Dynamic Salivary Gland Scintigraphy: Establishing Normative Data through a Healthy Cohort

Abstract

Purpose:

The primary objective was to establish the normative data of dynamic salivary gland scintigraphy (dSGS). Secondary objectives included comparing salivary gland function across different glands, examining gender differences, and correlations with age.

Materials and Methods:

Twenty-nine consecutive healthy adults (19 females) with a mean age of 46.5 ± 11.9 years underwent dSGS. Scintigraphy images were analyzed visually and quantitatively, generating dynamic salivary scintigrams for each gland. Quantitative indices such as maximum percent uptake (MU%) and excretion fraction (EF%) were calculated. Comparisons between the parotid and submandibular glands, as well as between genders, were performed using the Mann–Whitney U-test or Independent Samples t-test. Spearman’s rank correlation was used to analyze the relationship between age and the quantitative indices. Reference values for MU% and EF% were determined using the 5^th percentile and the mean – 1.645 × standard deviation, respectively.

Results:

On visual analysis, parotid glands showed higher radiotracer uptake than submandibular glands. The dynamic scintigram displayed three distinct phases: accumulation, excretion, and re-accumulation. EF% was significantly higher in parotid than submandibular glands (P < 0.0001). No significant gender differences were found in MU% (P ≥ 0.422), but females had higher EF% in the parotid glands (P = 0.004) and a trend toward higher EF% in submandibular glands (P = 0.058). A weak positive correlation was noted between age and the MU% of the submandibular glands (Spearman’s ρ = 0.391, P = 0.036). The reference cutoffs for MU% and EF% were determined to be ≥0.23% and ≥50% for parotid glands and ≥0.18% and ≥32% for submandibular glands, respectively.

Conclusion:

This study established normative data of dSGS for evaluating salivary gland function. Further research with larger cohorts is recommended to explore age and gender variations and to validate these findings across diverse populations.

Introduction

Dynamic salivary gland scintigraphy (dSGS) provides valuable insights into salivary gland function and excretion dynamics. The main benefit of dSGS over other imaging modalities is its ability to quantify the parenchymal and excretion functions of the salivary glands with a single intravenous injection of radiotracer. Furthermore, dSGS is simple to conduct, reproducible, objective, quantitative, and patient-friendly. It has demonstrated clinical significance in various functional salivary disorders, including Sjögren’s syndrome, obstructive sialadenitis with or without parenchymal damage, and iatrogenic irradiation-related sialadenitis resulting from radiotherapy for head-and-neck tumors or radioiodine treatment for thyroid cancer.[1] Unlike visual interpretation, which is observer dependent, quantitative evaluation of dSGS is more objective and reproducible with high sensitivity to detect abnormalities with as little as 25% gland parenchyma destruction although few studies demonstrated no significant superiority of the latter over the former.[1,2,3,4,5,6] Nevertheless, combining visual and quantitative analysis for dSGS is anticipated to be corroborative and provide more comprehensive information. In addition, quantitative analysis can be highly beneficial in evaluating conditions that affect multiple salivary glands.

Despite its clinical utility, the use of dSGS is limited by the lack of normative data and standardized methodologies, as no consensus guidelines exist for dSGS methodology and normative values. This study aimed to fill these gaps in the literature. The primary objective was establishing the normative data for salivary gland function on dSGS. Secondary objectives included comparing quantitative indices of salivary gland function between different glands, assessing gender-based variations, and examining the relationship with age.

Materials and Methods

Study design and population

This prospective, noninterventional cross-sectional study was conducted from March 2023 to February 2024. Healthy controls aged ≥18 years were prospectively recruited after obtaining written informed consent. The exclusion criteria were: Individuals aged <18 years, individuals with oral discomfort or xerostomic symptoms, eye discomfort or dry-eye symptoms, known Sjogren’s syndrome, sarcoidosis, or Immunoglobulin G4-related disease, history of cervical radiotherapy or head–neck surgery, use of drugs affecting salivary gland function, a history of thyroid disorders or radioiodine/prostate-specific membrane antigen radioligand therapy, pregnant or lactating women, and those refusing to give informed written consent. Ethical approval was obtained from the Institute Ethics Committee (Ref. No.: IECPG-259/07.06.2023, RT-19/20.07.2023).

Dynamic salivary gland scintigraphy protocol

Participants fasted for a minimum of 2 h and were ensured adequate hydration before the procedure. The baseline preinjection syringe count was measured (camera based) for 10 s. Then dynamic imaging of the head–neck region in anterior view was performed immediately after intravenous administration of 5 mCi (185 MBq) of Tc-99 m sodium pertechnetate with the participant lying supine and hands by the side. Dynamic imaging acquired sequential images at 30-s intervals over 30 min (60 frames). At 20 min, participants received 5 mL of lemon juice (sialagogue) intraorally, held for 30 s before swallowing, and imaging continued for an additional 10 min. Postinjection syringe count was then measured for 10 s to measure residual activity. Imaging was performed using a dual-head gamma camera (Mediso Anyscan SC) with a low-energy high-resolution collimator and a symmetrical 20% energy window around 140 keV photopeak, matrix size 128 × 128, zoom 1.45, and pixel size 3.34 mm/pixel.

Image analysis

Regions of interest (ROIs) were delineated on dynamic images of the parotid and submandibular glands, with a background ROI drawn over the supraclavicular fossa contralateral to the limb where radiotracer was injected. Decay-corrected time-activity curves (TACs) were generated for each gland. Reformatted scintigraphy images were displayed on the InterView XP 3.06 workstation for visual and quantitative analyses. Images were visually analyzed for symmetry and patterns of radiotracer accumulation and excretion in the salivary glands. The time to minimum uptake (T_min), defined as the time from sialagogue stimulation (at 20 min) to the point when salivary gland counts or uptake reaches a minimum, was also assessed. In addition, quantitative indices were generated based on the counts derived from the ROIs and injected activity. These were:

1. Maximum percent uptake (MU%): It is defined as the percent uptake of the total injected activity in each salivary gland at peak uptake level. It was calculated for each pair of major salivary glands (parotid and submandibular) and the averages were derived. It is given by the formula:

   MU% = (maximum activity/injected activity) ×100

2. Excretion fraction (EF%): It is also known as secretion fraction, or simply secretion, and is defined as the fraction of the maximum activity in the salivary gland that is excreted or secreted on sialagogue stimulation. It was calculated for each pair of major salivary glands (parotid and submandibular) and the averages were derived. It is given by the formula:

   EF% = (1 − T [P + 3]/T [P − 1]) ×100; where

   T (P + 3) is the measured activity value 3 min after sialagogue stimulation

   T (P − 1) is the measured activity value 1 min before sialagogue stimulation

Statistical analysis

Categorical variables were described as frequency (percentage). Continuous variables were described using mean ± standard deviation (SD), median (minimum – maximum), and percentiles (2.5^th, 5^th, 95^th, and 97.5^th). The Kolmogorov–Smirnov test was used to check for normality for continuous variables. Comparisons of quantitative indices of dSGS between the parotid and submandibular glands were made using either the Mann–Whitney U-test or the Independent Samples t-test, as appropriate. Similarly, comparisons between females and males were conducted using the same tests where applicable. Box plots visually represented these comparisons. Spearman’s rank correlation was performed between age and the quantitative indices, with the results plotted in scatter plots. Reference values for MU% were based on the 5^th percentile value (assuming 95% of the healthy population had values ≥ this cut off), and for EF%, they were derived using the formula, mean – 1.645 × SD (assuming 95% of the healthy population had values ≥ this cut off). A two-tailed P < 0.05 was considered statistcally significant. Statistical analyses were conducted using IBM SPSS Statistics 26 (IBM Corp., Somers, New York, USA), MedCalc 19.6.4 (MedCalc Software, Ostend, Belgium), and XLSTAT 2022.5.1 (Addinsoft Inc., New York, USA).

Results

A total of 29 consecutive healthy controls (19 females, 65.5%; 10 males, 35.5%) with a mean age of 46.5 ± 11.9 years were prospectively enrolled. All subjects fulfilled the enrolment criteria and provided informed written consent.

Visual analysis of dynamic salivary gland scintigraphy

The visual pattern of dSGS observed in the current study can be described under two headings:

Visual appearance of radiotracer concentration

On visual inspection of multiple frames of dynamic images, there was a gradually increasing symmetrical radiotracer uptake in the salivary glands over time. The parotid glands generally exhibited relatively higher tracer concentration compared to the submandibular glands. During the early dynamic study, both parotid and submandibular glands showed radiotracer concentration lower than that of the thyroid. This difference diminished over time, and by 20 min (just before sialagogue stimulation), the uptake in the salivary glands was at its maximum, appearing equivalent to or slightly less than the thyroid gland [Figure 1a and b, black arrow]. After sialagogue stimulation, radiotracer uptake in the salivary glands abruptly decreased, becoming equivalent to the background blood pool activity at approximately 3 min [Figure 1a and c, red arrow]. Gradual radiotracer re-accumulation then occurred in the salivary glands until the end of the study at 30 min [Figure 1a and d, green arrow].

Figure 1.

Dynamic salivary gland scintigraphy images of a 59-year-old healthy male subject. Reformatted frames of dynamic anterior view images reveal gradual accumulation of radiotracer in the salivary glands (a). Later in the last image before sialagogue (lemon) stimulation at 20 min, the uptake in the salivary glands is maximum and almost equivalent to that of the thyroid (black arrow in a and b). After sialagogue stimulation, the uptake in the salivary glands becomes minimum at approximately 3 min (red arrow in a and c). Afterward, gradual re-accumulation of tracer uptake is evident in the salivary glands until the study is complete at 30 min (images after the red arrow in a, while the image with green arrow in a and d represent the last image at 30 min)

Dynamic salivary scintigram

The TAC of the dSGS, referred to as the dynamic salivary scintigram, consisted of three phases [Figure 2]:

Figure 2.

Dynamic salivary scintigram. Time-activity curves for parotid (a) and submandibular (b) glands show three phases of dynamic salivary scintigram: Accumulation phase (black arrows), excretion phase (red arrows), and re-accumulation phase (green arrows)

Accumulation phase

The accumulation phase began immediately after radiotracer injection [Figure 2, black arrows]. This phase, marked by a gradually rising curve with a positive slope, continued until the maximum height was reached at 20 min (sialagogue stimulation). The curves for the submandibular glands, in general, were lower in height compared to the parotids, lasting about 20 min.

Excretion phase

Starting immediately after sialagogue stimulation, this phase was characterized by a sharp downward curve with a negative slope toward the baseline [Figure 2, red arrows]. The radiotracer uptake in the salivary glands fell to a minimum approximately 3 min (T_min, 3 min; range, 2–4 min) after sialagogue stimulation, lasting about 3 min.

Re-accumulation phase

Beginning approximately 3 min (2–4 min) after sialagogue stimulation, this phase was evident as a gradually rising curve with a positive slope until the end of the study at 30 min [Figure 2, green arrows]. This phase lasted for approximately 7 min.

Quantitative indices of dynamic salivary gland scintigraphy

Table 1 displays the summary statistics of quantitative indices of dSGS. The parotid and submandibular glands had MU% of 0.45 ± 0.20 (median, 0.37; range, 0.21–0.91) and 0.36 ± 0.15 (median, 0.35; range, 0.14–0.68), respectively. The difference was not statistically significant (P = 0.205). The EF% of the parotid and submandibular glands were 68 ± 11 (median, 72; range, 43–86) and 50 ± 11 (median, 49; range, 30–70), respectively. The difference was statistically significant (P < 0.0001) [Figure 3].

Table 1.

Summary statistics of quantitative indices of dynamic salivary gland scintigraphy

Parameter	Mean±SD	Median (range)	Skewness	Kurtosis	Probability of normality	2.5th percentile	5th percentile	95th percentile	97.5th percentile
MU%
Parotid	0.45±0.20	0.37 (0.21–0.91)	0.959	−0.142	<0.001	0.22	0.23	0.84	0.89
Submandibular	0.36±0.15	0.35 (0.14–0.68)	0.763	−0.109	0.017	0.15	0.18	0.68	0.68
EF%
Parotid	68±11	72 (43–86)	−0.694	−0.144	0.099	44	49	83	85
Submandibular	50±11	49 (30–70)	0.065	−0.865	0.200	30	31	67	69

MU%: Maximum percent uptake, EF%: Excretion fraction, SD: Standard deviation

Figure 3.

Box plots showing the comparison of quantitative indices of dynamic salivary gland scintigraphy between parotid and submandibular glands. MU%: Maximum percent uptake, EF%: Excretion fraction, SM: Submandibular

There was no significant gender-based variation in the MU% of the parotid and submandibular glands (P = 0.694 and 0.422, respectively). However, females had higher EF% in the parotid glands (P = 0.004) and a trend toward higher EF% in submandibular glands (P = 0.058) compared to males [Figure 4].

Figure 4.

Box plots showing the comparison of quantitative indices of dynamic salivary gland scintigraphy between females and males. MU%: Maximum percent uptake, EF%: Excretion fraction, SM: Submandibular

We observed no significant correlation between the MU% of the parotid glands and age. However, there was a weak positive correlation between the MU% of the submandibular glands and age (Spearman’s ρ = 0.391, P = 0.036). The EF% of both the parotid and submandibular glands showed no significant correlation with age [Table 2 and Figure 5].

Table 2.

Spearman’s correlation between age and quantitative indices of dynamic salivary gland scintigraphy

	Age
	Spearman’s ρ	P
MU% (parotid)	0.226	0.239
MU% (SM)	0.391	0.036*
EF% (parotid)	0.060	0.755
EF% (SM)	0.029	0.882

*Significant at P < 0.05 MU%: Maximum percent uptake, EF%: Excretion fraction, SM: Submandibular gland

Figure 5.

Scatter plots between age and quantitative indices of dynamic salivary gland scintigraphy. MU%: Maximum percent uptake, EF%: Excretion fraction, SM: Submandibular

The reference values of MU% and EF% established in the present study are highlighted in Table 3.

Table 3.

Proposed reference values of quantitative indices of dynamic salivary gland scintigraphy

Parameter	Parotid gland (%)	SM (%)
MU%	≥0.23	≥0.18
EF%	≥50	≥32

MU%: Maximum percent uptake, EF%: Excretion fraction, SM: Submandibular gland

Discussion

Salivary scintigraphy is a specialized diagnostic procedure available at a limited number of centers globally, with significant variations in methodologies and no standardized guidelines for the technique or reference values. Existing literature on dSGS normative data reveals wide variations in methodology, indices, and reported reference values, primarily based on older gamma cameras, which may have nonuniform counting rates.[7,8,9,10,11] This study aimed to address this gap by establishing normative data of dSGS in a cohort of healthy controls.

The 20-min mark for sialagogue stimulation was selected as an optimal time point between peak radiotracer accumulation and the onset of spontaneous excretion, as supported by findings from earlier studies.[1] Our study revealed a consistent pattern of radiotracer uptake and excretion in healthy individuals, aligning with the existing literature. The visual analysis of dSGS images showed symmetrical uptake in the salivary glands, with parotid glands typically displaying higher tracer concentration than submandibular glands. In our study, 21/29 subjects (72.4%) showed more uptake in the parotids than the submandibular gland. The observed differences in visual uptake between parotid and submandibular glands may partly be attributed to anatomical and physiological factors. Parotid glands are generally larger in volume and more superficial in location, which may enhance count detectability and tracer uptake compared to the deeper, more compact submandibular glands. The difference in spatial positioning relative to the detector may also influence count recovery. In addition, functional differences in glandular physiology, including baseline secretory activity, may contribute to the uptake variability. The dynamic salivary scintigram depicted three phases: Accumulation, excretion, and re-accumulation, providing a clear pattern for interpreting scintigraphy results. Anjos et al. used thyroid uptake as a reference for salivary gland function assessment, classifying gland uptake compared to thyroid uptake.[11] Our study diverged from this approach due to the variability in thyroid uptake, focusing instead on the dynamic phases of radiotracer concentration in the salivary glands without comparing it to thyroid uptake.

Various quantitative indices for salivary scintigraphy have been reported in the literature, including the partitioned percentage of total activity, percentage uptake, maximum net uptake ratio, prestimulatory oral radioactivity index, poststimulatory oral radioactivity index, uptake rate, salivary gland to thyroid ratio, washout fraction, maximum accumulation, excretion speed, uptake index, MU%, and EF%.[5,6,7,8,9,10,11,12,13,14,15] Our study focused on MU% and EF% as quantitative indices due to their reproducibility and consistent reporting in diseases involving the salivary glands, such as Sjogren’s syndrome.[16] Although the MU% did not differ significantly between the parotid and submandibular glands, the EF% was significantly higher in the parotid glands. This may be due to the intrinsic differences in secretory behaviors between the two glands – parotid glands, composed predominantly of serous acini, highly responsive to gustatory stimulation, whereas submandibular glands, which contain a mix of serous and mucinous acini, generally exhibit a slower and less pronounced response.[17] The established reference cutoffs for MU% and EF% are ≥0.23% and ≥50% for parotid glands and ≥0.18% and ≥32% for submandibular glands, respectively. The values of MU% and EF% obtained in our study are majorly in line with findings from the literature.[7,8,10,11] Using the 5^th percentile or mean – 1.645 × SD (reflecting the 95% confidence interval for a healthy population) to derive reference values provides a robust and statistically significant cutoff for healthy individuals – a key strength of our study.

The analysis of gender differences in quantitative indices found that females had a significantly higher EF% in the parotid glands compared to males (P = 0.004). For the submandibular glands, there was a trend toward significance (P = 0.058), indicating potential physiological differences in salivary gland function between genders. This finding deviates from the study by Firat et al.,[14] who reported no significant gender differences but highlighted the need for larger studies to validate their observations. However, no significant gender differences were observed in MU% in our study, indicating that peak uptake levels are consistent across genders. Our study lacks the power to fully assess these gender variations in quantitative indices, indicating the need for further research with larger sample sizes.

The literature presents mixed findings on the effects of aging on salivary gland function. Some studies, including those by Parvinen and Larmas,[18] Tylenda et al.,[19] Ship et al.,[20] and Jones and Ship,[21] reported no significant age-related changes in salivary gland output or flow rates. In contrast, other studies, such as those by Pedersen et al.[22] and Percival et al.,[23] found reduced salivary flow rates in older adults. Notably, these studies used conventional saliva-collection methods rather than scintigraphic techniques to assess salivary gland function. Firat et al.[14] observed that age significantly affects salivary gland function in some quantitative dSGS indices, with a stronger impact on women than men. However, our study found no significant correlation between age and dSGS quantitative indices, except for a weak positive correlation between age and MU% of the submandibular glands (Spearman’s ρ = 0.391, P = 0.036), suggesting a minor influence of aging on specific indices. Further research with larger sample sizes is needed to confirm these findings.

This study has several strengths. First, it prospectively established the normative data of dSGS by enrolling a cohort of consecutive healthy controls. The study also presented the visual pattern of dSGS and the phases of the dynamic salivary scintigram in a simplified manner. In addition, we streamlined the quantitative analysis by selecting MU% and EF% as the primary indices. While many indices reported in the literature are cumbersome, superfluous, or technically challenging to calculate, these two indices are the most consistently documented, and reproducible and are reliably affected by salivary gland diseases.[16]

Despite its valuable insights, the study has certain limitations. The small sample size may restrict the generalizability of the results, particularly the possible variations in quantitative indices related to gender and age. In addition, the normative data established may not apply to children or individuals under 18 years old. Further research with larger, more diverse cohorts is needed to validate these findings and explore potential variations across different demographics.

Conclusion

The study successfully established the normative data of dSGS by providing reference values for quantitative indices and visual patterns for healthy individuals. The results confirm that dSGS effectively captures symmetrical radiotracer uptake and excretion in the salivary glands, with distinct phases of accumulation, excretion, and re-accumulation. Key quantitative indices, including MU% and EF%, were defined with reference values, offering a corroborative benchmark for assessing salivary gland function. While the findings contribute significantly to the existing body of knowledge, further research with larger sample sizes and diverse populations is suggested to enhance the generalizability of these results, particularly exploring the possible age and gender variations on salivary gland function.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.