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.
CT scans are created using a series of x-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 density tissue (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.
Depending on the structure being imaged, CT scans can be used with and/or without contrast. 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
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 are looking up the body from the patient’s toes.
A helpful way to get your bearings is the acronym RALP. Starting at the 9 o’clock position and moving clockwise in 90 degree intervals, we are looking at the Right, Anterior, Left and Posterior aspects of the patient.
Radiologists will often use images reconstructed in the coronal and sagittal plane to help supplement their diagnosis.
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 with high attenuation coefficients (strong absorption) show up white, and those which absorb with low attenuation coefficients (weak absorption) show up black. This is quantified by the Hounsfield Scale of radiodensity. Tissues with a high Hounsfield score have a high attenuation coefficient, and so appear white:
Clinical Relevance: Intracranial bleeds
Intracranial bleeds are potentially life-threatening conditions, and occur most commonly as an acute or delayed response to trauma. They can occur spontaneously from the rupture of cerebral aneurysms, but this is less common.
CT scanning has evolved to become to mainstay of investigation of patients with a suspected intracranial bleed. There are four broad types of intracranial bleed:
Extradural: A bleed outside the dura mater, which creates a lentiform (lemon shaped) bleed on CT. These are arterial, and frequently related to blunt trauma. There may be a midline shift.
- Subdural: A bleed between the dura and arachnoid layers, most commonly due to a tearing of the bridging veins in the elderly. These can become symptomatic up to a month after the initial insult. These create crescenteric lesions on head CT, with a possible midline shift.
- Subarachnoid: A ruptured aneurysm in the subarachnoid space leads to a subarachnoid haemorrhage, although they can be a consequence of trauma. Their radiographic features can be subtle; the CSF becomes paler as it becomes tinted with blood, and the normally dark subarachnoid cisterns turn white.
- Intracerebral haemorrhage: Causes include hypertension, diabetes, and trauma. They present on CT angiography as localised lesions, with surrounding oedema due to inflammation.
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.
|Factor||CT (CT abdo used as example)||MRI||X-ray (CXR used as example)||Ultrasound|
|Duration||3-7 minutes||30-45 min||2-3 min||5-10 minutes|
|Soft tissue||Poor detail||Excellent detail||Poor detail||Poor detail|
|Bone||Excellent detail||Poor detail||Excellent detail||Poor detail|
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