Computed Tomography of the Head

Over the last decades, we have seen a steady growth in the number of computed tomography (CT) examinations [1‐3]. New technological developments, broader availability of the required hardware and software, as well as physician and patient demands are the main reasons for increased clinical application of CT [1, 2]. Although magnetic resonance imaging (MRI) has developed tremendously over the last decades, in particular for head imaging, and has become more available, CT still is the workhorse of head imaging for multiple indications, including non-contrast CT (NCCT) in cerebral ischemia and stroke, assessment of intracranial hemorrhage (ICH), headaches, or acute neurologic deficits, as well as for first-line diagnostics in loss of consciousness and trauma evaluation in the emergency setting [4]. Detailed assessment of cerebral blood supply has become the clinical standard through the use of CT angiography (CTA) and CT perfusion (CTP) [5, 6]. Therefore, CT of the head is essential for fast and accurate diagnosis, optimized patient management and treatment. Besides MRI, which is generally used in less acute clinical settings, CT makes up a large proportion of the daily neuroradiological workload in head imaging.

However, CT comes with the inherent downside of ionizing radiation, which may cause radiation-induced malignancies [7, 8]. In the USA, approximately 2% of future cancer cases are assumed to be attributable to the current application of medical imaging [3, 9], and CT exposure was estimated to be responsible for 1% of total cancer mortality [10]. Therefore, the principle to keep radiation exposure as low as reasonably achievable (ALARA) is fundamental [11, 12].

Unfortunately, CT-related radiation exposure in daily clinical routine still shows strong inter-institutional and intra-institutional variations, as well-defined and universal reference standards are usually not available [13, 14]. General recommendations are complicated to determine considering different scanner models and technologies, which considerably influence radiation exposure. Using the technique only when the clinical value outweighs risks and costs, and restricting the scan volume to the clinical task are undoubtedly the most effective ways to limit CT-related radiation exposure in order to protect patients. Besides a careful risk-benefit assessment, developments on both the acquisition and the reconstruction side have led to an optimized trade-off between image quality (IQ) and radiation exposure [15, 16]. A widely applied dose reduction approach is the optimization of acquisition parameters, including tube voltage, tube current, and contrast medium (CM) volume. Tube current is expressed either directly (as mA) or indirectly in terms of tube current-time product (as mAs). Modern clinical CT systems use automatic exposure control (AEC) by means of automatic tube current modulation (ATCM), which is based on patient habitus, z‑axis modulation, and rotational modulation [17‐19].

Increased image noise and artifacts are the drawbacks of most dose reduction methods but could at least partially be compensated by adequate image reconstruction techniques. Developments in iterative reconstruction (IR) techniques have led to major improvements in recent years, which have therefore become an essential component of CT dose reduction [16]. Before the emergence of IR, filtered back projection (FBP) was the standard image reconstruction for clinical CT. This fast and robust method is based upon the exact mathematical relation between measured projection and reconstructed image data [16, 20]. However, particularly for dose-reduced acquisitions, FBP-reconstructed images can suffer from low quality, as noise-free data are assumed and image noise is amplified in the filtering process. In contrast, IR can reduce image noise through iterative filtering or modeling of data acquisition physics [16]. Currently, hybrid IR (HIR), such as iDose (Philips Healthcare, Best, The Netherlands), ASIR (GE Healthcare, Milwaukee, WI, USA), or SAFIRE (Siemens Healthineers, Erlangen, Germany), is the most frequently used IR technique, providing increased reduction of image noise and reconstruction time through iterative filtering of both projection and image data. Benefits in terms of achievable IQ, in particular at low doses, are only exceeded by model-based IR (MBIR) algorithms, which use advanced models in an iterative process of backward and forward projections. However, the exceptional level of noise reduction and achievable dose reduction comes at the cost of high computational efforts, limiting accessibility and usability in daily clinical practice. Yet, artificial intelligence (AI) could help to overcome this limitation by using a convolutional neural network (CNN) trained with (simulated) low-dose (LD) data to reconstruct standard-dose (SD) high-quality CT images [21‐24].

Although the use of dose reduction techniques for head CT is increasing, they have not yet been systematically reviewed. Therefore, the purpose of this article was to review clinical applications of dose reduction and LD techniques for NCCT and CTA of the head. The objective was to analyze achievable dose reductions, effects on IQ, and diagnostic performance with respect to the most relevant pathologies that are frequently assessed by head CT applications. Availability and quality of the included literature has a high degree of heterogeneity with respect to dose reduction methods, reported dose parameters, and clinical applications. Therefore, we concentrated on five major clinical indications: (i) NCCT of cerebral ischemia and stroke, (ii) NCCT of intracranial hemorrhage, (iii) NCCT of the head without specified indications, (iv) CTA of intracranial aneurysms, and (v) CTA of other cerebrovascular diseases and carotid artery disease.

In this article, dose reduction techniques for NCCT and CTA of the head were systematically reviewed and 37 studies representing the most common clinical indications were included. LD-CT and SD-CT were most frequently compared between a different combination of tube settings and image reconstruction techniques.

For NCCT of cerebral ischemia and stroke, achieved dose reductions ranged from 10 to 36%. The majority of studies reduced the dose by at least 22% and maintained objective and subjective IQ, while the use of more advanced IR improved visualization of ischemic demarcation. For NCCT of intracranial hemorrhage, slightly higher dose reductions of 18–66% with sufficient IQ were achieved, while simulated LD-CT suggested a dose reduction potential of 60%. Achieved dose reductions for NCCT with unspecified indication were in a comparable range of 20–40% (Fig. 4).

In the context of NCCT, it has to be mentioned that the required dose for differentiation of normal from pathological tissue depends on the contrast difference. For the detection of cerebral ischemia, the contrast differences are very small and therefore a higher dose is required compared to the assessment of ICH, where the contrast difference is significantly higher.

Given the only slightly decreased or even equivalent IQ and diagnostic acceptability of LD protocols and ongoing improvements in image reconstruction, there could be more potential for future dose reduction beyond the reported 20–40% in NCCT of the head. However, increased dose reductions must be used with care. In the abdominal region, the diagnostic performance of low contrast lesions has been reported to be impaired when dose reduction exceeded 25% [75, 76]. Such low contrast lesions are key diagnostic questions in NCCT, particularly in cerebral ischemia and stroke. Diagnostic performance must therefore be thoroughly evaluated before higher dose reductions, facilitated by advanced IR or other novel reconstruction techniques, are implemented.

For CTA of intracranial aneurysms, dose reduction ranged from 16 to 82%, while 6 of 10 studies even achieved maximum values of 65% or higher (Fig. 4). Despite the substantial dose reduction, subjective IQ and diagnostic performance for aneurysm detection were non-inferior. Hence, CTA could be a less invasive and dose-intensive alternative to DSA for accurate evaluation of intracranial aneurysms. Achieved dose reduction in CTA of other cerebrovascular diseases and carotid artery disease was at a comparable level. Similarly, subjective IQ and diagnostic performance with respect to arterial stenosis were not negatively affected. Overall, it can be concluded that tube voltage reduction, advanced IR, and CM volume optimization may represent the most effective combination for dose reduction in head CTA. Beyond radiation exposure, patient care can be improved via lower contrast media-related morbidity, as long as long as CM volume is reduced with care, in order to not impair the visibility of intracranial vessels and associated diagnostic performance.

When comparing dose reductions of NCCT and CTA, one important difference regarding the required contrast has to be acknowledged: in comparison to NCCT of cerebral ischemia or ICH, blood vessels in CTA are high-contrast objects which therefore require a significantly lower dose per se.

Until now, dose reduction in head CT has mainly been achieved through modified tube settings while advanced reconstruction techniques ensured acceptable IQ and high diagnostic performance. Most of these advances have been enabled by software innovations, but current and future developments in CT hardware will also very likely increase dose reduction. The clinical introduction of photon-counting CT (PCCT) represents a revolution in CT imaging, also with respect to radiation exposure [77, 78]. At the same radiation dose as conventional CT, better gray to white matter differentiation due to higher CNR and less image noise has been demonstrated. The ensuing high potential of PCCT for IQ improvement in NCCT of the head could translate into a radiation dose reduction of approximately 40% [79]. At a comparable dose as conventional single-energy CT, PCCT has been reported to reduce beam-hardening artifacts and improve IQ in arterial segments close to surrounding bone [80]. In combination with its ability of spectral material decomposition, it thus bears great potential to further reduce radiation dose for intracranial and carotid CTA, particularly in the presence of calcifications and plaques. Furthermore, the increasing clinical use of PCCT implies the routine acquisition of spectral data. With this evolution, virtual non-contrast (VNC) scans could make NCCT scans redundant in stroke or other head CT protocols requiring non-contrast and contrast-enhanced images, and thereby save a considerable amount of radiation dose. Clinical translation of AI in CT imaging is steadily increasing, which can be expected to unveil additional dose reduction opportunities for head CT, primarily on the image reconstruction side [25‐29]. Beyond the application of CNNs to accelerate MBIR algorithms, deep-learning techniques can enable the implementation of more complex functions in IR models [16]. Given that existing study results can be validated and reliably translated into the clinical routine, the use of AI will further reduce the required dose in head CT.

The present review article is not without limitations. Methodology and design of the included studies are quite heterogeneous, mostly due to the development of CT hardware and software over time, different CT manufacturers and models, and the fact that different departments have established different head CT protocols. As a result, different dose parameters (CTDI_vol, DLP, E) with different ranges of values have been used to determine the amount of dose reduction. Furthermore, absolute definitions of SD and LD protocol cannot be reasonably established in the context of the included studies. Consequently, the start parameters for dose reduction quantification could not be normalized which potentially compromises objectivity and comparability of dose reduction.

In conclusion, considerable dose reduction in NCCT and CTA of the head can be realized by global approaches, such as decreased tube voltage and ATCM, while advances in IR algorithms ensure diagnostic image quality. For CTA examinations, the combination with lower CM volume can be particularly effective for improved patient care. In the upcoming years, the ongoing clinical transition of novel CT acquisition and reconstruction techniques can be expected to enable additional dose reductions of 40% or more, and potentially further CM volume reduction. However, it will be essential that the image quality and diagnostic performance is thoroughly evaluated to guarantee that the benefits of head CT are not compromised.