Organ doses and normalized organ doses for various age groups in ultralow dose pediatric C-arm cone-beam CT

Development of a TOPAS Monte Carlo (MC) model extension to simulate the automatic exposure control function of a C-arm CBCT

PURPOSE

Accurate simulation of organ doses in C-arm CBCT is critical for estimating personalised patient dosimetry. However, system complexities such as automatic exposure control (AEC) and the incorporation of DICOM images into simulations are challenging. The aim of this study was to develop a model for mimicking the operation of an AEC system, which maintains a constant dose to the detector through mA modulation in order to facilitate more accurate MC dosimetry models for C-arm CBCT.

METHODS

A Siemens Artis Q Interventional Radiology (IR) C-arm system [Siemens, Erlangen, Germany] was modelled in TOol for PArticle Simulation (TOPAS) by incorporating system specifications such as rotational speed, number of projections and exam protocol parameters. A novel threshold scorer, AECScorer, was developed to model the AEC functionality. MC simulations were performed using a variety of imaged volumes including a CTDI phantom, an anthropomorphic phantom and a patient DICOM dataset.

RESULTS

The AECScorer extension provides a framework for a conditional scoring function within TOPAS which allows for the simulation of an AEC system. The AECScorer successfully equalises the dose to the detector for simple phantoms and DICOM imaging datasets by adjusting the number of histories simulated at each CBCT projection. This AECSCorer tool is applicable to other medical imaging systems requiring AEC simulation.

CONCLUSIONS

We demonstrate a novel threshold scorer in TOPAS for a C-arm CBCT setup. The presented AECScorer is the first step towards providing a system-, patient- and protocol-specific dose estimates from CBCT dosimetry applications.

Implementation and evaluation of the AAPM TG 111 for radiation dosimetry in a state-of-the-art C-arm cone beam CT system

The aim of this study was to assess the dosimetric characteristics of a state-of-the-art C-arm cone beam computed tomography (CBCT) system using the methodology proposed by the American Association of Physicists in Medicine (AAPM) Task Group (TG) 111. The dose measurement methodology described in AAPM TG 111 for wide cone beam acquisitions without table translation was employed to estimate equilibrium beam length (αeq‾) and equilibrium dose (feq‾) in various interventional task-specific protocols with different tube arc projection geometries. Dose profiles were derived from point dose measurements in the centre and peripheral locations of the ICRU/AAPM and standard polymethyl methacrylate (PMMA) body phantom. Dose measurements were performed in phantom and free-in-air using a solid-state point detector. Monte Carlo (MC) based simulation dosimetry was performed to quantify the inhomogeneous dose patterns imparted in the phantoms. Estimatedαeq‾andfeq‾on the ICRU/AAPM phantom was up to 49.4 cm and 6.17 mGy/100 mAs, respectively. Corresponding values determined on the PMMA phantom were 139 cm and 8.8 mGy/100 mAs, respectively. Free-in-air dose measurement ranged from 1.43 mGy/100 mAs to 5.93 mGy/100 mAs. Per cent difference inαeq‾andfeq‾between MC simulation and solid-state point detector measurement methods in the ICRU/AAPM phantom were within 16% and 18%, respectively. Manufacturers can use the presented methodology to characterize the dosimetric properties of C-arm CBCT systems. Clinical medical physicists may follow this methodology to verify corresponding data provided by the manufacturer and check for C-arm CBCT system performance dosimetric consistency.

The effect of added filtration on radiation dose and image quality in digital radiography of newborns

BACKGROUND

Repeated chest and/or abdomen radiographs are needed on mobile digital radiography (DR) units to assess infants' health status. Optimization of DR tube peak kilovoltage (kVp) and tube current time product (mAs) to derive images of diagnostic quality at as low as reasonably achievable radiation dose has been a challenging task.

OBJECTIVE

To investigate the effect of exposure parameters and additional filtration on entrance skin dose and image quality in newborn DR imaging.

MATERIALS AND METHODS

A physical anthropomorphic phantom that simulates the average full-term neonate was used. Chest and chest/abdomen DR images were acquired at the manufacturer's recommended kVp/mAs exposure parameters followed by a series of image acquisitions at different kVp/mAs and beam filtration combinations. Entrance skin dose (ESD) and signal difference to noise ratio (SdNR) for soft tissue, bone and feeding gastric tube were estimated in raw unprocessed images. A figure of merit (FOM) analysis provided the kVp/mAs and filtration that generated images of adequate quality at the lowest ESD.

RESULTS

Signal difference increased with kVp and progressively decreased with increasing filtration. Compared to the manufacturer's recommended 53 kVp/1.6 mAs exposure parameters, ESD was reduced by 76% (47.61 μGy versus 11.3 μGy) in the chest and 66% (47.61 μGy versus 16.14 μGy) in the chest/abdomen when the exposure parameters and additional beam filtration provided by the FOM analysis were used.

CONCLUSION

The results derived from this phantom study suggest that additional beam filtration along with the appropriate leverage of exposure parameters can lower ESD while maintaining image quality in full-term newborns.

Assessment of abdominal organ dose and image quality in varying arc trajectory interventional C-arm cone beam CT

PURPOSE

The aim of this study was to investigate the effect of varying arc exposure trajectory on radiation dose to radiosensitive organs and to assess image quality in abdominal C-arm cone beam computed tomography (CBCT) interventional procedures using a latest generation system.

METHODS

An anthropomorphic phantom that simulates the average adult individual was used. Individual-specific Monte Carlo (MC) simulation dosimetry was performed to estimate organ doses (OD) in abdominal C-arm CBCT. Seven examination protocols prescribed by the system for vascular and soft tissue CBCT, were simulated. These protocols are differentiated in the range of the arc exposure trajectory and the level of radiation dose delivered to the patient. OD was estimated for liver, adrenals, kidneys, pancreas, stomach, gall bladder, spleen, bone and skin. Image noise, signal to noise ratio (SNR), contrast to noise ratio (CNR) and in-plane spatial resolution were assessed using CT-specific image quality assessment phantoms.

RESULTS

OD was found to depend on the range of arc trajectory and was higher for posterior located organs. In vascular protocols OD ranged from 4.75 mGy for skin to 0.60 mGy for bone. Image noise was higher in vascular protocols than in soft tissue ones. SNR and CNR were significantly modified among different soft tissue protocols (P < 0.05). In-plane spatial resolution was found 0.80 lp/mm in vascular as opposed to 0.41 lp/mm in soft tissue protocols.

CONCLUSIONS

The current results may be used to estimate OD for different examination protocols and enable operators choose the appropriate acquisition protocol on the preprogrammed interventional task.