Acute ischemic stroke (AIS) is a leading cause of mortality and permanent disability [1]. Computerized tomography (CT) based neuroimaging techniques continue to be the mainstay for selecting patients eligible for thrombolytic therapy. Non-contrast CT (NCCT) and CT angiography (CTA) of cervico-cerebral vessels are routinely performed for AIS, while CT perfusion (CTP) of the brain is performed in selected patients. One important reason for selecting CT-based imaging is the fast data acquisition in multi-detector row CT systems [2]. Recent clinical trials have demonstrated that CTP imaging of the brain using an increased detector width can detect additional potentially salvageable ischemic lesions even in the extended therapeutic time window [2, 3].

American Heart Association recommends NCCT to evaluate patients presenting with stroke-like symptoms to exclude various mimicking conditions [2]. An NCCT of the head excludes intracranial hemorrhage with high reliability. However, multimodal imaging with NCCT, CTA and CTP significantly increase diagnostic accuracy compared to NCCT alone. CTA depicts the presence and site of arterial occlusion as well as various collateral flow patterns. CTP further helps in cases with an uncertain time of symptom-onset, especially to evaluate the amount of salvageable penumbra and eligibility for various endovascular procedures in AIS [2, 3]. Importantly, whole-brain perfusion measurement can be achieved very fast and avoid any significant delays in the initiation of definitive therapy [2, 3].

CT scanners are the significant contributors to the radiation exposure received by patients in radiology departments, predisposing them to an increased risk of somatic and genetic effects of ionizing radiation [4, 5, 6, 7]. In addition to the brain, red bone marrow, thyroid, and the eyes’ lens are the most radiosensitive tissues for these CT procedures. Patients and clinicians often raise concerns about radiation exposure to various organs during computerized tomography (CT) based imaging. Therefore, various strategies are employed to minimize the radiation exposure associated with CT protocols for brain imaging. Some of these strategies involve changes in the acquisition parameters (kVp, mill-ampere, pitch, etc.) [5, 6]. Although described scarcely, the risks due to the radiation exposure are believed to be related to the dose received by various radiosensitive organs. Hence, evaluating patient doses during various imaging procedures is essential to justify repeated exposure and optimize them to balance the benefit against radiation risk [7, 8, 9]. We evaluated the amount of radiation exposure to the brain and various organs in our stroke patients during various CT-based imaging studies. We created a low-dose protocol to hypothesize that a low-dose protocol would be associated with less radiation exposure, specifically to individual organs, without reducing diagnostic accuracy. We compared radiation exposure between the low-dose protocol and the standard protocol used at our institution using the low-dose protocol. Additionally, we compared to control the image quality between low-dose and standard protocol.

The radiation data for standard-dose protocol for brain imaging for NCCT (n = 391), CTA (n = 149) and CTP (n = 61) were retrieved from the institutional dose tracking system attached to a dedicated CT scanner (Phillips iCT256 with iDose4). Data for the optimized low-dose protocol for NCCT, CTA, and CTP were obtained prospectively for 32 adult AIS patients (mean age 63 $\pm$ 17 years vs. 62 $\pm$ 17 years for standard-dose patients; p = 0.098). Details of various scan parameters such as kVp, mAs, collimation, scan length, rotation time and pitch for standard dose scanning of the head (AAPM) [11] and low-dose optimized stroke protocols for NCCT, CTA and CTP procedures are summarized in Table 1.

Our findings demonstrate that radiation exposure to the sensitive organs during standard dose neuroimaging protocols employed in AIS patients is within acceptable limits. However, the radiation doses to these organs may be reduced further by using low-dose CT imaging protocols and dose tracking software, without any significant deterioration in the image quality.

The preferred neuroimaging in AIS remains CT-based, mainly due to the widespread availability, rapid acquisition, and acceptable image quality [2, 3]. An NCCT is the first diagnostic test for rapid exclusion of a hemorrhagic stroke, identifying early ischemic signs as well as a quick estimation of tissue damage. Intravenous thrombolysis is often initiated in eligible patients immediately after the completion of NCCT. In patients without any contraindications to radio-contrast, CTA is performed according to the ‘bolus tracking’ technique and shows the arterial tree from the aortic arch to the circle of Willis. Identification of thrombotic occlusion of a relevant artery helps in early prognostication, prediction of response to intravenous thrombolysis, and early planning for endovascular treatment. Most comprehensive stroke centers employ additional parenchymal imaging to estimate the amount of salvageable penumbra, especially in patients presenting beyond the thrombolysis window. The preferred imaging in this subset of AIS patients is CTP [2]. Furthermore, additional CT scans may be needed in patients with clinical fluctuations, elevating the cumulative radiation dose exposure. Therefore, various radiation dose reduction strategies are often explored, such as modifying exposure factors like tube voltage, lowering tube current, and applying iterative reconstruction of CT images [12, 13, 14, 15]. Such strategies may carry a risk of degrading the image quality. This work estimated the radiation dose delivered to various radiosensitive organs in the head and neck region. Furthermore, we demonstrated that employing low-dose imaging protocols for NCCT, CTA, and CTP can reduce the radiation dose delivered to sensitive organs without compromising the image quality. An example of the image quality with standard radiation dose protocol and the low dose protocol for plain CT, CTA and CTP is shown in Fig. 1.

Fig. 1.

Comparative Image quality with standard radiation dose protocol and the low dose protocol for non-contrast-enhanced computed tomography (NCCT), CT angiography (CTA) and CT perfusion (CTP) in a 62 years old patient who presented with acute ischemic stroke. The top panel shows NCCT, CTA and CTP images performed using the standard radiation dose protocol. As a part of a research project, this patient underwent repeat imaging studies using the low radiation dose protocol. The resultant representative images are shown in the lower panel.

Decreasing the peak voltage (kV) results in a lower dose exposure to the imaged tissues proportional to the square of the change in tube voltage. Furthermore, CT manufacturers have also introduced new techniques of dose reduction, such as dose modulation and iterative reconstruction (iDose4, ADMIRE, and ASIR) to achieve these aims [15, 16, 17]. While evaluating the feasibility of low-dose imaging protocol by reducing tube current or current and tube voltage together. We ensured that the level of image quality remained acceptable for diagnostic purposes. We decreased the tube current during stroke imaging protocols for NCCT, CTA as well as CTP. Additionally, we decreased kVp with lower mAs for CTA only to increase the effectiveness of contrast enhancement [15, 16, 17].

CTDI vol and DLP are routinely reported on scanner consoles in clinical practice. They do not quantify the patients’ dose metrics and provide only the output radiation of the CT scanners determined by many factors, including kVp, mAs, collimation, pitch, slice thicknesses and reconstruction techniques, etc. CTDI vol is a good index when comparing protocols and technical parameter settings. However, it overestimates skin dose if the scan is performed without table movement. We estimated that the CTDI vol value was less than the reference value of 60 mGy for NCCT brain set by ACR guidelines based on AAPM and ICRP recommendations [10, 11, 18]. We used lower mAs in the low-dose protocols (decrease tube current from 330 to 280 mAs) that resulted in a 15% reduction (2.36 mSv) of the radiation dose during NCCT. While trying the low-dose scanning protocols, we found acceptable image quality for CTP at 80 kV and 100 mAs, whereas CTA required100 kV and 200 mAs for similar results. All CT protocols at our center used the iDose4 iterative reconstruction algorithm as a standard scanning procedure, which has been described¹ (¹ACR Practice Guideline for the Performance of Computed Tomography (CT) Perfusion in Neuroradiologic Imaging 2017 (https://www.acr.org/-/media/ACR/Files/Practice-Parameters/ct-perfusion.pdf); accessed on 16 May 2020.) to ensure the better image quality of CTA and CTP in dynamic enhancing imaging within a reduced radiation dose [16].

Our work provides detailed organ-specific radiation dose values associated with the standard and low-dose scanning protocol for CT-based studies in AIS. Doses to individual organs may be more appropriate in estimating cancer risk from CT exposure [17, 18, 19]. Eye lenses are susceptible to radiation as they are directly in the radiation beam and exposed to higher radiation doses. ICRP (2012) has recently set the thresholds in absorbed dose to 500 mGy for the eye lens [19]. The radiation exposure to eye lens was only 197.58 mGy in the low-dose group when NCCT, CTA, and CTP were performed in a patient. Other sensitive organs such as skin and red bone marrow showed significantly reduced exposure in the low-dose protocol. For example, the estimated dose exposure to the thyroid gland was 25.35 mGy, which is similar to the reported literature [19, 20]. Importantly, the total effective dose during comprehensive scanning (NCCT + CTA + CTP) was 6.128 mSv in the low-dose protocol as compared 11.48 mSv in the standard scanning, a reduction of 46.6%. Furthermore, we used the statistical Iterative Reconstruction IDose4 algorithm for CT reconstruction, which ramps the image clarity and reduces noise compared to the standard filtered back projection (FBP) technique. This enabled comparable image quality even with significantly reduced radiation dose in all CT-based imaging protocols.

Some limitations need to be acknowledged. First, the standard dose and low-dose CT scans were not acquired in the same patient. Standard dose CT studies would have served as a reference against which the diagnostic quality and imaging findings of low-dose scans could have been assessed. However, it would have been unethical to expose the human subjects to additional radiation exposure. Second, the blinded neuroradiologists and stroke neurologists had a good agreement in reading various imaging studies. The low-dose CT studies might have failed to detect tiny parenchymal lesions in some patients. Last, there was no reference standard for CTP studies. We strongly feel that faster MRI brain perfusion studies (with no ionizing radiation) in a smaller subset of the study population could have served as the standard control to evaluate the scan quality for CTP. However, this approach would have exposed our patients to an additional dose of a nephrotoxic contrast agent. Acute stroke care had changed considerably since this research work was performed. An increasing number of endovascular interventions are being performed, especially among patients with large vessel occlusions. This certainly increases the amount of radiation exposure. However, we cannot comment on this aspect since no acute stroke interventions were performed.

Furthermore, reduction of radiation exposure in CTP did not degrade the quality of images. However, one should be careful in optimizing the post-processing of the source images since the reduced dose might lead to clinically relevant infarct core overestimation in individual cases [21]. Lastly, Dose Area Product (DAP) size or Kerma Area Product (KAP) size are currently used to represent patient exposure during diagnostic X-ray examinations and interventional procedures. DAP is greatly affected due to scattered radiation being the primary source of exposure (mostly during interventional procedures). However, this information was not collected.

Radiation dose is a significant concern for clinicians and patients with the increased use of CT-based examination. Our data show that although the standard dose techniques deliver an acceptable radiation dose, a low-dose scanning protocol may be adopted while maintaining acceptable image quality. Continuous radiation monitoring, controlling and optimizing stroke CT protocols with dose tracking software will help implement best practices for the safe use of ionizing radiation in medical imaging.

AIS, acute ischemic stroke; ALARA, as low as reasonably achievable; CT, computerized tomography; CTA, computerized tomographic angiography; CTP, computerized tomographic perfusion; DLP, dose length product.

SCK and VKS conceived and designed the study; SCK and JK performed the radiation dose measurements; SCK wrote the initial draft of the paper; JK and VKS provided critical revision and final approval.

The study was approved by institutional ethics committee (NHG DSRB 2012/00517).

We thank Arvind K Sinha for technical guidance during the study. We also thank the anonymous reviewers for their constructive comments and suggestions.

The study was supported by a research grant from National Medical Research Council, Singapore (NMRC//CSA/037/2012).

The authors declare no conflict of interest.