[Protection of eye lens in computed tomography--dose evaluation on an anthropomorphic phantom using thermo-luminescent dosimeters and Monte-Carlo simulations]

CONTRIBUTION OF ORGAN-BASED TUBE CURRENT MODULATION TO THE REDUCTION OF LENS EXPOSURE DOSE IN HEAD 4D CT IMAGING: A PHANTOM STUDY

The purpose of this study was to evaluate the effectiveness of organ effect modulation (OEM) in reducing the lens dose in 4D computed tomography (CT) of the head in volume-acquisition (NVA) mode. Six radiophotoluminescent dosemeters were placed on the head of a RANDO phantom. The doses absorbed by the organs and image noise change rate were determined. The lens doses without OEM (i.e. in the OEMoff case) were higher than those with the same target standard deviation and volume-computed tomography dose index (CTDIvol) as in the OEMoff case (p < 0.01). The image noise change rate was 11%. OEM reduced the lens dose during head 4D CT imaging in the NVA mode by 18%. Furthermore, the feasibility of lens dose reduction while ensuring sufficient image quality was confirmed under the condition in which OEM was employed with the same CTDIvol as in the OEMoff case.

MEASUREMENT OF RADIATION DOSE TO THE EYE LENS IN NON-ENHANCED CT SCANS OF THE BRAIN

Radiation can have undesirable effects on the eye, including a gradual loss of vision. Unnecessary radiation can reach the eye lens during non-enhanced computed tomography (CT) scans of the brain. The International Commission on Radiological Protection states the threshold for acute and chronic eye lens exposure is 500 mGy and the equivalent dose limit for the eye lens for public exposure is 15 mSv per year. Therefore, we measured the direct radiation dose to the eye lens during head CT scans using NanoDots in 216 adults. The mean absorbed dose to both eyes was 33.62 mGy (standard deviation ±12.442). The averages for the other variables were: tube current-time product: 260 mAs; dose-length product: 708 mGy cm and CT dose index: 35.5 mGy. Our findings encourage further study of radiation exposure and modifications in CT imaging protocols.

Frequency and Diagnostic Implications of Image Artifacts by Eye-Lens Shielding in Head CT

OBJECTIVE

The eye lens is one of the most radiosensitive organs, and medical radiation is one of the main causes of cataracts. To protect the lens during head CT examinations, protectors have been developed; however, they can lead to image artifacts, which is a major disadvantage of their use. This study retrospectively evaluates the frequency and extent of artifacts caused by these protectors related to three anatomic regions (eye, brain, and bone) and their dependence on protector positioning.

MATERIALS AND METHODS

Datasets from 261 consecutive head CT examinations obtained during 3.5 months of routine clinical imaging were assessed. Diagnostic quality of the images was evaluated by objective measuring and subjective scoring on a 5-point Likert scale. Furthermore, the position of the lens protector in correlation to the eye lens and the intensity and frequency of artifacts were analyzed.

RESULTS

Only 4.6% of all analyzed examinations were completely free from artifacts; 95.4% showed artifacts at least in the orbital cavity. Although the brain was affected in 27.8% of cases, in only 5.8% of cases was there a risk of misinterpretation, such as suspected intracranial bleeding. In 24.9% of cases, the lens was not properly covered by the protector. A too cranial position of the protector was identified as the main risk factor for cerebral artifacts.

CONCLUSION

Eye shielding for brain CT examinations often leads to artifacts. However, in only a small percentage of cases do these artifacts affect tissue depiction in regions beyond the eye (i.e., brain or bones). Correct positioning is mandatory to minimize artifacts.

[Practical radiation protection of the patient in radiological diagnostics]

BACKGROUND

The use of radiation protection equipment can reduce the radiation exposure of patients.

OBJECTIVES

The aim was to show which patient shields should be used for the different types of examination.

METHODS

The results of multiple studies were compiled and analyzed and recommendations made for the use of patient shields. The absolute dose values and the protective effect were considered.

RESULTS

Radiological protection should be used in many investigations; particularly in the case of CT investigations, a reasonable dose reduction potential exists due to the higher radiation dose.

CONCLUSIONS

Based on these recommendations, workflow changes in some types of investigation are expected due to the use of additional patient shields.

Clinical evaluation of a dose monitoring software tool based on Monte Carlo Simulation in assessment of eye lens doses for cranial CT scans

INTRODUCTION

The aim of this study was to verify the results of a dose monitoring software tool based on Monte Carlo Simulation (MCS) in assessment of eye lens doses for cranial CT scans.

METHODS

In cooperation with the Federal Office for Radiation Protection (Neuherberg, Germany), phantom measurements were performed with thermoluminescence dosimeters (TLD LiF:Mg,Ti) using cranial CT protocols: (I) CT angiography; (II) unenhanced, cranial CT scans with gantry angulation at a single and (III) without gantry angulation at a dual source CT scanner. Eye lens doses calculated by the dose monitoring tool based on MCS and assessed with TLDs were compared.

RESULTS

Eye lens doses are summarized as follows: (I) CT angiography (a) MCS 7 mSv, (b) TLD 5 mSv; (II) unenhanced, cranial CT scan with gantry angulation, (c) MCS 45 mSv, (d) TLD 5 mSv; (III) unenhanced, cranial CT scan without gantry angulation (e) MCS 38 mSv, (f) TLD 35 mSv. Intermodality comparison shows an inaccurate calculation of eye lens doses in unenhanced cranial CT protocols at the single source CT scanner due to the disregard of gantry angulation. On the contrary, the dose monitoring tool showed an accurate calculation of eye lens doses at the dual source CT scanner without gantry angulation and for CT angiography examinations.

CONCLUSION

The dose monitoring software tool based on MCS gave accurate estimates of eye lens doses in cranial CT protocols. However, knowledge of protocol and software specific influences is crucial for correct assessment of eye lens doses in routine clinical use.

Optimized imaging of the midface and orbits

A variety of imaging techniques are available for imaging the midface and orbits. This review article describes the different imaging techniques based on the recent literature and discusses their impact on clinical routine imaging. Imaging protocols are presented for different diseases and the different imaging modalities.

Eye-lens bismuth shielding in paediatric head CT: artefact evaluation and reduction

BACKGROUND

CT scans of the brain, sinuses and petrous bones performed as the initial imaging test for a variety of indications have the potential to expose the eye-lens, considered among the most radiosensitive human tissues, to a radiation dose. There are several studies in adults discussing the reduction of orbital dose resulting from the use of commercially available bismuth-impregnated latex shields during CT examinations of the head.

OBJECTIVE

To evaluate bismuth shielding-induced artefacts and to provide suggestions for optimal eye-lens shielding in paediatric head CT.

MATERIALS AND METHODS

A bismuth shield was placed over the eyelids of 60 consecutive children undergoing head CT. Images were assessed for the presence and severity of artefacts with regard to eye-shield distance and shield wrinkling. An anthropomorphic paediatric phantom and thermoluminescence dosimeters (TLDs) were used to study the effect of eye lens-to-shield distance on shielding efficiency.

RESULTS

Shields were tolerated by 56/60 children. Artefacts were absent in 45% of scans. Artefacts on orbits, not affecting and affecting orbit evaluation were noted in 39% and 14% of scans, respectively. Diagnostically insignificant artefacts on intracranial structures were noted in 1 case (2%) with shield misplacement. Mean eye-lens-to-shield distance was 8.8 mm in scans without artefacts, and 4.3 mm and 2.2 mm in scans with unimportant and diagnostically important artefacts, respectively. Artefacts occurred in 8 out of 9 cases with shield wrinkling. Dose reduction remained unchanged for different shield-to-eye distances.

CONCLUSION

Bismuth shielding-related artefacts occurring in paediatric head CT are frequent, superficial and diagnostically insignificant when brain pathology is assessed. Shields should be placed 1 cm above the eyes when orbital pathology is addressed. Shield wrinkling should be avoided.

Dose exposure of patients undergoing comprehensive stroke imaging by multidetector-row CT: comparison of 320-detector row and 64-detector row CT scanners

BACKGROUND AND PURPOSE

Recently introduced 320-detector row CT enables whole brain perfusion imaging compared to a limited scanning area in 64-detector row CT. Our aim was to evaluate patient radiation exposure in comprehensive stroke imaging by using multidetector row CT consisting of standard CT of the head, CTA of cerebral and cervical vessels, and CTP.

MATERIAL AND METHODS

Organ doses were measured by using LiF-TLDs located at several organ sites in an Alderson-Rando phantom. Effective doses were derived from these measurements. Stroke protocols including noncontrast head CT, CTA of cerebral and cervical vessels, and CTP were performed on 320- and 64-detector row scanners.

RESULTS

Measured effective doses for the different scanning protocols ranged between 1.61 and 4.56 mSv, resulting in an effective dose for complete stroke imaging of 7.52/7.54 mSv (m/f) for 64-detector row CT and 10.56/10.6 mSv (m/f) for 320-detector row CT. The highest organ doses within the area of the primary beam were measured in the skin (92 mGy) and cerebral hemispheres (69.91 mGy). Use of an eye-protection device resulted in a 54% decrease of the lens dose measured for the combo protocol for whole-brain perfusion with the 320-detector row CT scanner.

CONCLUSIONS

Phantom measurements indicate that comprehensive stroke imaging with multidetector row CT may result in effective radiation doses from 7.52 mSv (64-detector row CT) to 10.6 mSv (320-detector row CT). The technique of 320-detector row CT offers additional information on the time course of vascular enhancement and whole-brain perfusion. Physicians should weigh the potential of the new technique against the higher radiation dose that is needed. Critical doses that would cause organ damage were not reached.