starting at the 9 o’clock position and moving clockwise in 90 degree intervals, we are looking at theRight,Anterior,Left andPosterior aspects of the patient.

Radiologists will often use images reconstructed in the coronal and sagittal plane to help supplement their diagnosis.

The Image

The density of the body tissue determines the degree to which the x-rays are attenuated. In turn, this affects the brightness and contrast of the imaged tissues.

Those tissues withhighattenuation coefficients(strong absorption)show upwhite, and those which absorb withlow attenuation coefficients(weak absorption)show upblack.This is quantified by the Hounsfield Scale of radiodensity. Tissues with a high Hounsfield score have a high attenuation coefficient, and so appear white:

Comparison to Other Imaging Techniques

CT scanning is the ideal imaging modality in emergency cases. It is often the choice of examination for trauma patients in the emergency room (due to its quick scan times). It is more efficient when an immediate diagnosis is required such as intracranial bleeds, dissection of a blood vessel, or renal stones.

The biggest drawback of CT is that it utilises radiation that can potentially be harmful, especially with younger patients and children. However, the benefits often outweigh the risk, and there has been an upward trend in the use of CT in diagnostic imaging.

Technological advancements in CT have paved the way for more advanced applications – such as virtual colonoscopy– which is quickly replacing traditional barium enema studies. Cardiac gating on CT scanners have allowed institutions to perform studies dedicated to the cardiac arteries and to perform measurements of ejection fraction. Specialised software has advanced 3D applications in CT, allowing for better visualisation of certain pathologies.

Below is a summary table of the common imaging modalities. Depending on the tissue being imaged, the urgency of the investigation and the level of detail required, any of these techniques may be preferred.