CT Scanning

CT Scanning

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Computed Tomography (CT) Imaging: Basic Principles, Contrast Imaging, and Interpreting a CT Scan

Computed tomography (CT) scanning is an essential imaging modality for modern medicine, rapidly replacing many diagnostic radiographic procedures with technological advancements. In this article, we will outline the basic science underlying CT scans, explain the principles of interpretation, and compare their advantages and drawbacks 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 sends x-rays towards the patient from various directions, and the detectors in the scanner measure the difference between the x-rays that are absorbed by the body and the x-rays that are transmitted through it. This phenomenon is known as attenuation.

The amount of attenuation is determined by the density of the imaged tissue, and each one is assigned a Hounsfield Unit or CT Number. Thus, high density tissue (such as bone) absorbs more radiation, resulting in a reduced signal detected by the scanner on the opposite side; conversely, low density tissue (like the lungs) absorbs less radiation, and the scanner will pick up a greater signal.

Conventional x-rays provide the radiographer with two-dimensional images, and require the patient to be moved manually in order to image the same region from a different angle. On the other hand, thanks to the complex mathematical algorithms employed by CT scans, three-dimensional planes of the human body can be imaged and displayed on a monitor as stacked images, providing a full overview of the desired field.

Contrast Imaging

Depending on the structure being imaged, CT scans can be used with or without contrast. Introducing a radiofluorescent contrast into the bloodstream via intravenous injection serves a variety of diagnostic purposes. For example, it can be used to visualize the cardiovascular system (for example, in searching for potential aneurysms, dissections, or atherosclerosis) or to identify whether a tumor is malignant.

Around 7 minutes after an injection of iodinated CT contrast, it begins to excrete from the body through the urinary system. The contrast can be seen in the ureters going into the bladder, forming a CT Urogram - a procedure that has largely replaced the traditional intravenous pyelogram used in radiography. Likewise, oral contrast can be administered if it is necessary to investigate the digestive system (for example, for Crohn's disease, bowel obstruction, diverticulitis, or appendicitis).

Interpreting a CT Scan

When interpreting a CT scan, it is important to determine the orientation. Images are usually presented in the transverse plane, and they are oriented so that the viewer is looking up the body from the patient's toes. An easy way to get acquainted with the exam is by using the acronym RALP. Starting at 9 o'clock and moving clockwise in 90 degree intervals, this stands for Right, Anterior, Left, and Posterior, meaning they represent the respective parts of the patient.

Radiologists will often make use of images reconstructed in the coronal and sagittal planes to help with their diagnosis. The density of the body tissue determines the extent to which the x-rays are attenuated. As a consequence, this affects the brightness and contrast of the imaged tissues. High attenuation coefficients (strong absorption) correlate to a high Hounsfield score, meaning they appear white on the scan; on the other hand, tissues with low attenuation coefficients (weak absorption) will show up as black.

  • SubstanceAir Hounsfield Value: -1000
  • SubstanceFat Hounsfield Value: 70
  • SubstanceWater Hounsfield Value: 0
  • SubstanceBlood Hounsfield Value: 70
  • SubstanceBone Hounsfield Value: 1000

Clinical Relevance: Intracranial Bleeds

Intracranial bleeds are potentially life-threatening conditions, often manifesting as an acute or delayed response to trauma. Less commonly, they can occur spontaneously due to the rupture of cerebral aneurysms. CT scanning is now the primary method of investigation for suspected intracranial bleeds. There are four main types of intracranial bleed:

  • Extradural Bleeds - located outside of the dura mater, this type of bleed appears as a lentiform (or lemon-shaped) structure on CT scans. They are arterial and generally caused by blunt trauma, and they may lead to a midline shift.
  • Subarachnoid Bleeds - these bleeds occur in between the arachnoid and pia mater (the two innermost layers of the meninges), and may manifest as a star pattern on CT scans.
  • Intraparenchymal Bleeds - these occur in the brain parenchyma, where the primary reason for presentation is an acute or subacute neurological deficit, and may manifest as a hyperdensitiy on CT scans due to the accompanying edema in the brain.
  • Subdural Bleeds - this type of bleed is located between the dura mater and the arachnoid, and is typically seen in the elderly as a consequence of a head trauma. It appears as a crescent-shaped hyperdensity on CT scans.

CT scans can be an invaluable help in diagnosing intracranial bleeds. They can detect the type of bleed, as well as potential associated conditions such as midline shifts or neurological deficits. The earlier a diagnosis is made, the better the chances of a successful treatment.

CT Scans and Imaging Techniques: A Comparison

Computed tomography (CT) scanning is a widely used imaging modality in modern medicine. It has revolutionised radiology and its constantly evolving technology has enabled it to rapidly replace many diagnostic radiographic procedures. In this article, we will delve into the basic science behind CT scans, the principles of interpretation and assessment, and its advantages and drawbacks compared to other imaging techniques.

Basic Principles

CT scans are created using a series of x-rays, a form of radiation on the electromagnetic spectrum. The scanner emits radiation from various angles which are then detected by the detectors in the scanner, and the amount of x-ray absorption is measured. This is called attenuation, and the amount of it is determined by the density of the tissue and is assigned a Hounsfield Unit or CT Number. High density tissue such as bone absorb radiation to a higher degree, and a reduced amount is detected by the scanner on the opposite side of the body. Low density tissue such as the lungs absorb radiation to a lesser degree, and there is a greater signal detected by the scanner. Conventional x-rays provide radiographers with two-dimensional image, and require the manual movement of the patient to image the same region from a different angle. Because advanced mathematical algorithms are involved in CT, three-dimensional planes of the body can be displayed on a monitor as stacked images, giving a complete view of the field of interest. This is accomplished by acquiring data from different angles, and the process of reconstruction translates three-dimensional data to be viewable on a two-dimensional monitor. Though the data collected will never be a perfect replica of the scanned object, it is a close enough representation to be used for medical diagnosis.

Contrast Imaging

In certain cases, the use of intravenous radiofluorescent contrast is necessary for a variety of diagnostic purposes. This contrast can be used to visualise the cardiovascular system (for example, to investigate for suspected aneurysms, dissections, or atherosclerotic diseases), or to identify whether a tumour is malignant. Alternatively, CT scans can be used without contrast.

Subdural Bleeds

Subdural bleeds occur between the two layers of the dura and arachnoid which are typically caused by the tearing of bridging veins in the elderly. Symptoms may not show up to a month after the initial insult, and they will manifest as a crescent-shaped lesion on the head CT which may cause a midline shift.

Subarachnoid Haemorrhage

Subarachnoid haemorrhages are caused by ruptured aneurysms in the subarachnoid space, but may also be a consequence of trauma. On x-ray, the CSF may become paler as it is tinted with blood, and the dark subarachnoid cisterns will become white.

Intracerebral Haemorrhage

Intracerebral haemorrhages can be caused by hypertension, diabetes, or trauma, and they will present on CT angiography as localized lesions with surrounding oedema due to inflammation.

Advantages and Disadvantages of CT Scanning

CT scanning is often the choice of examination for trauma patients in the emergency room as it has quick scan times and provides an immediate diagnosis, such as intracranial bleeds, dissection of a blood vessel, or renal stones. Nevertheless, it should be taken into consideration that CT scans involve radiation, which may be detrimental, particularly if used on young patients or children. However, the benefits typically outweigh the risks, and the use of CT has increased steadily in the last few years. Thanks to technological advancements, CT scans can now be used for applications such as virtual colonoscopy, cardiac gating studies, and 3D reconstruction of certain pathologies.


In summary, it is up to the discretion of the radiographer/clinician to decide which imaging modality should be used depending on the tissue being imaged, the urgency of the diagnosis, and the level of detail required. Nevertheless, CT scans are an extremely common imaging modality and their advanced features, such as contrast imaging, make them ideal for many medical cases.

CT Scanning and the Use of Contrast Agents

After an intravenous injection of iodinated CT contrast, it will take about seven minutes for the contrast to expel from the body via the urinary system. This is occurring during a CT Urogram, which is a procedure that is nowadays commonly replacing the traditional intravenous pyelogram seen in radiography. In addition to the intravascular administration of contrast, oral contrast can be administered if investigation of the digestive system is necessary (conditions such as Crohn's disease, bowel obstruction, diverticulitis, and appendicitis, for example).

Interpreting a CT Scan

When attempting to interpret a CT scan, it is helpful to first determine the orientation of the images, as they are most often presented in the transverse plane. Orientating oneself in this way with the help of the acronym RALP (Right, Anterior, Left, Posterior) can be very effective. Viewing images reconstructed in the coronal and sagittal plane can help supplement the diagnosis as well.

The density of body tissue determines the degree to which x-rays are attenuated, which in turn affects the brightness and contrast of the related tissues. The Hounsfield Scale of radiodensity can quantitatively measure the absorption of x-rays.

CT scanning is now the mainstay of investigation of patients with a suspected intracranial bleed. These bleeds can be categorised into four broad types: extradural, subdural, subarachnoid, and intracerebral. Extradural bleeds are arterial, often resulting from blunt trauma, and they can result in a lentiform (lemon shaped) bleed on CT with a possible midline shift. Subdural bleeds are most commonly due to a tearing of the bridging veins in the elderly, and can present as crescenteric lesions on head CT, again with a potential midline shift. A subarachnoid haemorrhage is usually caused by a ruptured aneurysm, although they can result from trauma as well; the CSF becomes paler and the dark subarachnoid cisterns turn white. Intracerebral haemorrhages can occur from various causes, such as hypertension, diabetes, and trauma. They are typically seen as localised lesions on CT angiography, with noticeable oedema from inflammation.

CT scanning is the favoured imaging modality in emergency cases, especially for trauma patients in the emergency room, as it can produce speedy scan results. Nevertheless, radiation from the procedure can potentially be harmful, particularly for children and younger patients. Despite this, there is an upward trend in the use of CT for diagnostic imaging, as the benefits often outweigh the risks. Furthermore, technological advancements in CT have paved the way for more advanced applications, such as virtual colonoscopy and cardiac gating, as well as 3D applications that allow for better visualisation of certain pathologies.

In many situations, any of the following imaging modalities can be preferred depending on the tissue being imaged, the urgency of the investigation, and the level of detail required:

  • X-ray
  • Ultrasound
  • Magnetic resonance imaging (MRI)
  • Positron emission tomography (PET)
  • Single photon emission computed tomography (SPECT)
  • Computerised tomography (CT)

CT Scans: An Overview

Computed tomography (CT) scans are one of the most utilized imaging technologies used in the medical field today. CT scans are so popular due to their ability to generate two-dimensional cross-sectional images based on three-dimensional source information. This allows healthcare providers to quickly orient and interpret a CT scan in order to gain clinical insight into intracranial bleeds, brain structure changes, and other medical conditions. In addition to the fast imaging and thorough details provided, contrast imaging is also available, which is an intravenous injection of a contrast agent that further improves the visibility of anatomy for the scan.

However, CT scans also come with radiation dangers to the patient, and they are the most potentially hazardous of all imaging modalities. Nonetheless, recent advances in technology such as virtual and augmented reality CT scan imaging have made CT scan imaging even more detailed and practical for clinical applications.

When compared to other imaging modalities such as x-ray, ultrasound, and MRI, CT scans provide a significantly faster image, and offer more details. To compare the different imaging modalities for factors such as duration, cost, dimensions, soft tissue details, and bone details, refer to the following table.

Comparison of Imaging Modalities

Factor CT (CT Abdo Used as Example) MRI X-Ray (CXR Used as Example) Ultrasound Duration 3-7 Minutes 30-45 Minutes 2-3 Minutes 5-10 Minutes Cost Cheaper Expensive Cheap Cheap Dimensions 3 3 2 2 Soft Tissue Poor Detail Excellent Detail Poor Detail Poor Detail Bone Excellent Detail Poor Detail Excellent Detail Poor Detail Radiation 10mSv None 0.15mSv None

In conclusion, CT scans are the gold standard for imaging technology and provide a faster image than other common imaging modalities, as well as more detailed information. Although risk of radiation exposure is a factor in CT scans, advances in technology have greatly improved the safety and accuracy of CT scans, making them more accessible and practical than ever.

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