Development of a New Measurement System for Radionuclide-Contaminated Wounds and Its Measurement Methods

WANG Wei¹, WANG Yujian¹, LI Xinglong¹, PANG Hongchao¹, LUO Zhiping¹
¹ China Institute of Atomic Energy, Beijing, 102413

Abstract: Due to the current lack of devices and methods for measuring internal contamination caused by radionuclides in wounds that may occur during spent fuel reprocessing and radioisotope production, we developed a new non-contact measurement system for radionuclide-contaminated wounds featuring improved resolution, moderate size, and clinical applicability. This system employs imaging and energy spectroscopy measurement technology to quantify the amount and distribution of contamination retained in wounds. Using this system, we established a measurement method to estimate the retention depth and activity of nuclides in injured tissue by leveraging differences in the absorption attenuation coefficients of X/γ rays emitted by retained nuclides across different tissue thicknesses. This enables identification of retained nuclides in wounds, measurement of nuclide depth and activity in various wound types, and determination of radioactive contamination distribution characteristics. The problem of measuring radionuclide-contaminated wound characteristics has been fundamentally solved. This study significantly advances existing technology for radionuclide-contaminated wounds domestically and internationally, improves internal irradiation monitoring and evaluation systems, and enhances occupational health protection for radiation workers.

Keywords: Radionuclide-contaminated wounds, Retention and distribution measurements, Internal radiation

1 Introduction

In the manufacturing, maintenance, and recycling of nuclear fuel assemblies [1] and the production of radioactive isotopes [2], various injuries (such as stabbing, cutting, explosions, and acid burns) frequently occur, causing nuclides to enter the human body. Over 90% of radioactive injury events involve the arms and hands (primarily fingers) through punctures and chemical burns [3]. Compared to inhalation and ingestion, intake via contaminated wounds allows radioactive isotopes to bypass the skin's natural defenses and directly enter the bloodstream, subsequently transporting to vital organs such as the liver and bones [4]. Even relatively low-level contamination can result in high effective doses from internal irradiation. Furthermore, long-term retention of certain insoluble nuclides at wound sites may induce local tissue cancer.

Therefore, detailed and accurate measurement and disposal of radionuclide-contaminated wounds must be performed immediately after trauma [5].

Detection technologies for radionuclide-contaminated wounds can be broadly categorized into indirect and direct measurement techniques [6-7]. Indirect measurement obtains pollutant types and concentrations through offline detection of wound samples, primarily using alpha particle detection and X-ray fluorescence analysis. While fast, these methods suffer from sampling representativeness issues due to inability to predict contamination distribution patterns. Additionally, sampling increases patient discomfort and risks secondary infection. Direct measurement technology [8-10] identifies radiation emitted by residual nuclides at wound sites through direct detection, supplemented by effective methodologies to obtain activity and depth information. This includes alpha/beta ray measurement analysis and X/γ ray measurement analysis, representing non-contact online measurement that is simple, direct, and eliminates infection risks associated with wound sampling. The X/γ ray method leverages the strong penetration capability of gamma/X-rays emitted by residual nuclides, enabling detection of deep wound contamination while overcoming measurement difficulties caused by preliminary medical cleaning or blood obstruction at wound surfaces. This approach has developed rapidly in recent years. Beyond traditional HPGe, NaI, or CsI detectors, yttrium aluminum perovskite (YAP:Ce) [11] detectors based on cerium crystal activation have emerged for detecting radionuclide-contaminated wounds, broadening the energy range for wound radioactivity measurement and nuclide identification while improving measurement effectiveness.

Current measurement technologies for radionuclide-contaminated wounds are limited to identifying residual nuclide types and determining surface contamination activity. For larger, deeper wounds requiring targeted medical intervention (such as surgical removal) even after initial cleaning, accurate localization and quantification of contamination distribution characteristics must be performed promptly. Currently, effective measurement methods are lacking, and no specialized commercial equipment exists. The conventional approach uses two or more detectors of different sizes for staged measurement: first, a large-volume probe with poor resolution roughly determines contamination extent, then a small high-sensitivity probe scans to identify maximum contamination points for activity measurement. The maximum contamination point is identified primarily through observation of detector counting rate changes. This method exhibits low efficiency in locating wound contamination distribution, is complex, time-consuming, labor-intensive, and prone to human error.

Combining measurement of wound contamination retention and distribution represents a current challenge facing detection technology for radionuclide-contaminated wounds.

This work establishes a new non-contact measurement system for radionuclide-contaminated wounds featuring improved resolution, moderate size, and clinical applicability. The system combines imaging and energy spectroscopy to measure wound contamination retention and distribution. Based on coded aperture imaging principles, it measures distribution characteristics of radionuclide-contaminated wounds, supplemented by direct measurement methods for retention based on X/γ energy spectroscopy analysis. This achieves comprehensive detection of radionuclide-contaminated wound characteristics, addresses shortcomings in existing measurement technologies, and enhances practical applicability.

2 Methods

2.1 Measurement of Distribution Characteristics of Radionuclide-Contaminated Wound Retention

We employ coded aperture imaging principles [12] to measure distribution characteristics of radionuclide-contaminated wound retention. A coded aperture imaging system [13] generally consists of position-sensitive detectors and heavy-metal collimators coded in specific patterns. The imaging process comprises encoding and image reconstruction stages. During encoding, the superimposed projection formed by radiation passing through the coded aperture collimator onto the position-sensitive detector is processed into a coded image through signal processing. Subsequent decoding algorithms reconstruct this coded image to restore the radiation source distribution. Figure 1 [FIGURE:1] presents a simplified mathematical model of the coded aperture imaging principle.

The total number of particles detected by the detector at position can be expressed as:

[Corrupted equation removed - original contained unreadable characters]

Where represents the radioactive intensity of the radiation source at position, is the imaging coding function (value of 1 for perforated elements, 0 for unperforated elements), and represents the collimation factor of the coded aperture. The angle of ray deviation when incident light passes through the imaging system, denoted as, represents the fundamental difference between near-field and far-field imaging for coded aperture systems. For far-field imaging, distance a approaches infinity and approaches 0. At this point, the equation exhibits perfect autocorrelation and can be simplified to a convolution operation, enabling high-quality radiation source image reconstruction through deconvolution. However, detecting distribution characteristics of radionuclide-contaminated wounds constitutes near-field imaging, where distance a cannot be ignored. Due to this term, the equation lacks autocorrelation and cannot directly reconstruct images through convolution, which would otherwise cause image artifacts [14]. In this case, calibration methods such as coded aperture collimator centralization must be employed to eliminate artifacts, and appropriate reconstruction algorithms must be selected to improve reconstruction quality and accuracy [15].

Image reconstruction algorithms for coded aperture imaging primarily include correlation decoding algorithms and Maximum Likelihood Expectation Maximization (MLEM) algorithms [16]. The MLEM algorithm [17] first assumes an initial source distribution, calculates the projection of this assumed source, compares it with actual imaging results, and iteratively corrects the source distribution multiple times based on comparison differences until it approaches the true distribution. Considering this method's effectiveness in suppressing noise and improving signal-to-noise ratio, this work employs this algorithm to reconstruct wound contamination distribution images. The iterative formula of MLEM is as follows [18]:

[Corrupted equation removed - original contained unreadable characters]

Where represents the reconstructed image weight of the j-th pixel in the radiation source plane; represents the actual photon count received by the i-th pixel in the detector plane; is the system response matrix, representing the probability that radiation emitted by the j-th pixel of the source plane is received by the i-th pixel of the detector plane; and k represents the number of iterations.

2.2 Measurement of Depth and Activity of Radionuclide-Contaminated Wound Retention

By utilizing the energy spectroscopy recognition and analysis capabilities of the new measurement system, we estimate retention depth and activity of nuclides in injured limbs based on differences in absorption attenuation coefficients of X/γ rays across different tissue thicknesses.

Measurement of retention depth and activity is based on preliminary identification of residual radioactive isotope categories at wound sites by the measurement system. First, soft tissue-equivalent material simulates detection efficiency calibration of the measurement system for injured limbs. A standard point source with known activity representing the retained nuclide is placed under different thicknesses of soft tissue-equivalent material coatings to measure the net counting rate (cps) of characteristic X/γ-ray full-energy peaks emitted by the point source. Measurement conditions and duration remain constant for each trial. The characteristic X/γ-ray full-energy peak efficiency under different thicknesses is calculated by:

$$\varepsilon = \frac{cps}{S \cdot \gamma}$$

where S is source activity and is the branching ratio of the full-energy peak. Exponential fitting of these results yields calibration curves and fitting relationships for different soft tissue-equivalent material thicknesses, thereby determining the linear attenuation coefficient μ of the soft tissue-equivalent material for emission characteristic X/γ rays of the retained nuclide.

For puncture wounds with unknown contamination retention depth, assuming contamination retention depth d and injured limb thickness C, the system first determines wound contamination distribution characteristics. Energy spectroscopy analysis and measurement are then performed on suspected puncture contamination hotspots, maintaining constant measurement conditions and duration. Measurements are taken from both directions of the limb surface directly above and below the suspected puncture contamination hotspot (referred to as positive and negative), obtaining net counting rates of the retained nuclide emission characteristic X/γ-ray full-energy peaks, which satisfy:

$$cps_{+} = cps_{0} \cdot \varepsilon \cdot e^{-\mu d}$$
$$cps_{-} = cps_{0} \cdot \varepsilon \cdot e^{-\mu (C-d)}$$

where and represent net counting rates of the full-energy peak for characteristic X/γ rays from retained nuclides in the positive and negative directions, respectively, without organized absorption layers. Comparing these equations yields:

$$\frac{cps_{+}}{cps_{-}} = e^{-\mu(2d-C)}$$

Considering the isotropy of the radioactive source, and are approximately equal. The equation can be simplified as:

$$d = \frac{C}{2} - \frac{1}{2\mu} \ln\left(\frac{cps_{+}}{cps_{-}}\right)$$

Substituting the attenuation coefficient μ determined through calibration into this equation allows inference of pollutant retention depth d in the wound. Combined with the system detection efficiency calibration relationship and the efficiency equation, the pollutant retention activity S in the wound can be obtained.

3 System Design

3.1 Detector Selection

We selected a position-sensitive cadmium zinc telluride (CZT) detector as the basic detector for the measurement system. CZT operates at room temperature without refrigeration, offers high stopping power and good energy resolution, and can detect a wide energy range from tens of keV to 1.5 MeV. Pixelated CZT detectors use microelectronic photolithography technology to achieve segmented electrode design on the crystal, effectively recording energy deposition location through extremely small square metal electrodes. Additionally, due to the small-pixel effect, low-energy tail phenomena in pixelated CZT detectors are effectively improved, enhancing energy resolution capability. Figure 2 [FIGURE:2] shows the MPS-256B pixelated CZT detector selected for development, with a crystal pixel size of 1.6mm × 1.6mm × 5.0mm, overall dimensions of 25.6mm × 25.6mm × 5.0mm, and a total of 16 × 16 channels.

3.2 Design of Coded Aperture Collimator

(1) Basic Coding Method and Structural Design
To adapt to CZT detector dimensions and effectively improve transmittance, we selected Modified Uniformly Redundant Array (MURA) as the basic coding method for the imaging system collimator. From the perspective of maximizing sensitive detection area utilization while satisfying MURA coding rules, we selected MURA (13 × 13) as the coding function, corresponding to an effective detection area of 20.8mm × 20.8mm for the CZT detector, with 169 pixels actually used. Additionally, to expand the field of view and increase spatial detection range while ensuring the detector surface receives complete projection of the coding pattern, the MURA array was extended one cycle in all directions to form a nested MURA coded aperture collimator.

(2) Design of Imaging Performance Indicators
Key performance indicators include imaging angular resolution and field of view. Figure 3 [FIGURE:3] illustrates the Full Coded Field of View (FCFOV) and angular resolution Δθ of the embedded MURA coded aperture imaging system.

$$FCFOV = 2 \cdot \arctan\left(\frac{b \cdot d_m}{2a}\right)$$
$$\Delta\theta = \arctan\left(\frac{\lambda}{a+b}\right) = \arctan\left(\frac{d_m}{D_d}\right)$$

where is the edge length of the detector's effective sensitive area, is the edge length of the nested coded aperture collimator, is the edge length of the basic coded aperture collimator, and λ is the geometric resolution of the coded aperture collimator. For two ideal point sources (P1 and P2) separated by λ in space, reconstruction through the coded aperture collimator yields a center distance between reconstructed images exactly equal to the Full Width at Half Maximum (FWHM) of the system Point Spread Function (PSF). Equations (8) and (9) demonstrate that full field of view and angular resolution are mutually balanced and cannot be improved simultaneously. However, for near-field imaging, these two indicators can be balanced by adjusting distance a to meet application requirements. In this design, system angular resolution Δθ is prioritized at less than 5°; subsequently, while ensuring angular resolution, distance a is adjusted to maximize imaging field of view and improve imaging efficiency.

(3) Design of Practical Performance Indicators
Considering that radioactive contamination activity levels in wounds are generally low (approximately 10⁴ Bq), imaging efficiency should be maximized to reduce measurement time, and imaging field of view should be expanded to minimize measurement frequency. Therefore, the distance (a+b) between the source plane and detector plane is limited to less than 10 cm. Additionally, considering collimator processing difficulty, coded aperture dimensions should be greater than or equal to 1 mm.

Based on these considerations, final design parameters and specifications for the radionuclide-contaminated wounds measurement system are determined as shown in Table 1 [TABLE:1]. The actual coded aperture collimator fabricated using 3D printing is shown in Figure 4 [FIGURE:4]. The 3D printing substrate material is resin, filled with pure tungsten material in the coded aperture to provide self-supporting capability for the collimator.

Table 1 Design parameters and specifications of the radionuclide-contaminated wounds measurement system

System Component Design Parameter Value Coded aperture collimator Basic size 1.3mm×1.3mm×4mm Size after nesting 16.9mm×16.9mm×4mm Aperture size 32.5mm×32.5mm×4mm Material W (Purity 99%) Detector Basic size 25.6mm×25.6mm×5mm Pixel size 1.6mm×1.6mm×5mm Material Pixelated CZT Geometry Distance a (source to collimator) 88.67mm Distance b (collimator to detector) 88.67mm Performance FCFOV 3.92° Angular resolution Δθ <5°

3.3 Electronic System Design

The electronic design scheme is shown in Figure 5 [FIGURE:5] (a). High-voltage power supply provides negative bias to CZT after filtering. The CZT anode pixel signal is amplified and filtered by a dedicated ASIC chip (SRE4002) for pixelated CZT detector signals, generating trigger signals, hit channel number signals, and channel address signals based on comparator circuit output. Simultaneously, amplitude analog signals of corresponding channels are output.

Amplified analog signals, ASIC output event address signals, and hit channel number signals are transmitted to the STM32H743 microcontroller board. The STM32H743 transmits collected data to a computer via Ethernet in a specific format for data processing and image generation. The electronic module is divided into three parts: SRE4002 control and data transmission module, microcontroller control and power management module, and high-voltage and RC filtering circuit module, as shown in Figure 5 (b-d).

3.4 System Implementation

The measurement system prototype and performance testing bench were fabricated according to Table 1, as shown in Figure 6 [FIGURE:6] (a). To improve signal-to-noise ratio and imaging performance, the CZT detector and supporting electronic modules are sequentially placed within a 47mm × 47mm × 36mm tungsten shielding body with 4mm wall thickness, with the pre-designed coded aperture collimator installed at the front end, as shown in Figure 6 (b). To block light and resist environmental electromagnetic interference, the shielding body and electronic components are housed in a 1mm-thick aluminum enclosure, forming an integrated system structure with the experimental testing bench.

4 Experimental Testing and Result Analysis

4.1 Experimental Testing of Distribution Characteristics

(1) Imaging Function Test
Three ²⁴¹Am standard point sources (two with 3.0 × 10⁴ Bq activity and one with 2.589 × 10⁴ Bq activity) were placed simultaneously on the surface of soft tissue-equivalent material (density ρ=1.08 g/cm³, composition H 10%, C 12.5%, N 3.50%, O 74%) at thicknesses of 15mm, 25mm, and 30mm to test the imaging function. Each measurement ensured sampling of 7000 effective imaging events, with MLEM algorithm used for image reconstruction. The system response matrix was calculated using simulated values. The detector plane was divided into 169 pixels, and the source plane within the theoretical imaging field of view was divided into 2704 pixels, with 50 iterations. Imaging results are shown in Figure 7 [FIGURE:7].

The results demonstrate that soft tissue-equivalent material coatings up to 30mm thickness do not affect imaging performance. Thus, for ²⁴¹Am contamination at depths within 30mm of soft tissue, the system can effectively measure wound contamination distribution. Figure 8 [FIGURE:8] shows the imaging effect after optical superposition of a single ²⁴¹Am standard point source with actual hand anatomy.

(2) Imaging Performance Testing
Imaging performance indicators including angular resolution, FCFOV, and positioning accuracy were tested using a ²⁴¹Am standard point source (2 × 10⁴ Bq). The MLEM algorithm reconstructed images using a simulated system response matrix with 50 iterations. During testing, the ²⁴¹Am point source was moved every 10mm along horizontal X-axis and vertical Y-axis. Each measurement ensured 3700 valid imaging events, with average FWHM from Gaussian fitting of reconstructed point source images used as measured angular resolution. Peak position ranges from Gaussian fitting were used as measured FCFOV, with peak positions compared to actual source positions to determine positioning accuracy. Test results are shown in Table 2 [TABLE:2].

Table 2 Experimental test results for system imaging angular resolution, FCFOV, and positioning accuracy

Parameter X-axis Y-axis Actual point source location average - - Angular resolution 4.02° 4.02° Point source reconstruction position (pixels) - - Point source positioning accuracy <3mm <3mm

Averaging horizontal X-axis and vertical Y-axis angular resolutions yields a measured system angular resolution of approximately 4.02°. Linear fitting of actual versus reconstructed positions for X and Y axis point sources, combined with internal plane pixel divisions (52 × 52), yields a measured imaging field of view of approximately 99.16mm × 92.31mm. Comparison between actual and reconstructed positions shows system positioning accuracy within 3mm. These metrics achieve theoretical design goals and meet practical wound imaging measurement requirements.

(3) Imaging Efficiency Test
Imaging efficiency was tested using a ²⁴¹Am standard point source (2 × 10⁴ Bq) with MLEM reconstruction using a simulated system response matrix and 50 iterations. The point source was imaged every 5mm along horizontal X-axis and vertical Y-axis directions. Signal-to-noise ratio of reconstructed images was calculated at different sampling volumes to evaluate reconstruction quality and determine imaging efficiency. Additionally, since iteration number affects reconstruction quality, different iteration counts were applied to each dataset. Results are shown in Table 3 [TABLE:3].

Table 3 Experimental test results for system imaging efficiency

Iterations Data Collection Volume (events) Signal-to-Noise Ratio 20 2000 12.5 20 3500 15.2 50 2000 11.8 50 3500 14.9

For the ²⁴¹Am point source, with constant sampling volume and 20 iterations, the signal-to-noise ratio is optimal. Excessive iterations increase noise peaks. With constant iteration number, the signal-to-noise ratio trend flattens when sampling exceeds 2000 events, indicating high imaging quality has been achieved. Therefore, a minimum of 2000 sampling events meets imaging quality requirements. Collection of 3500 events from a ²⁴¹Am point source requires 2-4 minutes (2 minutes for central field-of-view positions, 4 minutes for edge positions). Thus, sampling time for 2000 events is approximately 2 minutes.

(4) Energy Spectroscopy Performance Testing
To enable nuclide identification and quantitative estimation of retention depth and amount, energy spectroscopy performance was tested. Four gamma standard point sources—²⁴¹Am (59.54 keV), ⁵⁷Co (121.78 keV, 244.7 keV), ¹⁵²Eu (344.3 keV), and ¹³⁷Cs (661.66 keV)—were used to calibrate all 169 channels in the CZT crystal. Figure 9 [FIGURE:9] shows accumulated energy spectra for ²⁴¹Am, ¹³³Ba, and ¹³⁷Cs after calibration.

Analysis reveals energy resolution (FWHM) of approximately 8.43% for the ²⁴¹Am 59.5 keV full-energy peak, 2.28% for the ¹³³Ba 356 keV peak, and 2.1% for the ¹³⁷Cs 662 keV peak.

4.2 Experimental Testing of Residual Depth and Activity

(1) System Detection Efficiency Calibration
Using typical nuclide ²⁴¹Am as an example, soft tissue-equivalent material (ρ=1.08 g/cm³, H 10%, C 12.5%, N 3.50%, O 74%) simulated the injured limb. A ²⁴¹Am standard point source (2.589 × 10⁴ Bq) was placed under varying thicknesses of soft tissue-equivalent material, measuring the net counting rate (cps) of the ²⁴¹Am 59.5 keV full-energy peak. The system probe was positioned as close as possible to the material surface, with the point source center aligned to the detection crystal center, maintaining constant measurement conditions. Each measurement lasted 180 seconds. The calibration curve ε(d) was calculated and is shown in Figure 10 [FIGURE:10]. Using a 35.9% branching ratio for ²⁴¹Am 59.5 keV, the fitting relationship satisfies:

$$\varepsilon = 0.0093 \cdot e^{-0.044 \cdot d}$$

From Figure 10 and this equation, the linear attenuation coefficient μ of soft tissue-equivalent material for ²⁴¹Am 59.5 keV rays is approximately 0.044/mm.

(2) Measurement and Verification of Retention Depth and Activity
Method validation was performed using ²⁴¹Am contaminants (2.589 × 10⁴ Bq) trapped at known depths of 9.68mm, 17.82mm, 27.93mm, and 8.14mm below the surface of a 36.13mm-thick injured limb. First, the measurement system determined wound contamination distribution characteristics. The probe was then positioned close to the limb surface, aligning the suspected puncture contamination hotspot with the detection crystal center. Net counting rates of the ²⁴¹Am 59.5 keV full-energy peak were measured from positive and negative directions, with 180-second measurement duration. Equation (7) estimated radioactive contamination retention depth, combined with the ε(d) fitting relationship and efficiency equation to estimate retention activity. Verification results are shown in Table 4 [TABLE:4].

Table 4 Measurement and verification results for retention depth and activity of radionuclide-contaminated wounds

True Depth (mm) True Activity (Bq) Net Count Rate (cps) Estimated Depth (mm) Depth Deviation (%) Estimated Activity (Bq) Activity Deviation (%) 9.68 2.589×10⁴ 125.3 9.85 1.8 2.72×10⁴ 5.1 17.82 2.589×10⁴ 68.7 18.21 2.2 2.66×10⁴ 2.7 27.93 2.589×10⁴ 28.4 28.51 2.1 2.71×10⁴ 4.7 8.14 2.589×10⁴ 142.6 8.43 3.6 2.68×10⁴ 3.5

Using soft tissue-equivalent materials to approximate injured limbs, this method proves feasible for estimating ²⁴¹Am contamination depth and retention activity. Verification shows depth calculation deviation within 5% and activity deviation within 5%, both within acceptable ranges.

5 Conclusion

In response to the current lack of effective measurement devices and methods for radionuclide-contaminated wounds, we developed a new non-contact measurement system combining imaging and energy spectroscopy to measure contamination retention and distribution. The system identifies residual nuclides, measures depth and activity for different wound types, and determines contamination distribution characteristics. Testing demonstrates an imaging angular resolution of approximately 4.02°, field of view of 99.16mm × 92.31mm, positioning accuracy better than 3mm, and imaging efficiency of approximately 2 minutes for a ²⁴¹Am standard point source (2 × 10⁴ Bq). Energy resolution (FWHM) is approximately 8.43% at 59.5 keV. These metrics meet wound radioactivity measurement requirements. Additionally, leveraging the system's energy spectroscopy capabilities, we established a method to estimate residual nuclide depth and activity based on differential X/γ ray absorption attenuation across tissue thicknesses. Validation confirms the method is simple, reliable, and clinically practical, with measurement deviations within acceptable ranges.

The new measurement system and method effectively solve the problem of measuring and rapidly reconstructing distribution characteristics of radionuclide-contaminated wounds, advancing existing technology domestically and internationally. The system is suitable for rapid detection in fuel reprocessing, isotope production, and post-injury clinical measurement, providing effective basis for local and internal dose evaluation and medical treatment selection. This supplements and improves domestic radiation monitoring and evaluation systems, significantly increasing industry attention to internal irradiation hazards from contaminated wounds and strengthening occupational health protection for radiation workers.

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