Author: Tanja Neumann, Radiographer, Asklepios Klinikum Hamburg, Germany

(Translated from German)

A cranial CT scan (computed tomography of the skull) is one of the most common examinations in the daily routine of medical radiologic technologists or radiographers. This article serves as a knowledge refresher, incorporating the latest recommendations from the German Radiation Protection Commission (SSK) on the "Use of Patient Radiation Protection Devices in Diagnostic X-ray Applications on Humans," which has been in effect since late 2022. Additionally, we will explore the differences between spiral CT technology and sequential CT scanning and their respective applications.

Contents

1. Indications: When is a cranial CT scan necessary?

2. Contraindications: When should a cranial CT scan be avoided?

3. Procedure: Patient positioning, topogram, scanning, and reconstruction

Side note – Proper positioning for key examinations

4. Radiation protection: What are the current guidelines?

5. Critical findings: How should they be handled?

6. Conclusion

1. Indications: When is a Cranial CT Scan Necessary?

There are numerous clinical reasons to perform a cranial CT (CCT). The most common indications come from neurology and neurosurgery. Given the short scan duration of CT imaging, it is highly effective for quickly detecting life-threatening conditions and enabling timely interventions, such as thrombolytic therapy after a stroke. Because of this, it is crucial to conduct an initial review of the images during the scan itself.

Some of the most frequent indications for a CCT include:

• Intracranial hemorrhages
• Ischemic stroke
• Seizures (status post epileptic attack)
• Tumor assessment or exclusion
• Head trauma
• Microangiopathy (often relevant in dementia evaluations within geriatric psychiatry)

A significant proportion of patients undergo cranial CT scans in the emergency department due to symptoms such as:

• Severe headaches (with or without vomiting)
• Loss of consciousness (of unknown cause or due to drug/alcohol intoxication)
• Dizziness
• Vision disturbances
• Numbness or sudden unilateral paralysis
• Speech disorders

CCT is also used for follow-up monitoring of certain conditions, including:

• Hemorrhages: Early detection of worsening or increasing bleeding
• Stroke: Evaluating treatment success (e.g., thrombolysis, thrombectomy) or identifying further infarctions
• Postoperative monitoring: After brain surgeries (e.g., tumor resection, hematoma evacuation) to assess complete tumor removal or detect complications like rebleeding or brain herniation
• Post-cardiac arrest evaluations: To check for hypoxic brain damage, which occurs when prolonged circulatory failure deprives brain tissue of oxygen

2. Contraindications: When should a cranial CT scan be avoided? 

The primary reason to carefully consider whether a CT scan is necessary is radiation exposure. This is particularly critical for children, young adults, and pregnant patients. However, if there is a suspected life-threatening intracranial condition, the benefits of rapid diagnosis and treatment outweigh the risks of radiation exposure. For patients who are not in immediate danger—for instance, those with symptoms that have persisted for a longer period—it is advisable to consider an alternative imaging method such as magnetic resonance imaging (MRI) to avoid unnecessary radiation exposure.

There are no absolute contraindications for a cranial CT scan. However, the benefit of the scan must outweigh the potential radiation risk. Therefore, young age and pregnancy are considered relative contraindications. Another important factor is patient movement during the scan. Restless or uncooperative patients should be calmed or sedated if necessary, as motion artifacts can render the images undiagnostic. In cases where a high likelihood exists that the scan cannot be properly evaluated, exposing a patient to radiation could be considered unethical or even harmful. To prevent involuntary head movement, an inflatable positioning aid, such as the CT HeadFix, can be used (see video).

Video 1. Stabilising shaky patients using an inflatable pad

3. Procedure: Patient positioning, topogram, scanning, and reconstruction

Before performing a cranial CT scan (CCT), the medical indication for the examination must be verified. Physicians request the scan, and radiologists assess whether the exposure to radiation is justified. Once this justification is confirmed by the radiologic technologist or the radiographer, the examination can proceed. To ensure optimal image quality, all foreign objects in the scan area must be removed to prevent artifacts. These objects include hearing aids, hair clips, hair ties, glasses, wigs, and any other metallic or non-metallic items within the scanning field. Artifacts can complicate the diagnosis or even lead to misdiagnosis, as they may resemble a hemorrhage. Additionally, they can obscure actual findings, making them "invisible." The most severe artifacts typically result from metallic objects.

The patient is positioned in a supine (lying on the back) position for a CCT. Their head is placed in the CT head cradle, which may contain a head cradle or cushion depending on the CT model. This setup allows the head to be slightly inclined downward to minimize radiation exposure to the sensitive eye lenses, keeping them outside the scan field whenever possible. To further stabilize the patient, a forehead strap can be used to secure the patient's head within the cradle, preventing movement during the scan. Once the patient is positioned correctly, radiation protection measures are applied.

Spiral CT (helical CT) technology is preferred in many cases as it offers lower radiation exposure compared to sequential CT scanning. It produces a lower local radiation dose and allows for image reconstruction in all three planes (axial, sagittal, and coronal) during post-processing. Another major advantage of spiral CT is its speed, which reduces the likelihood of motion artifacts in uncooperative patients and makes it particularly useful in emergency cases requiring rapid diagnosis. However, spiral CT does have some drawbacks. Unlike sequential CT, it cannot be angled, which can be a disadvantage in certain imaging protocols. Additionally, the scan process may require a brief "ramp-up" phase, meaning that radiation exposure begins slightly before reaching the intended scan area.

Figure 1: Spiral CT is particularly advantageous for restless patients or in cases where a sequential CT scan cannot be angled sufficiently to keep the eye lenses out of the scan field. Due to its lower local radiation dose, spiral CT is the preferred option in these situations.

In contrast, the sequential CT technique allows the gantry to be tilted, maximizing protection for the eye lenses. The downside of this method is that sagittal and coronal reconstructions appear more angular, as the scan is performed in discrete steps. Since the patient table moves incrementally rather than continuously, the scan takes longer, making it less suitable for uncooperative patients. However, some CT systems allow for manual, layer-by-layer acquisition. In these cases, sequential CT can still be an option for restless patients, provided they can remain still for short periods and the radiation is applied only during those moments of stability.

In summary, spiral CT should be used for patients who are either optimally positioned or prone to movement. It is especially recommended when a large hemorrhage is suspected, as it enables reconstructions in all three planes. On the other hand, sequential CT is preferable for calm patients who cannot tuck their chin adequately to protect the eye lenses or for restless patients if the system allows for single-shot imaging.

Figure 2 shows a topogram on the left, illustrating the planning of a spiral CT scan with optimal positioning. On the right, a sequential CT planning example is shown, designed to protect the eye lenses by positioning the scan from the philtrum to the upper skullcap. The scan is then performed from the skull base to the upper calvaria, ensuring that the eyes remain outside the scan area.

Figure 2: On the left, the planning of a spiral CT scan with optimal patient positioning. On the right, the planning of a sequential CT scan designed to protect the eye lenses

Side Note – Proper Positioning for key Examinations

Figure 3: Overview of positioning for the most important examinations

For a temporal bone CT (petrous bone CT), the patient lies in a supine position, and their exact positioning depends on their dental status. Patients with fixed dental implants are positioned in the CT head cradle, and a wedge cushion is placed under the head to incline it until the teeth are no longer within the scan area of the petrous bone, similar to the positioning shown in Figure 2. In this case, eye protection is deprioritized since the presence of artifacts from the implants could render the scan unreadable. For patients without dental implants, the head is hyperextended until the eyes are positioned outside the scan field of the petrous bone.

For a head and neck CT angiography (CTA), the patient lies in a supine position with their head secured in the CT head cradle. The shoulders should be maximally pulled down to ensure optimal visualization of the cervical arteries

Figure 4: Positioning for a head and neck CTA (Image: Alex Riemer)

A perfusion CT scan is performed with the patient in a supine position, with the head secured in the CT head cradle. A wedge cushion is placed under the head to protect the eyes, following the same positioning principles as a standard cranial CT scan (CCT)

For a CT scan of the paranasal sinuses (PNS) or facial skeleton, the patient lies supine in the CT head cradle. A wedge cushion is positioned to ensure that the Frankfort horizontal plane (an imaginary line connecting the lowest point of the eye socket and the external auditory canal) is perpendicular to the CT table.

A dental CT scan is performed with the patient in a supine position, with the head placed in the CT head cradle and hyperextended until the eyes are positioned outside the scan field, above the mandibular condyles. No additional positioning cushions are required for this examination.

Figure 5: Dental CT (Image: Alex Riemer)

To enhance stabilisation during any head CT examination, the CT HeadFix inflatable pad and a small blue stabilisation band (ProBelt 50) can be used to reduce movement.

Figure 6 : Inflatable Pad (CT Headfix) with a blue stabilisation band (ProBelt 50)

In some cases, standard positioning in the CT head cradle may not be possible, such as for intensive care patients with an ECMO (extracorporeal membrane oxygenation) device or unstable patients in trauma shock rooms, where excessive movement should be avoided. In such situations, the following alternative positioning technique can be used as a emergency solution:

Figure 7 : Emergency solution when it is not possible to position using the CT head cradle (Image: Alex Riemer) (Head mould: ProFoam Head, white cushion: PearlFit Cushion 50x30x10, blue wedge: ProFoam Wedge 15, 26x25x7)

4. Radiation Protection: What Are the Current Guidelines?

In September 2022, the German Radiation Protection Commission (SSK) issued an updated recommendation on the “Use of Patient Radiation Protection Devices in Diagnostic X-ray Applications on Humans.” These recommendations cover all X-ray examinations in projection radiography as well as computed tomography (CT), including cranial CT (CCT).

According to the SSK guidelines, the most effective radiation protection measure is to tilt the gantry, as this helps minimize radiation exposure to sensitive tissues. Additionally, for patients up to approximately 40 years old, a thyroid shield should be used whenever possible, since the thyroid gland is located near the scanning area. However, breast shielding is not explicitly recommended for women, as the breast tissue typically lies outside the radiation field during a cranial CT scan.

With these updated guidelines, the use of radiation protection devices has changed significantly, and many protective measures that were previously standard are no longer part of the official recommendations. However, the SSK emphasizes that if a patient requests additional radiation protection measures, they can still be used as long as there are no overriding medical or technical reasons against them.

5. Critical findings: how should they be handled?

Certain findings on a cranial CT scan require immediate medical attention. Examples include intracranial hemorrhages, especially those accompanied by a midline shift (Figures 8 and 9), as well as acute ischemic strokes with a dense artery sign or large chronic infarcts (Figure 10). In such cases, the reporting radiologic technologist or radiographer must promptly inform the referring clinician for further evaluation and urgent medical intervention.

Figure 8: Subdural hematoma in the right hemisphere with a maximum thickness of 2 cm. Midline shift 18 mm to the left with compression of the internal and external CSF spaces on the right

Figure 9: Intraparenchymal hemorrhage in the right hemisphere with signs of multiphase bleeding and mass effect. Additional subarachnoid hemorrhage in the left parieto-occipital region. Slight midline shift to the left.

Figure 10: Left: Dense artery sign in the left M1 segment, indicating MCA occlusion with a large left hemispheric perfusion deficit, a good level of mismatch, and only mild early infarct signs. Right: Old, extensive MCA infarction on the left, involving the basal ganglia, internal capsule, and insular cortex.

6. Conclusion

By taking the outlined factors into account, radiation protection, patient safety, and interdisciplinary collaboration can be effectively combined to ensure the successful execution of a cranial CT scan.

Originally published in Radiologie Technologie, 37th edition, Issue 4, December 2024.

For further information, the author can be contacted at: t.neumann@mit-dem-durchblick.de.

Source on radiation protection:
SSK Recommendation on Patient Radiation Protection Devices (last accessed: September 1, 2024, 15:53).