Original Research
Medical Physics and Informatics
April 23, 2014

Evaluation of Patient Effective Dose of Neurovascular Imaging Protocols for C-Arm Cone-Beam CT

Abstract

OBJECTIVE. The purpose of this study was threefold: to estimate the organ doses and effective doses (EDs) for seven neurovascular imaging protocols, to study the effect of beam collimation on ED, and to derive protocol-specific dose-area product (DAP)–to-ED conversion factors.
MATERIALS AND METHODS. A cone-beam CT system was used to measure the organ doses for seven neurovascular imaging protocols. Two datasets were obtained: seven protocols without beam collimation (FOV, entire head) and four with beam collimation (FOV, from the base to the top of the skull). Measurements were performed on an adult male anthropomorphic phantom with 20 metal oxide semiconductor field-effect transistor (MOSFET) detectors placed in selected organs. The DAP values were recorded from the console. The EDs of five protocols were also estimated using Monte Carlo simulations software. The ED values were computed by multiplying measured organ doses to corresponding International Commission on Radiological Protection tissue-weighting factors.
RESULTS. Without collimation, the EDs ranged from 0.16 to 1.6 mSv, and the DAP-to-ED conversion factors ranged from 0.035 to 0.076 mSv/Gy·cm2. For the four protocols investigated with beam collimation, the ED was reduced by a factor of approximately 2, and the DAP-to-ED conversion factors were reduced by approximately 30%. For the five protocols also estimated with the Monte Carlo method, the estimated EDs were in agreement (< 20% deviation) with those determined by the MOSFET method.
CONCLUSION. We have estimated ED for standard adult neuroimaging protocols in a 3D rotational angiography system. Our results provide a simple means of ED estimation using DAP console readings.
The C-arm cone-beam CT (CBCT) system is a relatively new x-ray imaging modality in interventional neuroradiology and endovascular procedures. In C-arm CBCT brain imaging, the patient's head is positioned in the center of the x-ray beam. During a rotation of the beam around the head, a large number of 2D images at different projection angles are acquired to reconstruct CT-like or 3D volumetric images. The 3D imaging capability of the C-arm CBCT has been shown to be valuable for evaluating low-radiopacity stents and assessing coil packing in small aneurysms [1] and for high-contrast assessment of the brain parenchyma and can be life-saving in situations where immediate detection is needed [2, 3].
Although the clinical benefit is evident, one cannot overlook the radiation-induced health risk for patients undergoing interventional procedures involving a C-arm CBCT system. For C-arm CBCT scans, studies have reported the effective dose (ED) values associated with cardiac imaging protocols, which involve irradiation primarily in the chest region of the patient [4, 5]. To the best of our knowledge, limited information is available about the ED associated with neurovascular imaging protocols of C-arm CBCT.
Fig. 1 —Anthropomorphic phantom. Photographs show anthropomorphic phantom (model 701-D, CIRS) loaded with metal oxide semiconductor field-effect transistor dosimeters (left) and placed on patient table of scanner (right).
In this study, we investigated the ED values of neurovascular imaging protocols of a C-arm CBCT system at our facility by physical measurements in an anthropomorphic phantom (Fig. 1) and by Monte Carlo simulation. In addition, protocol-specific dose-area product (DAP)–to-ED conversion factors for the neurovascular imaging protocols were derived for convenience in estimating ED during neurovascular interventional procedures.

Materials and Methods

C-Arm CBCT System and Neurovascular Scan Protocols

The C-arm CBCT system we investigated is an Allura Xper FD20/20 system (Philips Healthcare). Five cerebral CBCT protocols (XperCT series, Philips Healthcare) and two 3D rotational angiography protocols (3D-RA series, Philips Healthcare) were investigated (Table 1). The direct results were evaluated as ED (in millisieverts) per scan; these protocols have defined exposure times and numbers of images. To study the effect of beam collimation, we measured four of the five XperCT protocols with two collimation settings: uncollimated beam (FOV, entire head) and collimated beam (FOV, from the base to the top of the skull). See Figure 2 for sample scan images. The fifth XperCT protocol was not included because of the small default FOV, which was only 22 cm diagonal. Finally, protocol-specific DAP-to-ED conversion factors were determined for all five XperCT and 3D-RA protocols, both with and without beam collimation.
TABLE 1: Neurovascular 3D Imaging Protocols of Allura Xper C-arm CBCT (Philips Healthcare) Using Manufacturer's Default Settings
ProtocolXperCT Cerebral High DoseXperCT Cerebral Low DoseXperCT Cerebral Fast High DoseXperCT Intracranial Stent High DoseaXperCT Intracranial Stent Low Dosea3D-RA Cerebral Propeller3D-RA Cerebral Roll
Tube voltage (kV)12012012080808080
Tube current (mA)250250170260260137138
Exposure time (ms)5557777
Frames/second30306030303015
No. of images621312624621621120120
Scan time (s)20.810.410.420.820.848
C-arm motionPropeller movementbPropeller movementbPropeller movementbPropeller movementbPropeller movementbPropeller movementbRoll movementc
Degrees/second10101010105530
Detector field (FD)28.8 × 38.4 cm (48 cm diagonal)28.8 × 38.4 cm (48 cm diagonal)28.8 × 38.4 cm (48 cm diagonal)28.8 × 38.4 cm (48 cm diagonal)16 × 16 cm (22 cm diagonal)28.8 × 38.4 cm (48 cm diagonal)28.8 × 38.4 cm (48 cm diagonal)
Filters0.4 mm Cu + 1.0 mm Al0.4 mm Cu + 1.0 mm Al0.4 mm Cu + 1.0 mm Al0 mm Cu + 0 mm Al0 mm Cu + 0 mm Al0.1 mm Cu + 1.0 mm Al0.1 mm Cu + 1.0 mm Al

Note—All XperCT and 3D-RA protocols are manufactured by Philips Healthcare. FD = from/flat detector.

a
XperCT intracranial stent high-dose and low-dose protocols differ only in field size.
b
Starting at right anterior oblique 120° and ending at left anterior oblique 120° with C-arm tube side below the patient.
c
From right 90° to left 90° with C-arm tube side below the patient.
Fig. 2 —Sample scan images of phantom (model 701-D, CIRS) using XperCT (Philips Healthcare) protocols without collimation (left) and with collimation (right).

Estimation of Effective Dose With Metal Oxide Semiconductor Field-Effect Transistor Dosimeters and an Anthropomorphic Phantom

An adult male anthropomorphic phantom (model 701-D, CIRS) was used to simulate an adult patient (Fig. 1). The phantom is made of materials equivalent to bone, lung, and soft tissue, with compositions based on previously published data [6] and using tissue substitutes for radiation dosimetry [7]. The phantom consists of 39 contiguous 2.5-cm-thick slices, with several 5-mm-diameter holes in each slice for dosimeter placement. Its specifications are listed in Table 2.
TABLE 2: Anthropomorphic Phantom Specifications
SpecificationValue
Height (cm)173
Weight (kg)73
Thorax dimensions (cm)23 × 32
Head thickness (cm) 
 Anteroposterior21
 Lateral17.1
Neck thickness at thyroid (cm) 
 Anteroposterior13
 Lateral14.6
Twenty high-sensitivity metal oxide semiconductor field-effect transistor (MOSFET) dosimeters (model TN-1002RD, Best Medical) were placed in selected organs and tissue types in the phantom (Fig. 1 and Table 3). Most MOSFETs were placed in the head and neck region (e.g., brain, skull, and thyroid), to measure organ doses from direct exposure in the FOV, and the others were placed in the chest region, to measure organ doses from scatter outside the FOV. The MOSFET dosimeters were calibrated by a 6-cm3 ion chamber (model 10X5-6, Radcal) at the beam qualities of the imaging protocols investigated. The general experimental techniques and validation of MOSFET dosimetry in CT have been described by Yoshizumi et al. [8].
TABLE 3: Metal Oxide Semiconductor Field-Effect Transistor (MOSFET) Dosimeter Placement in the Phantom
MOSFET No.OrganPhantom SliceLocation No.
1Skin3Posterior
2Skin3Anterior
3Bone marrow—skull32
4Bone marrow—skull33
5Brain—left36
6Brain—left39
7Brain410
8Brain411
9Lens of the eye514
10Lens of the eye515
11Salivary glands (bone surface)719
12Bone marrow—cervical spine822
13Thyroid1026
14Esophagus1232
15Lung1234
16Bone marrow—clavicle1238
17Bone marrow—sternum1339
18Thymus1341
19Bone marrow—thoracic and lumbar spine1348
20Breast1794
The ED values were estimated according to International Commission on Radiological Protection recommendations [9]. Because the skin, red bone marrow, and bone surface were partially irradiated in all of the investigated scan protocols, readings from the dosimeters placed in these organs were corrected to estimate average organ doses. For the skin, relative exposed area of exposure was estimated to be 9% of the total skin area, according to the “rule of nines,” which is often used to estimate the percentage of burn of the skin. For the red bone marrow, dosimeter readings from various locations were multiplied by the distribution of red bone marrow in different bones of a standard man [10]. For the bone surface, a 20% correction factor was applied according to the skeletal mass distribution in the head and neck region [11]. The reading correction can be expressed by the following formula:
Daverage = vDmeasured,
where Daverage is the average organ dose, v is the volumetric correction factor for the partially irradiated organs, and Dmeasured is the average dosimeter reading for the bone surface and red bone marrow and the maximum of the two skin dosimeter readings for skin (for conservative consideration). Similar dosimeter reading corrections were performed by Fujii et al. [12] in their pediatric CT dosimetry study.

Evaluation of Effective Dose With Monte Carlo Method

A commercial clinical x-ray simulation software program (PCXMC, STUK Radiation and Nuclear Safety Authority) was used as a parallel ED evaluation to the MOSFET method for the five XperCT protocols. The program uses Monte Carlo calculation and simulates the organ dose and ED of a mathematic hermaphrodite patient model [13].
To estimate the organ dose and the ED for each x-ray image during an XperCT scan, we entered into PCXMC projection geometries (e.g., x-ray source-to–rotational isocenter distance, source-to-image distance, projection angles, anatomy position, and detector field), exposure factors (tube voltage and filtration, with inherent filtration of 2.5 mm Al at 75 kVp) (Table 1), DAP readings (Table 4), and the patient size (weight and height). Technical parameters were obtained from the manufacturer (Philips Healthcare).
TABLE 4: Evaluated Effective Dose (ED), Dose-Area Product (DAP) Readings, and DAP-to-ED Conversion Factors, Evaluated by Metal Oxide Semiconductor Field-Effect Transistor (MOSFET) and Monte Carlo Methods
ProtocolMOSFET MethodPCXMC
Uncollimated BeamCollimated BeamUncollimated Beam
ED (mSv)DAP (mGy · cm2)DAP-to-ED Conversion Factor (mSv/Gy−1 · cm−2)ED (mSv)DAP (mGy · cm2)DAP-to-ED Conversion Factor (mSv/Gy−1 · cm−2)ED (mSv)
XperCT cerebral       
 High dose1.6 ± 0.024,3960.065 ± 0.0010.80 ± 0.0316,4430.048 ± 0.0021.85 ± 0.09
 Low dose0.80 ± 0.0212,2580.062 ± 0.0020.38 ± 0.0382540.047 ± 0.0040.93 ± 0.05
 Fast high dose1.1 ± 0.0116,2770.070 ± 0.040.51 ± 0.0310,9860.046 ± 0.0031.27 ± 0.06
XperCT intracranial stent       
 High dose1.7 ± 0.147,4250.035 ± 0.0010.80 ± 0.0430,3960.025 ± 0.0011.87 ± 0.09
 Low dose0.52 ± 0.0410,1290.052 ± 0.004 0.62 ± 0.03
3D-RA cerebral propeller0.16 ± 0.0135990.043 ± 0.003 
3D-RA cerebral roll0.24 ± 0.0131940.076 ± 0.003 

Note—PCXMC software is manufactured by STUK Radiation and Nuclear Safety Authority. XperCT and 3D-RA protocols are from Philips Healthcare. Dashes indicate no data were available.

The DAP, an important parameter for dose monitoring, was manually recorded for every XperCT run from the Allura Xper system's reported values. XperCT brain imaging protocols do not apply an automated dose control model; instead, the tube voltage and current are kept constant during C-arm rotation. Thus, the DAP values are independent of the patient size and are identical for each image frame. To calculate the DAP value for each frame used in PCXMC, we used the reported DAP per run in the system and divided it by the number of frames. With a rotation of 240° around the head and a number of image acquisitions, the organ dose and the ED of each projection angle and the total for the entire rotation were calculated.

Results

The ED, DAP readings, and DAP-to-ED conversion factors of the investigated scan protocols, with and without collimation, estimated by the MOSFET and Monte Carlo methods, are listed in Table 4.

Discussion

The ED values for the XperCT cerebral protocols behaved well according to their difference in scan parameters; for example, the XperCT cerebral high-dose protocol had twice the sampling (number of images) rate than did the XperCT cerebral low-dose protocol. Consequently, the estimated ED value of the former protocol is twice that of the latter. The XperCT intracranial stent protocols differ in detector size by only an approximate ratio of 4:1 (48-cm-diagonal rectangle vs 22-cm-diagonal square), and their ED values had an evident 4:1 ratio.
Beam collimation reduced the ED by approximately 50% for the four protocols investigated, both with and without collimation. All investigated protocols target the same FOV so the organ dose distributions are similar. The following six organs receive the most significant organ dose: skin, brain, red bone marrow, lens of the eye, salivary glands, and thyroid (Fig. 3). Furthermore, beam collimation greatly reduced the dose to the salivary glands and thyroid, which came out of the directly irradiated area with the applied collimation, and slightly reduced the dose to other organs, which were still within the directly irradiated area but received less scatter radiation with collimation. Therefore, if it is acceptable in procedural circumstances, beam collimation should be applied to reduce the patient's risks of cancer induction in the salivary glands and thyroid.
Fig. 3A —Main organ doses measured by metal oxide semiconductor field-effect transistor dosimeters using XperCT and 3D rotational angiography (3D-RA) protocols (both from Philips Healthcare). For all protocols, skin dose is maximum entrance skin dose with no volumetric reading correction.
A, Graph shows collimated and uncollimated data for XperCT cerebral high-dose protocol.
Fig. 3B —Main organ doses measured by metal oxide semiconductor field-effect transistor dosimeters using XperCT and 3D rotational angiography (3D-RA) protocols (both from Philips Healthcare). For all protocols, skin dose is maximum entrance skin dose with no volumetric reading correction.
B, Graph shows collimated and uncollimated data for XperCT cerebral low-dose protocol.
Fig. 3C —Main organ doses measured by metal oxide semiconductor field-effect transistor dosimeters using XperCT and 3D rotational angiography (3D-RA) protocols (both from Philips Healthcare). For all protocols, skin dose is maximum entrance skin dose with no volumetric reading correction.
C, Graph shows collimated and uncollimated data for XperCT cerebral fast high-dose protocol.
Fig. 3D —Main organ doses measured by metal oxide semiconductor field-effect transistor dosimeters using XperCT and 3D rotational angiography (3D-RA) protocols (both from Philips Healthcare). For all protocols, skin dose is maximum entrance skin dose with no volumetric reading correction.
D, Graph shows collimated and uncollimated data for XperCT intracranial stent high-dose protocol.
Fig. 3E —Main organ doses measured by metal oxide semiconductor field-effect transistor dosimeters using XperCT and 3D rotational angiography (3D-RA) protocols (both from Philips Healthcare). For all protocols, skin dose is maximum entrance skin dose with no volumetric reading correction.
E, Graph shows data for XperCT intracranial stent low-dose protocol. Only uncollimated data are shown.
Fig. 3F —Main organ doses measured by metal oxide semiconductor field-effect transistor dosimeters using XperCT and 3D rotational angiography (3D-RA) protocols (both from Philips Healthcare). For all protocols, skin dose is maximum entrance skin dose with no volumetric reading correction.
F, Graph shows data for 3D-RA cerebral propeller protocol. Only uncollimated data are shown.
Fig. 3G —Main organ doses measured by metal oxide semiconductor field-effect transistor dosimeters using XperCT and 3D rotational angiography (3D-RA) protocols (both from Philips Healthcare). For all protocols, skin dose is maximum entrance skin dose with no volumetric reading correction.
G, Graph shows data for 3D-RA cerebral roll protocol. Only uncollimated data are shown.
The 3D-RA cerebral propeller and 3D-RA cerebral roll protocols showed considerable difference in ED values (0.16 vs 0.24 mSv; Table 4), despite their comparability in most key imaging parameters. The difference in ED resulted largely from the different thyroid doses (0.3 vs 1.8 mGy; Fig. 3). The most likely reason is that, when the C-arm is under the roll mode, the rectangular detector rotates 90° and thus covers a longer length in the axial direction of the patient, resulting in the inclusion of thyroid in the FOV.
The lens of the eye must always be considered a critical organ when imaging the head region. In the latest International Commission on Radiological Protection guidelines, the threshold for cataractogenesis is 0.5 Gy [14]. For the seven imaging protocols studied, all involved x-ray tube rotation from the back of the patient to minimize direct unshielded irradiation to the lens of the eye. The doses to the lens of the eye measured for all protocols in our study are well below the 0.5-Gy threshold (Fig. 3). However, if a patient undergoes a combination of different protocols in each procedure, or multiple procedures in a prolonged period, such dose-reduction imaging specifications become crucial in the long term [15].
The DAP-to-ED conversion factors displayed great protocol specificity, as well as collimation specificity, except among the three XperCT cerebral protocols. For the first three protocols, the conversion factors are in general agreement, likely because they share the same key parameters, such as tube voltage, C-arm motion, field size, and beam filtration; hence, the variations in the conversion factors of the other protocols may be primarily due to the variation of these parameters. The DAP-to-ED conversion factors offer a means of estimating patient ED values when DAP values are available. Likewise, clinicians can use these factors to anticipate the level of radiation dose when choosing from different imaging protocols. On the other hand, one must ensure the exact matching of scan parameters before applying these conversion factors for patient-specific ED estimations. In addition, it must also be noted that the conversion factors are valid for the investigated protocols of this particular system only.
The ED values evaluated by the Monte Carlo method showed good agreement with those by the MOSFET method, with less than 20% deviation. The slight differences may be because the MOSFET method involved an anthropomorphic phantom, whereas the Monte Carlo simulations were performed on a medical international radiation dose–based mathematic phantom [6]. Because the head region and its organs' geometric orientations are relatively simple and very similar for both phantom models, the two methods still served as good validations for each other in the ED evaluations.
Currently, there has been a lack of dosimetry studies for C-arm CBCT, especially for head and neck imaging. To the best of our knowledge, ours is the first dosimetry study on the Allura Xper C-arm CBCT system. Bai et al. [4] have reported the ED of head protocols of the DynaCT (Siemens Healthcare), another C-arm CBCT system, to be 1.18 mSv (70 kV, 216 mA, and 20-second scan time) and 0.85 mSv (90 kV, 115 mA, and 8-second scan time). Our ED findings are in general agreement with those of Bai et al., with comparable key scan parameters (tube voltage, tube current, and scan time); however, because other scan parameters were not listed in their study, only general comparisons could be made.
In conclusion, C-arm CBCT is playing an increasingly crucial role in interventional neuroradiology procedures because of its advanced in-procedure CT-like and 3D imaging capabilities. The results reported from this study provide clinicians with important information on the radiation-induced risk to patients, such as the effect of beam collimation and source-to-image distance on ED. Furthermore, the determined protocol-specific DAP-to-ED conversion factors will aid in patient-specific dose estimation for this system in the future.

Acknowledgments

Technical support was received from Debora Morren, Amanda Morris, and David Enterline (Division of Interventional Radiology, Duke University Medical Center) and from Zhigang Qian, William Haynes, and Sjirk Boon (Philips Healthcare).

Footnote

This work was funded in part by a Philips Healthcare research grant.

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Information & Authors

Information

Published In

American Journal of Roentgenology
Pages: 1072 - 1077
PubMed: 24758663

History

Submitted: April 1, 2013
Accepted: July 31, 2013

Keywords

  1. effective dose
  2. interventional radiology
  3. metal oxide semiconductor field-effect transistors
  4. neurovascular imaging

Authors

Affiliations

Chu Wang
Medical Physics Graduate Program, Duke University, Durham, NC.
Duke Radiation Dosimetry Laboratory, Duke University Medical Center, Durham, NC.
Giao Nguyen
Duke Radiation Dosimetry Laboratory, Duke University Medical Center, Durham, NC.
Greta Toncheva
Duke Radiation Dosimetry Laboratory, Duke University Medical Center, Durham, NC.
Department of Radiology, Duke University Medical Center, Box 3155, Durham, NC 27710.
Xianxian Jiang
Philips Healthcare, Best, The Netherlands.
Andrew Ferrell
Department of Radiology, Duke University Medical Center, Box 3155, Durham, NC 27710.
Tony Smith
Department of Radiology, Duke University Medical Center, Box 3155, Durham, NC 27710.
Terry Yoshizumi
Duke Radiation Dosimetry Laboratory, Duke University Medical Center, Durham, NC.
Department of Radiology, Duke University Medical Center, Box 3155, Durham, NC 27710.
Department of Radiation Oncology, Duke University Medical Center, Durham, NC.

Notes

Address correspondence to T. Yoshizumi ([email protected]).

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