Potentially the most critical experimental consideration for the acquisition of reproducible measurements of radiation induced radical density is the positioning of the resonator relative to the tooth. The effect of the resonator position is reflected in both the filling factor and quality factor terms. The filling factor reflects the overlap of the B1
magnetic field established by the resonator with the locations of radicals in the tooth sample. The B1
magnetic field distribution, which defines the sensitive volume of the resonator, is nonuniform for surface loop resonators (He, et al., 2002
). Experimental measurements of the EPR signal amplitude recorded for a tooth sample with variable position relative to the resonator are shown in . Measurements were performed with a single molar tooth that had been irradiated to a dose of 30 Gy, as the center of the biting surface was translated vertically and horizontally with respect to the center of a 12 cm diameter detection loop. For each measurement, nine 10 second scans were averaged and were collected with an incident power of 40 mW, 22 G scan width, and a 4.8 G modulation amplitude. These data show a strong dependence of the EPR signal amplitude on the vertical position of the detection loop with a maximum at Δx = -4 mm, where the tooth is placed within the resonator and the biting surface is slightly above the central plane of the loop. There is a weaker dependence of the amplitude on the horizontal position of the resonator, with a broad maximum occurring at roughly +3 mm, where the tooth is under the detection loop with its center slightly biased toward the distal tip of the detection loop.
Figure 1 The EPR signal amplitude of a tooth irradiated to 30 Gy was measured as the location of the center of the resonator detection loop was varied both (a) vertically and (b) horizontally from the center of the biting surface of the tooth. The maximal signal (more ...)
Dosimetric measurements with the whole body clinical spectrometer are made with the subject in a lying position on a dedicated custom-made stretcher (). It is crucial that subjects are comfortably immobilized during the measurements, as any motion adds noise to the spectrum and can alter the position of the resonator on the tooth surface. Importantly, this setup allows all of the positioning procedures for placing the resonator on the teeth of the subject to be carried out with the subject outside of the magnet, where there is ample visual and tactile access. In addition to allowing precise positioning, this would facilitate the throughput of these measurements by limiting the time in the magnet to that needed for the actual measurements.
Measurements are made with the subject in a lying position on a stretcher that can be placed within the magnet. The resonator is mounted on a lockable articulating arm that is fixed to the subject bed.
Once the subject is lying comfortably in the desired position, the resonator is positioned on the surface of the tooth. A double ended cheek retractor (Hager & Werken GmbH & Co. Duisburg, Germany) is used to hold the lips and cheeks out of the way (). Prior to positioning the resonator, the maximal width of the tooth of interest and the width along the orthogonal dimension are measured using a set of dental calipers and a standard intra-oral dental mirror and recorded for use in normalizing for the tooth size prior to dose estimation. One or more small absorptive pads (Cotton Rolls No. 2 Medium, Crosstex International, Hauppauge, NY)(Dry Tips, Molnlycke Health Care AB, Goteborg, SWE) are positioned in the mouth to absorb saliva during the experiment (). The surface loop resonator is affixed to the end of an articulating arm that is mounted to the subject bed. (). The articulating arm (Fisso, Baitella AG, Zurich), was customized in house to replace several ferrous parts. It allows full 3D translation and rotation of the resonator so the detection loop can be precisely placed on the tooth of interest and robustly locked in position using a unified single-handed locking mechanism.
The placement of the resonator is further guided and stabilized using an individualized pair of dental casts (). These casts are constructed using commercial Exafast dental putty (GC America Inc., Alsip, IL). This putty requires approximately 5 minutes to harden and has been observed to have no confounding EPR signal or lead to significant reduction in the quality factor (Q) of the resonator. One piece of the cast is placed over the tooth of interest, mating with the row of teeth on one of its sides and with the detection loop on the other. A hole is cut in the center of this piece between the tooth and the detection loop allowing placement of the resonator directly on the tooth surface. A second cast piece is placed over the detection loop and couples with the first piece. The subject holds the resonator and the pair of casts in place by lightly biting on the combination (). This arrangement has several advantages in addition to the stability it confers in the placement of the detection loop on the tooth surface. The casts push both the tongue and cheek away from the resonator, which reduces the amount of lossy tissue near the resonator. This prevents the Q of the resonator from being further reduced and decreases the amount of variability in this factor during measurements and in between subjects. This system increases the comfort of the subject by reducing fatigue associated with keeping the mouth open. If repeated measurements are to be made for a given subject, these casts are re-usable and enable the resonator to be placed in precisely the same position. Measurements made with volunteers with single irradiated teeth have used casts that isolate the resonator from the opposing teeth, but measurements made with subjects with complete sets of irradiated teeth may use casts that position the detection loop in close proximity to both upper and lower teeth for simultaneous measurement and increased sensitivity.
Figure 3 Dental casts are constructed to allow for accurate and reproducible positioning of the resonator on the tooth or teeth of interest. Panel (a) shows the base of a cast coupled to the dentition with the resonator in place, the tooth-mating surface of the (more ...)
Once the resonator has been installed, the subject bed is rolled into the magnet so the tooth of interest and the detection loop of the resonator are positioned near the center of the magnet (). A pair of modulation coils that are fixed on arms that mount to the magnet frame are positioned such that the tooth lies along the axis defined by the coils. These coils are capable of providing modulation amplitudes of up to 5 G peak-to-peak. As the current coils are far from the Helmholtz configuration, due to geometrical constraints, they are significantly nonuniform. The modulation amplitude increases as the region of interest gets closer to either coil, increasing by up to 25% within a 4 in diameter central volume. Such nonuniformity can lead to substantial variation in the signal amplitude as the location of tooth varies from side-to-side or between subjects. In order to address this potential source of error, we have implemented a reference standard measurement which provides the modulation amplitude at the base of the detection loop.
The reference standard consists of a single small crystal of lithium phthalocyanine (LiPc) with a natural Lorentzian linewidth of 40 mG contained in an evacuated capsule. The amount of LiPc is large enough that its signal is much larger than that of an irradiated tooth, but spectra can be acquired with all the same instrumental settings (e.g., sensitivity and gain) as those used for measurements of the tooth alone. This capsule is fixed at the end of retractable rod that runs parallel to the transmission lines of the resonator. At the onset of a tooth measurement the rod is pushed forward such that the LiPc sample is at the proximal edge of the detection loop. After installing and tuning the resonator, a small set of spectra are acquired with 20 dB attenuation of the 100 mW of incident power used for tooth measurements and a nominal modulation amplitude of 4.0 G. These spectra are then analyzed using a least-squares fit with a spectral model that incorporates the effects of modulation amplitude and frequency (Robinson, et al., 1999
). With a known fixed natural linewidth, the modulation amplitude is treated as an adjustable parameter during the spectral fitting, resulting in a direct measurement of the modulation amplitude at the detector loop. Using this measurement, the amplitude of the low frequency source can be adjusted to set the modulation amplitude at the tooth to the desired value of 4.0 G. With this adjustment completed, the LiPc sample is removed from the sensitive volume of the resonator by retracting the supporting rod and the RF power is returned to 100 mW for tooth measurement. The removal of the LiPc standard does not affect the position of the resonator detection loop or significantly affect the loaded Q
of the resonator. In addition to providing the modulation amplitude, development is underway to also use the LiPc reference standard to provide a measurement of the amplitude of the B1
magnetic field generated by the resonator. With online fitting of the LiPc spectra, this measurement and adjustment of the modulation amplitude can be completed in approximately 1 min.
Finally, a larger set of spectra are acquired of the dosimetric tooth signal. Typically, 60 scans are acquired with a 3-sec sweep time, 25 Gauss scan range, 100 mW of incident power, with 1024 points and a 30 ms time constant. Including the delays between scans, these scans are completed within 4 minutes. Longer measurements, with increased averaging, can be performed to increase the signal-to-noise ratio; this might be especially useful after an initial screening has placed most subjects into appropriate categories, enabling measurements to be made more precisely when the initial result was borderline. Using the data acquisition methodology described here and the data analysis techniques described by Demidenko et al. (Demidenko, et al., (In Press)
) in vivo
dosimetry measurements have been performed in normal volunteers with irradiated single-tooth dentures with standard error of prediction equal to ±184 cGy. Averaging similar measurements made on 3 days reduced this standard error of prediction to ±46 cGy. Refinements to this methodology are continuous, and it is expected that the error of prediction can be reduced substantially. The most straightforward improvement is the simultaneous measurement of multiple irradiated teeth to increase the signal to noise ratio. This approach is currently being investigated in measurements of volunteers who have completed courses of radiation therapy that resulted in significant doses to the teeth.