Radiation Effective Doses to Adults Undergoing Modified Barium Swallow Studies

Abstract

Modified Barium Swallow Studies (MBSSs) are an important test to aid the diagnosis of swallowing impairment and guide treatment planning. Since MBSSs use ionizing radiation, it is important to understand the radiation exposure associated with the exam. This study reports the average radiation dose in routine clinical MBSSs, to aid the evidence-based decision making of clinical providers and patients. We examined the MBSSs of 200 consecutive adult patients undergoing clinically indicated exams and used kilovoltage (kV) and Kerma Area Product (KAP) to calculate the effective dose. While 100% of patients underwent the exam in the lateral projection, 72% were imaged in the upper posterior-anterior (PA) projection and approximately 25% were imaged in the middle and lower PA projection. Average kVs were 63 kV, 77 kV, 78.3 kV and 94.3 kV, for the lateral, upper, middle and lower PA projections, respectively. The average effective dose per exam was 0.32 ± 0.23 mSv. These results categorize a typical adult MBSS as a low dose examination. This value serves as a general estimate for adults undergoing MBSSs and can be used to compare other sources of radiation (environmental and medical) to help clinicians and patients assess the risks of conducting an MBSS. The distinction of MBSS as a low dose exam will assuage most clinician’s fears, allowing them to utilize this tool to gather clinically significant information about swallow function. However, as an x-ray exam that uses ionizing radiation, the principles of ALARA and radiation safety must still be applied.

Introduction

The Modified Barium Swallow Study (MBSS) is the primary diagnostic test used to identify abnormalities in oropharyngeal swallowing function, detect the presence and etiology of aspiration, and test the effects of diet modifications and therapeutic interventions^1,2,3. While MBSSs are important diagnostic tests, they expose patients to ionizing radiation, which should be kept “as low as reasonably achievable” (ALARA) due to the associated cancer risks^4,5. We have previously reported on the cancer risks of adult patients who undergo a complete MBSS, defined as the full Modified Barium Swallow Impairment Profile (MBSImP) protocol⁶. However, results from that study do not represent the average dose / risk in clinical care as many patients are unable to complete the full MBSImP protocol. When communicating with patients and healthcare team members it is important to know the average effective dose expected in a typical clinical scenario and use that information to determine the risk / benefit of performing a MBSS.

Effective dose is a good descriptor of the total amount of radiation received by patients who undergo a radiological exam, and can be used to estimate the associated radiation risks^7,8,9,10. Computation of effective dose requires knowledge of the radiation dose received by all exposed organs and tissues, which can subsequently be combined using the relative radio sensitivities (i.e., tissue weighting factor w_T) for each of these organs. Effective doses permit the radiological exposure associated with an MBSS to be directly compared with any other type of exposure in the same individual. For example, a CT examination with an effective dose of 5 millisieverts (mSv) will result in a patient detriment that is about 100 times higher than that associated with a standard 1-view chest x-ray (0.05 mSv). Another major benefit of the effective dose concept is that patient effective doses in examinations such as an MBSS can be directly compared with other benchmark doses, also expressed as effective doses, such as regulatory dose limits and exposures to natural background radiation.

For any radiological examination, organ doses are directly related to both the quantity and quality of the radiation used to perform the examination. The quantity of radiation in the incident x-ray beam is expressed as a Kerma Area Product (KAP)¹¹. The x-ray beam quality, which relates to the average energy (i.e., penetrating power), is determined by the x-ray tube voltage (kV), and the total amount of filtration used to “harden” the beam. Patient factors that affect the patient effective dose include the body region exposed, the x-ray projection, and patient characteristics (i.e., height and weight). In this study, we investigated how typical MBSSs were performed at our institution, as well as the effect of the corresponding patient characteristics. Combining both the x-ray techniques with patient characteristics enabled us to perform a systematic assessment of patient effective doses for adult MBSSs.

Method

Patients and Modified Barium Swallow Studies

We examined the MBSSs of 200 consecutive adult patients. Inclusion criteria were: 1) all patients underwent a clinically indicated MBSS, 2) the MBSS was conducted under pulsed fluoroscopy (30fps), 3) the MBSImP protocol was utilized, but not always fully completed due to the severity of the patient, and 4) the exam was recorded. There are 12 swallow trials in the MBSImP protocol, 10 swallows in the lateral view and then 2 swallows in the posterior-anterior (PA) view. Patients with various medical diagnoses and levels of swallowing impairment were included. The clinician’s judgment was used to determine when it was in the best interest of the patient to stop the exam. Data were extracted from each MBSS to define the kV and KAP for the average adult patient undergoing an MBSS. Specifically, average kV was calculated from the kV used at 6 time points in each projection (lateral, upper PA, middle PA, lower PA)¹². Average KAP included values of total KAP for each projection of the MBSS as well as total KAP for the entire exam. Selected data from a subset of these patients who underwent the complete MBSImP protocol using the same procedures as this study have been used to determine radiation risk and were previously published⁶.

Effective Doses

We have previously published a detailed methodology of deriving effective dose per unit kerma area product (E/KAP) conversion factors¹³. This methodology allows for a quick estimation of the effective dose by simply multiplying the KAP for a given exam by the conversion factor. In this earlier work, we used PCXMC 2.0.1, a commercial software program to calculate organ doses to patients undergoing MBSSs¹⁴. PCXMC allows the operator to vary the x-ray beam penetrating power (quality) by changing the x-ray tube voltages over the range generally encountered in clinical practice. The operator can also select Aluminum (Al) and/or Copper (Cu) as added filter materials, and the appropriate filter thickness. The x-ray beam intensity incident on a patient is expressed as air Kerma (K_air) measured in Grays (Gy), and the KAP is obtained by multiplying the K_air by the corresponding x-ray beam area, which is measured in Gy-cm².

While the step-by-step derivation of the conversion factors can be found in this earlier manuscript¹³, it is important to mention that the work explicitly derives conversion factors based on the two key contributors that determine patient dose. The first conversion factor is based on the specific kV and filter combination. This factor was derived for a range of kV from 60 kV to 110 kV in combination with thickness of 2 to 3 mm of Al, and 0.1 to 0.2 mm of Cu. Note that this covers the typical range of kV and types of filters used during clinical exams. The second conversion factor is based on the patient size, namely the height and weight of the patient. This factor was modeled for patients between 40 and 120 kg and heights between 1.5 m to 2.0 m. Thus, knowing the technique factors of the MBSS (kV, filter), patient size (height, weight), and the KAP allows the estimation of the effective dose. Patient and exam-specific data were extracted for each exam and the effective doses were estimated using the aforementioned conversion factors.

Results

A. Radiographic Techniques

Table 1 shows the x-ray exam projections that were used for the 200 consecutive patients. All 200 patients (100%) had a lateral projection, and 72% also had an upper PA projection as well. Only about a quarter of patients additionally had middle PA (25%) and lower PA (25%) projections.

Table 1.

Distribution of number of patients undergoing the Modified Barium Swallow Study with the 4 different x-ray projections. N=number; PA=posterior-anterior projection.

	Lateral	Lateral + Upper PA	Lateral + Middle PA	Lateral + Lower PA
N (%)	200 (100%)	144 (72%)	51 (25%)	48 (24%)

Figure 1 shows the histogram distributions for the x-ray tube voltage selected for all four projections. The median tube voltage for the lateral projection was 63.8 ±4.5 kV, with most instances falling between 60 and 69.5 kV.

Fig. 1.

Tube voltage (kV) distribution for the four projections. Also shown are the values for the median, standard deviation, tenth and ninetieth percentiles. Y-axis represents number of patients.

The tube voltages were higher for PA projections and showed a broader distribution that reflects the range of patient sizes. The median tube voltages were 77.4 ± 6.3 kV, 78.5 ± 7.9, and 91.7 ± 12.7 kV for the upper, middle, and lower PA projections respectively. While the average and median kV values for the lateral, upper PA, and middle PA projections were almost the same, for the lower PA projection, the average value (94.2 kV) was higher than the median value (91.7 kV).

B. Kerma Area Product

Figure 2 shows the histogram distributions for the KAP values obtained for all four projections. The average KAP for the lateral projection was 0.96 ± 0.73 Gy-cm², for the upper PA projection was 0.29 ± 0.32 Gy-cm², for the middle PA projection was 0.07 ± 0.13 Gy-cm², and for the lower PA projection was 0.26 ± 0.29 Gy-cm². Table 2 shows statistics for the total KAP where the average value was 1.34 ± 0.89 Gy-cm². This average value is slightly higher than the median value (1.11 Gy-cm²) reflecting a skewed distribution.

Fig. 2.

Kerma area product (Gy-cm²) distribution for the four projections. Also shown are the values for the average, standard deviation, and relevant percentiles. Y-axis represents number of patients.

Table 2.

Statistics of the total KAP (Gy-cm²) and total effective dose (mSv) for the 200 patients.

	Total KAP (Gy-cm2)	Total Effective Dose (mSv)
Min	0.17	0.04
10%ile	0.49	0.11
25%ile	0.73	0.17
Median	1.11	0.25
75%ile	1.74	0.40
90%ile	2.33	0.53
Max	5.93	1.67
Avg	1.34	0.32
SD	0.89	0.23

C. Effective Doses

Figure 3 shows how the effective dose is related to KAP corresponding to each projection. For lateral projections, the slope shows that the conversion factor (slope of the best-fit line) from KAP to effective dose was 0.24 mSv per Gy-cm². The corresponding conversion factor was 0.16 mSv/Gy-cm² for upper PA projections, 0.22 mSv/Gy-cm² for middle PA projections, and 0.43 mSv/Gy-cm² for lower PA projections.

Fig. 3.

Effective dose (mSv) versus KAP (Gy-cm²) for the four projections. Also shown are the best-fit lines for a first order polynomial fit.

Figure 4 shows the effective doses for each of the four projections, together with the corresponding statistics. The average effective dose for the lateral projection was 0.233 ± 0.189 mSv. The corresponding values were 0.058 ± 0.052 mSv, 0.024 ± 0.028 mSv, and 0.146 ± 0.126 mSv for the upper, middle and lower PA projections.

Fig. 4.

Effective dose (mSv) distribution for the four projections. Also shown are the values for the average, standard deviation, and relevant percentiles. Y-axis represents number of patients.

Table 2 shows the statistical distribution of the effective doses to these 200 consecutive patients. The average effective dose was 0.316 ± 0.227 mSv. Note that this is lower than the sum of the average effective doses of the four separate projections, which is 0.461 mSv (0.233 + 0.058 + 0.024 + 0.146). This difference can be explained by the fact that while all 200 patients had lateral views, only 144 had an additional upper PA view, 51 had an additional middle PA view and 48 had an additional lower PA view. Thus, the effective dose contribution from each view is weighted differently in the statistical average for the whole cohort of 200 patients as compared to simply adding the average effective dose for each view (which assumes equal weighting of each view).

Discussion

The MBSS is a critical exam to evaluate swallowing function that drives patient diagnosis and treatment planning. The MBSS is often referred to as the gold-standard for the diagnosis of swallowing impairment. Despite its critical role for more than nine million patients in the United States with swallowing impairment¹⁵, this exam has historically been underused or used in a truncated manner due to radiation exposure concerns. One reason for this is that clinicians have not had access to data which quantifies the average radiation exposure to patients undergoing MBSS when determining the risk / benefit ratio for this exam causing clinicians to err on the side of caution. This has likely resulted in misdiagnosis, errors in treatment planning, worse patient health outcomes, and increased healthcare costs.

To estimate the radiation exposure to patients undergoing MBSS, we investigated clinical exams in 200 consecutive patients. Compared to our prior study that assessed radiation exposure only for patients who underwent the complete MBSImP protocol, the results of this study, are more relevant when estimating exposure for the general population of adults undergoing MBSS. This is because in this study the type of x-ray projections used varied according to the patient’s ability to complete the standardized protocol (MBSImP). While the MBSS was conducted using the lateral projection for all patients, only 72%, 25%, and 24% of exams used the upper PA, middle PA and lower PA projections, respectively. There are some practical reasons for this distribution. The portion of the protocol that employs the PA projection is at the end of the exam, after all swallows in the lateral projection have been completed. Therefore, for patients who are unable to complete the entire standardized exam, for example those who are sicker, the PA projection is sometimes excluded. This occurred in approximately 56 patients in our sample of 200. One difference between the set of patients who only underwent the upper PA projections versus those that underwent the full exam using all projections, was the patient’s ability to stand. Generally, the upper PA projection is able to be easily completed in the seated position, whereas the middle and lower PA projections are more easily complete in the standing position. Most patients (approximately 65%), largely inpatients, in our sample were unable to undergo both the middle and lower PA projections. We believe that the data based on this group represents a common clinical scenario when using the standardized MBSImP protocol.

Modern x-ray imaging systems will increase the x-ray tube voltage as the patient thickness (i.e., total attenuation) increases. The reason for this is x-ray beam penetration should increase with increasing patient attenuation otherwise patient radiation doses increase too much for optimal performance. In our study, the median x-ray beam tube voltage was lowest (64 kV) for lateral projections, and highest (92 kV) for lower PA projections. Based on our measurements of the x-ray tube output and technique, these voltages correspond to a patient thickness of approximately 10 cm and 20 cm respectively, correctly reflecting the typical body thickness in the neck and the abdominal regions.

The median total KAP for a typical MBSS of 1.1 Gy-cm² was lower than the sum of the median values for the four projections, which is 1.35 Gy-cm². The reason for this is that not all patients had all four projections. The total KAP in an MBSS is higher than that of a chest x-ray examination that has a typical KAP of 0.1 Gy-cm² (K_air ~ 0.1 mGy), but is lower than that of a typical abdominal x-ray with a typical KAP of 2.5 Gy-cm² (K_air ~ 3 mGy).

The slope of the curves shown in Figure 3 is the conversion factor from KAP used to perform a given type of examination into the corresponding patient effective dose, E. The lowest conversion factor was for upper PA examinations (0.16 mSv/Gy-cm²), and the highest conversion factor was for lower PA examinations (0.43 mSv/Gy-cm²). There are two important factors that influence this E/KAP conversion factor. The first relates to the fact that thicker body parts require an increase in tube voltage (penetrating power) that will increase the total energy deposited within the patient. The second factor is that for a given organ dose, the effective dose increases with organ radio sensitivity. The lower PA projection has the highest E/KAP conversion factor because it uses the highest x-ray tube voltages, and this part of the patient anatomy contains more radiosensitive organs (lung, breast, red bone marrow) than the upper gastrointestinal (GI) and neck regions.

The projection that resulted in the highest average effective dose was the lateral projection (0.23 mSv), and the middle PA projection had the lowest average effective dose (0.02 mSv). The upper and lower PA projections had average effective doses of 0.06 and 0.15 mSv, respectively. These effective dose values take into account both the amount of radiation used, and the corresponding E/KAP conversion factor. Lateral projections typically use four times the amount of radiation that are encountered for lower PA projections. This is because we use the lateral projection for approximately 75% of the duration of the MBSS¹². However, because the E/KAP conversion factors are only about half those of lower PA projections, the resultant effective dose for the lateral projections are less than twice those associated with the lower GI projections. This example illustrates that understanding patient dose requires more than a simplistic analysis of just the amount of radiation used (KAP) in MBSSs.

The average effective dose for the exam was 0.32 mSv, which makes a typical MBSS a “low dose” examination (typical effective dose between 0.1 and 1 mSv), “Very low dose” examinations have effective doses of less than 0.1 mSv, and include chest x-rays, bone mineral densitometry, and radiographic examinations of the extremities¹⁶. Moderate dose examinations with effective doses in the range of 1 to 10 mSv include most GI/GU (gastrointestinal / genitourinary) studies, simple CT examination of the head and body, as well as most Nuclear Medicine examinations which utilize ^99mTc labelled radiopharmaceuticals. High dose examinations with effective doses higher than 10 mSv include complex CT studies (e.g., multiphase whole-body scans), PET/CT examinations as well as most interventional procedures⁷. MBSSs have patient doses that are higher than some common radiological exams such as chest x-rays but are markedly lower than virtually all GI/ GU studies such as barium enemas and small bowel flow through (SBFT) studies that would have effective dose of about 5 mSv. We would like to emphasize that MBSS represents a “low dose” exam despite being conducted at a relatively high fluoroscopic pulse rate of 30 pps. It should be noted that dropping the pulse rate to a lower value (say 15 pps) does not always reduce the dose by a half, since in many systems, the individual pulses would typically require higher tube current (mAs) in order to compensate for image noise. Thus, in such a case, the dose savings at lower pulse rates would be minimal, while likely compromising on the clinical efficacy of the procedure, due to clinicians missing observations of swallowing impairment. As such, pulse rates of 15 pps and less could compromise patient care and should be avoided.

A typical inhabitant of North America is exposed to about 3 mSv (300 millirem (mrem)) each year, or the equivalent of ten MBSSs¹⁷. It is also of interest to compare the effective dose of an MBSS with current US regulatory dose limits. Radiation workers currently have an annual effective dose limit of 50 mSv/year¹⁸. However, it is important to appreciate that the most highly exposed radiation workers, such as workers in Interventional Radiology, receive 2 - 4 mSv¹⁹. Aircrew are not considered radiation workers, but receive about 5 mSv each year because they spend about 1000 hours at an elevation greater than 30,000 ft where cosmic radiation levels are markedly higher than those at sea levels²⁰. The dose limit to members of the public in the US are currently 1 mSv/year, and the limit to the fetus of a pregnant radiation worker is currently 0.5 mSv/month¹⁹.

The clinical implications of the determination that the MBSS for an adult patient is a low dose examination are many. Clinicians and patients can now feel confident in the information they use for risk/benefit assessment. We have previously shown that the median effective dose related to the full MBSImP protocol MBSS is about 0.27mSv⁶. The related cancer risks are less than a mammogram (effective dose of 0.4mSv) and in the most radiosensitive group (younger adult females) the MBSS has a related cancer risk of 0.0032%⁶. Note that these values should not be assumed true for MBSSs conducted without a standardized, validated protocol similar to the MBSImP. Our results indicate an extremely low risk when weighed against the benefit of acquiring information about a patient’s swallowing function that will define their diagnosis and determine their treatment. This result also indicates that alternative methods of acquiring diagnostic information (such as with the use of the flexible endoscopic examination of swallowing (FEES) or bedside swallow studies) versus the use of MBSS to acquire information directly regarding the swallowing physiology should not be used based on the rationale of reducing radiation exposure. Despite the ‘good news’ that MBSS is a low dose examination, radiation protection and safety principles still apply and no exam should be undertaken if the information gained is not important for patient care.

Conclusions

The results of this study indicate that the MBSS is a low dose examination, with an average dose for an adult of 0.32 mSv. This value serves as a general estimate for adults undergoing MBSSs and can be used in comparison to other sources of radiation (environmental and medical) to help clinicians and patients assess the risks vs the benefits of conducting an MBSS. The distinction of MBSS as a low dose exam will assuage most clinician’s fears, allowing them to utilize this tool to gather clinically significant information about swallow function. However, as an x-ray exam that uses ionizing radiation, the principles of ALARA and radiation safety must still be applied.