ARAS: an automated radioactivity aliquoting system for dispensing solutions containing positron-emitting radioisotopes
© Dooraghi et al. 2016
Received: 16 November 2015
Accepted: 19 February 2016
Published: 9 March 2016
Automated protocols for measuring and dispensing solutions containing radioisotopes are essential not only for providing a safe environment for radiation workers but also to ensure accuracy of dispensed radioactivity and an efficient workflow. For this purpose, we have designed ARAS, an automated radioactivity aliquoting system for dispensing solutions containing positron-emitting radioisotopes with particular focus on fluorine-18 (18F).
The key to the system is the combination of a radiation detector measuring radioactivity concentration, in line with a peristaltic pump dispensing known volumes.
The combined system demonstrates volume variation to be within 5 % for dispensing volumes of 20 μL or greater. When considering volumes of 20 μL or greater, the delivered radioactivity is in agreement with the requested amount as measured independently with a dose calibrator to within 2 % on average.
The integration of the detector and pump in an in-line system leads to a flexible and compact approach that can accurately dispense solutions containing radioactivity concentrations ranging from the high values typical of [18F]fluoride directly produced from a cyclotron (~0.1–1 mCi μL−1) to the low values typical of batches of [18F]fluoride-labeled radiotracers intended for preclinical mouse scans (~1–10 μCi μL−1).
According to the ORAMED (Optimization of RAdiation protection for MEDical staff) study, nearly one in five workers in nuclear medicine is likely to receive more than the legal dose limit for the skin (500 mSv per year) . To better comply with regulations and to enhance the safety of employees, protocols must be developed that minimize radiation exposure. Automated tools for handling radiation provide a promising approach to reduce radiation exposure . Furthermore, well-implemented automated systems reduce human error and, thus, allow for a streamlined workflow. For clinical applications, systems have been developed such as Intego™ (MEDRAD, Warrendale, PA), which dispenses and automatically delivers a prescribed dose of a radiotracer to a patient. With this system in use for the injection step of PET procedures, whole-body and extremity radiation exposures to nuclear medicine workers were significantly reduced by 38 and 94 %, respectively . Customized tools similar to Intego™ but developed for preclinical PET radiotracer synthesis and usage can be implemented to provide corresponding reductions in whole-body and extremity radiation exposures to radiation workers.
In regard to the development and production of radiotracers, a tool that allows for automated aliquoting of user-specified amounts from a batch of [18F]fluoride solution will eliminate the need for radiation workers to manually draw radioactivity. Moreover, this automated dispenser can be implemented in any step of the radiotracer development and usage pipeline, including not only aliquoting of [18F]fluoride after cyclotron bombardment to support multiple research or production runs but also aliquoting the radiotracer for delivery into a subject. However, a technical challenge faced in both of these applications is the small volume of original stock solutions and the even smaller volume of individual aliquots. For example, [18F]fluoride from the cyclotron may be delivered in as little as ~1 mL (or even down to several hundred microliters, depending on the cyclotron target design) and a batch of a PET probe for preclinical imaging in mice may be concentrated in ~1 mL. The volume for mice should typically be no more than 100 μL, and since the batch is potentially used over the span of several half-lives of F-18, this means the initial aliquots will have a significantly smaller volume, down to a few 10s of microliters.
To address the opportunity of significantly increasing safety and accuracy, we have developed ARAS, an automated radioactivity aliquoting system for dispensing solutions containing positron-emitting radioisotopes with particular focus on fluorine-18 (18F). ARAS consists of a solid-state radiation detector in series with a peristaltic pump. The detector comprises two 3 × 30 mm2 anti-parallel PIN Si diodes operated in current mode. Two diodes are used in order to suppress the background from long-range 511-keV photons produced from positron-electron annihilation. These are present when handling positron-emitting radioisotopes like 18F which is commonly used in PET and is the radioisotope considered in this work. For each batch of radioisotope, the detector is used to perform a one-time calibration to determine the initial reference radioactivity concentration. The peristaltic pump is used to deliver prescribed volumes of [18F]fluoride solutions based on the decay-corrected radioactivity concentration and the desired amount of radioactivity. The automated design of this system promises to reduce exposure to the operator compared to manual dispensing operations and manual measurements using a dose calibrator. In this work, we describe the design of the prototype system and characterize the system performance. We also present preliminary examples of possible usage in radiochemistry and in mouse tail vein injections.
Figure 1b shows an implementation of the setup. Lead bricks placed in front of the system (not shown) were used to attenuate the exposed radiation emanating from the tubing. For radiochemistry usage, the entire setup was placed in a lead-shielded cabinet. In order to make the pump, optical sensor, and radiation detector footprint as compact as possible, these components were housed in an enclosure separate from power and computer control connections, which could be placed outside of the radiation shielding. A USB DAQ (NI USB-6215, National Instruments, Austin, TX) was used to interface with the peristaltic pump, radiation detector, and optical sensor. An RS-485 to USB converter provided a USB connection to interface with the stage controller. Control software was developed in LabVIEW following an event-driven machine state design pattern.
Radiation detection technique
Signals from both photodiodes were amplified by a current-to-voltage amplifying stage. The output of the current-to-voltage gain stage of photodiode 2 was subtracted from that of photodiode 1, and a low-pass filtering stage was applied to yield a current output. The output signal was then calibrated to translate measured voltage to radioactivity concentration.
Calibrated the optical sensor.
Pumped solution until the liquid sensor triggered, indicating arrival of the liquid at the sensor reference position.
Pumped a known volume of solution to completely cover the radiation detector.
Read the radiation detector and saved information to file.
Retracted the radioactivity solution to a shielded region behind the reference position which served as a home position.
Moved the solution from the home position to the location of the tip of the tubing based on the linear flow rate and the volume of tubing in between.
Dispensed the requested radioactivity/volume and appended data to a file for record-keeping purposes.
Raised the stage and retracted the radioactivity solution to home position.
The user must ensure that the linear stage was lowered before initiating the dispense radioactivity routine. The algorithm development for the dispenser system required independent calibration data from the radiation detector as well as the peristaltic pump. Specifically, the output voltage from the radiation detector was calibrated to known radioactivity concentration, as measured by a dose calibrator and a precision scale. Similarly, the speed control voltage from the peristaltic pump was calibrated to flow rate, as measured by a precision scale and the known density of the liquid solution. Calibration data from the peristaltic pump and radioactivity detector were incorporated into the dispensing algorithm.
Radiation detector response
Two variations of the readout electronics for the radiation detector were considered for possible applications of ARAS. Specifically, the dispenser was assessed as a tool (1) to dispense a desired radioactivity or volume of [18F]fluoride in [18O]oxygen-enriched water ([18O]H2O) for laboratories in which each batch of radioisotope is used for multiple research projects or production runs (typical radioactivity concentration ~0.1–1 mCi μL−1) and (2) to infuse a prescribed dose of an [18F]fluoride-labeled probe through a catheterized mouse tail vein (typical radioactivity concentration ~1–10 μCi μL−1). For these distinct applications, the readout electronics of the radiation detector were identical except for a change in the gain of the current-to-voltage amplifier stage (see “Radiation detection technique” section). The “high”-gain and “low”-gain configurations varied in gain by a factor of ~20. A higher-gain configuration allows for an increased sensitivity. However, the higher-gain configuration is also susceptible to electronic saturation when high radioactivity concentrations are used, setting an upper limit of useable concentrations. To avoid this situation, selection of the operation range of the radiation detector must precede its use, according to predetermined application specifications. In our case, these two configurations were deemed adequate for these two extreme examples of use.
Both configurations were characterized to determine the relationship between voltage signal and radioactivity concentration. C-FLEX tubing (ID = 0.508 mm) was filled with 3 and 60 μCi μL−1 of an [18F]fluoride solution at the start of measurement for the high- and low-gain configurations, respectively, and secured over the radiation detector’s sensitive area.
where V b is the average background voltage and σ b is the standard deviation in the background voltage. Based on the independent calibration of voltage versus radioactivity concentration, the minimum voltage was then converted to a radioactivity concentration to yield the MDA.
Validation of dispensed volume
where V d is the dispensed volume and V r is the requested volume. An Excellence Plus XP Analytical Balance (XP205, Mettler Toledo, Columbus, Ohio) was used to measure volume given the density of water of 1.00 g cm−3.
Assessment of sterility of dispensed solutions
To assess sterility of the system and dispensing procedure, several samples of [18F]FDG were dispensed into sterile empty vials and the samples were tested via standard United States Pharmacopeia methods. Aseptic handling procedures were used during installation of the source vial and the collection vial. After a 24-h period for radioactive decay, two 100-μL aliquots were taken from each sample and mixed with soybean casein digest medium and fluid thioglycollate medium, respectively, and incubated for 14 days at 37 °C.
Assessment of residual radioactivity after dispensing
Residual radioactivity was tested using a [18O]water/[18F]fluoride solution. After dispensing multiple samples, liquid was automatically retracted from the needles and tubing by running the pump in reverse. The tubing (including the needles) was then removed and assayed in a dose calibrator. The residual activity was compared with the starting activity (after correcting for radioactive decay). The starting radioactivity amounts were 237, 91.6, and 255 mCi, each in a volume of 1 mL.
Test application I
where R d is the dispensed radioactivity measured with the dose calibrator and R r is the requested radioactivity.
Test application II
ARAS was also evaluated for infusing a selectable amount of a PET probe into a mouse via the tail vein. [18F]FDG was loaded into the source vial of the dispensing system. The start calibration routine (“LabVIEW algorithm” section) was modified so that at the end of the routine, the tubing was primed with the [18F]FDG solution. The mouse tail vein was catheterized, and the catheterization tubing was connected to the C-Flex pump tubing, with care taken to avoid an air pocket. A total of 100 μCi of [18F]FDG was requested for dispensing. The dispense radioactivity routine (“LabVIEW algorithm” section) was modified to exclude linear stage motion as well as exclude retraction of the radioactivity after injection. The entire anesthetized mouse was placed in the dose calibrator, and the dispensed radioactivity was confirmed. For imaging, the mouse was then placed in a custom-designed holder  and scanned in an Inveon preclinical PET tomograph (Siemens Preclinical Solutions, Knoxville, TN). To properly quantify the total activity in the mouse, attenuation correction was performed. To achieve this, the mouse was scanned with an X-ray MicroCT (Siemens Preclinical Solutions, Knoxville, TN). PET emission images were reconstructed with OSEM with attenuation correction applied. The experiment was performed a total of three times on three different mice.
Results and discussion
Radiation detector response
The minimum detectable activity (MDA) specifies the lowest amount of radioactivity that can be measured . To better evaluate the behavior of the radiation detector at low radioactivity concentrations, Fig. 4b shows the calibration data plotted on a log scale. A visual comparison of the two curves confirms a lower MDA available with the high-gain configuration compared to the low-gain configuration. The minimum voltage was converted to a radioactivity concentration using the equations shown in Fig. 3a to yield the MDA, which was 0.02 μCi μL−1 for the high-gain configuration and 0.3 μCi μL−1 for the low-gain configuration.
The maximum detectable activity specifies the highest amount of radioactivity that can be measured reliably. The radiation detector output saturated at 5 V. Extrapolation of the calibration data to 5 V yielded a maximum detectable activity of 120 and 2830 μCi μL−1 for the high-gain and low-gain configurations, respectively.
Minimum and maximum detectable activities. The dynamic range of the radiation detector spans four orders of magnitude
ARAS radiation detection limits
Minimum detectable activity (μCi μL−1)
Maximum detectable activity (μCi μL−1)
Validation of dispensed volume
Assessment of sample sterility
Since, in some applications, the dispensed samples would be used in small animals, or potentially even human subjects, it is critical that sterility be preserved during operation. Sterility testing was performed for three dispensed samples, and no evidence of bacterial or fungal growth was observed after the incubation period. Furthermore, the tubing is in principle disposable, providing another way that sterility can be maintained.
Residual radioactivity after dispensing
If the dispenser is used to aliquot different source solutions (e.g., different batches of radioisotope, or different PET probes), one must consider the effect of carryover. The amount of carryover was characterized using [18F]fluoride solution. From measurements taken on three separate occasions, 0.20 ± 0.06 % (n = 3) of the initial activity (corrected for radioactive decay) remained in the tubing and needles after the dispensing process (including retraction of liquid after dispensing). This corresponds to a residual volume of 2.0 ± 0.06 μL (n = 3). Depending on the distribution of this residual liquid within the fluid path, it could impact the calibration process when the second source solution is loaded since the volume of the detector region is only 6.1 μL. A simple way to mitigate this problem is to replace the tubing when switching from one source solution to another.
Test application I
Test application II
We developed ARAS, an automated radioactivity aliquoting system for dispensing solutions containing positron-emitting radioisotopes with particular focus on fluorine-18 (18F). The key to the operation of this system was a solid-state detector integrated in line with a peristaltic pump and computerized control of the motion of liquids in the calibrated system. The system demonstrated volume accuracy within 5 % for volumes of 20 μL or greater. When considering volumes of 20 μL or greater, delivered radioactivity was in good agreement with the requested radioactivity as measured independently with the dose calibrator. The detector operates in a DC current mode, where the radiation-induced photo-current is simply averaged over time to produce a steady signal proportional to the average rate of energy deposited in the Si diode. Thus, the response is insensitive to changes in normal lab temperatures where extreme changes in temperature are not expected. Moreover, the dual-diode scheme provides a measure of self-correction, since both front and rear diode channels are subject to the same changes in temperature.
The integration of the detector and pump led to a flexible system that can accurately dispense solutions containing radiolabeled probes in radioactivity concentrations directly produced from a cyclotron (~0.1–1 mCi μL−1), to lower activity concentrations intended for preclinical mouse scans (~1–10 μCi μL−1). Such a system has the potential to significantly reduce the exposures of personnel handling radioactive solutions for biomedical research or clinical applications, while at the same time streamline the workflow. Its small size and low cost offer an opportunity for multiple copies of such a system to be installed at the many steps along experiments utilizing radioactive solutions, where manual operations currently take place. The implementation of ARAS within a protocol that ensures sterility of the disposable tubing demonstrates a promising approach for radiation handling in application related to PET involving patients or animal studies.
This study was supported in part by the Department of Energy Office of Biological and Environmental Research (DE-SC0001249), the UCLA Foundation from a donation made by Ralph & Marjorie Crump for the UCLA Crump Institute for Molecular Imaging, and the UCLA Scholars in Oncologic Molecular Imaging program, NIH grant R25T CA098010. We thank Saman Sadeghi, Umesh Gangadharmath, and the staff of the UCLA Biomedical Cyclotron for providing the samples of [18F]fluoride and [18F]FDG and for performing sterility tests on samples. Finally, we thank Waldemar Ladno, Mark Lazari, Brandon Maraglia, David Prout, Olga Sergeeva, and David Stout for their input in the design and development of ARAS.
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