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Transportable versus Fixed Platform CT Scanners: Comparison of Costs¹

¹ From the Department of Radiology, Rhode Island Hospital, Brown University School of Medicine, 593 Eddy St, Providence, RI 02903 (W.W.M.S., W.J.S., C.M.C., G.S.D.); Department of Radiology, Massachusetts General Hospital, Boston (J.T.R.); and Center for Statistical Sciences, Brown University, Providence, RI (I.F.G.). From the 2000 RSNA scientific assembly. Received December 14, 2001; revision requested January 25, 2002; revision received March 21; accepted May 13. Supported in part by a grant from the Philips Corporation. Address correspondence to W.W.M.S. (e-mail: wmayo-smith@lifespan.org).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
PURPOSE: To compare the aggregate hospital technical costs of a transportable computed tomographic (CT) scanner used to image patients in an intensive care unit with those of a fixed platform CT scanner in the radiology department.

MATERIALS AND METHODS: Direct fixed costs (ie, machine and service contract costs) and direct variable costs (ie, personnel costs) were calculated. Indirect costs, including space costs and departmental overhead, were calculated. Total costs were calculated as the sum of indirect, direct fixed, and direct variable costs. Personnel costs were calculated from time-motion analyses involving 95 patients who underwent brain CT with either a transportable (n = 51) or a fixed platform (n = 44) CT scanner. Costs per examination were calculated by using both low- and high-examination-volume models and compared with use of the Wilcoxon rank sum test.

RESULTS: The total cost per examination for the transportable scanner ranged from $108.98 to $167.20 for the high- and low-volume models. Total cost per examination for the fixed platform scanner ranged from $75.24 to $112.39 for the high- and low-volume models. For the transportable scanner, direct fixed, variable, and overhead costs were $87.05, $70.73, and $9.42 per examination, respectively, with the low-volume model. The corresponding costs for the fixed platform scanner were $46.66, $55.69, and $10.04, respectively.

CONCLUSION: The technical cost of using an in-hospital transportable CT scanner is higher than that of using a fixed platform scanner.

© RSNA, 2002

Index terms: Computed tomography (CT), utilization • Cost-effectiveness • Economics, medical • Radiology and radiologists, socioeconomic issues

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Economic pressures to reduce health care costs have led to shorter hospital stays and a more ill inpatient hospital population in the United States (1). Economic pressures have stimulated analyses of the costs of providing computed tomographic (CT) services (2–9). Rapid advances in CT technology have led to the development of transportable CT scanners. There has been an increasing demand for portable imaging examinations that allow the imaging device and personnel to be brought to the patient’s bedside rather than require the patient to go to the radiology suite to be imaged. This has been particularly true for portable radiography and more recently for ultrasonography.

Development of mobile CT equipment has allowed CT imaging to be performed at the bedside. Whereas prior studies have demonstrated potential applications for this new mobile CT technology (10–13), we know of no formal study evaluating the cost of performing portable CT examinations. Although there are advantages to imaging the patient at the bedside, including decreased risk of adverse events during travel for a critically ill patient, the difference in costs of transportable versus fixed CT scanners should be evaluated to determine the overall financial effect on the hospital of introducing a transportable scanner. The purpose of this study was to compare the aggregate hospital costs of imaging patients in an intensive care unit (ICU) with a transportable CT scanner with those of transporting the patient to the radiology suite.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
This study was performed at a single institution during the period of October 1997 to January 1999 and was approved by the institutional review board of Rhode Island Hospital. Informed consent was waived. The transportable CT scanner (Tomoscan M; Philips Medical Systems, Shelton, Conn) is a U.S. Food and Drug Administration–approved device that consists of a portable table, CT gantry, and computer console, all of which operate on a standard 110-V alternating current outlet.

Determination of Costs
The aggregate cost of performing a CT examination includes direct fixed, direct variable, and indirect costs (2). Examples of fixed costs include the cost of the machine and service contract. Variable costs include items that vary in proportion to the number of examinations performed. Examples of variable costs include supplies and technologist salaries. Indirect costs include costs that are not directly attributed to performing the examination but contribute to overall examination costs. Examples of indirect costs include departmental administration costs.

For this study, costs were determined on the basis of 1998 values, as discussed in detail in the following text. Equipment costs were based on purchase prices and the costs of service contracts. Personnel costs were based on time-motion studies and the rates of pay for the various jobs that would be affected by the use of transportable scanners. Indirect costs were determined from the hospital’s accounting system (Ecliypsys, Boca Raton, Fla), in which radiology cost centers for support functions are aggregated and allocated to the examinations performed. Hospital cost for space was allocated on the basis of square feet occupied by the scanners. This study did not include physician or professional costs since these would not change regardless of the type of scanner used. Also, items of hospital overhead that would not be affected to a substantial degree were excluded.

Fixed Costs
To determine equipment costs, the purchase prices of fixed and portable scanners were determined. The cost of the transportable CT scanner at the time of this study was $549,000. The price for the fixed platform scanner with comparable features in 1998 was approximately $400,000. Yearly equipment costs for the fixed and transportable scanners were based on 7-year straight-line depreciation. In 1998, the annual service contract was $80,000 for the transportable scanner and $90,000 for the fixed platform scanner.

The direct fixed cost per scan was determined by dividing the yearly cost by the annual volume. Because the portable scanner was not in routine use, the volume had to be estimated. Since the fixed cost of CT scanners is high, changes in volume have a considerable effect on the costs per examination. For this reason, we chose to estimate fixed cost by using two models. One was a low-volume model in which the machine was operational Monday through Friday, 8 hours per day. A high-volume model assumed the machines were operational 7 days a week, 24 hours per day. For the transportable CT scanner and the low-volume model, it was estimated that approximately one CT examination could be performed per hour or a total of seven transportable examinations per working day. This estimate of seven examinations per day was calculated from measured times to perform examinations as described below. Annual volume with use of this model was calculated by multiplying 52 weeks per year x 5 days per week x 7 examinations per day = 1,820 examinations per year.

Alternatively, if the transportable scanner were operational 24 hours per day and 365 days per year, assuming 15 examinations per day would be requested, then 5,475 examinations could be performed per year (15 examinations per day x 365 days per year = 5,475 examinations per year). For the fixed platform scanner, we used two different estimates of volume. The low-volume model was based on a nonhelical unit in our outpatient facility that operated 8 hours per day, 5 days per week. This scanner was used to perform 3,153 examinations in 1998. The high-volume model was based on a nonhelical scanner that operated 24 hours per day in our emergency department. This machine was used to perform 14,130 examinations in 1998.

Variable Costs
Variable costs consist primarily of personnel costs and supplies such as film, processors, and intravenous contrast material. Both scanners used the same volume of film per examination, and no intravenous contrast material was administered for the examinations in this study. For this reason, supply costs were not appreciably different between the two machines and were not included in this analysis. Thus, the main variable cost was the difference in personnel time. To determine personnel costs, time-motion data were collected prospectively for the transportable CT scanner used to examine 51 consecutive patients who underwent CT at their bedside in the neurosurgery ICU, and for the fixed platform CT scanner used to examine 44 different patients who traveled from the ICU to the radiology department.

All patients imaged with both scanners were from the ICU and thus represented similar patient populations. We did not attempt to measure the severity of illness between the two groups. The transportable scanner was available for this study for a 6-month period. During these 6 months, 51 consecutive nonenhanced brain CT examinations requested Monday through Friday during the day (when the research technologists were available) were performed with the transportable scanner. During the remaining 8 months, 44 CT examinations were performed (Monday through Friday during the day to match the transportable cohort) with the fixed platform scanner in the radiology department.

Personnel times for each type of scanner were determined by summing the times of each person required to perform the examination. Personnel involved with the transportable scanner included one or usually two research CT technologists (W.J.S. and C.M.C.) who performed the examinations. Their time included that required to travel to the patient, set up the scanner at the patient’s bedside, perform the examination, disassemble the scanner, and return to the radiology department with the computer console to print the films from the examination. Nursing time for the transportable scanner was calculated as the amount of time dedicated to preparing, monitoring, and returning the patient to his or her bed after the examination until the nurse could perform other responsibilities in the ICU. Unit secretary time was that required to schedule the examination and occasionally assist in moving the patient in the ICU. Transporters were not required to assist in imaging patients in the ICU.

For patients imaged with the fixed platform scanner, technologist time was that required to position the patient on the scanner, perform the examination, film the study, and prepare the room until the next examination could be performed. Nursing time was that required to prepare the patient for transport, transport the patient, monitor the patient in the radiology department while the examination was being performed until the nurse returned to the unit and could perform other responsibilities. Respiratory therapist and transporter times were defined as the time when they left their division until they returned and were available to assist another patient. Unit assistant time was the time to schedule the examination and call support personnel to arrange transport. At the time of both the transportable and fixed examinations, one of two research technologists (W.J.S. and C.M.C.) and the nurse involved with the preparation, transport, and imaging of the patient recorded, on a separate data sheet for each patient, the total time in minutes that they and support personnel devoted to performing the examination. Information from these data sheets was used to determine personnel times and costs for each scanner.

The personnel cost for each examination was derived by multiplying personnel time by the average hospital reimbursement rate for that position. In addition, 21% was added to the salary estimate to account for employee overhead and benefits. The salaries used to estimate cost in this study were as follows: nurses, $24.00/hour; respiratory therapists, $18.00/hour; transporters, $10.50/hour; unit assistants, $11.00/hour; and radiologic technologists, $21.00/hour. For each type of personnel, we estimated the mean and the 95% confidence limits for the number of hours spent and the cost using each type of CT scanner. We used the nonparametric Wilcoxon rank sum test to compare the time and costs for each scanner, since the data were not normally distributed. SAS software (Statistical Analysis Systems, Cary, NC) was used for this analysis.

Indirect Costs
Indirect costs consist of overhead costs incurred by the radiology department and overhead costs incurred by the hospital that are allocated for radiologic procedures. Radiology departmental overhead was determined by summing aggregate departmental costs (eg, administrative and support personnel salaries, physics department overhead, computers, information services support, film storage costs). The allocation of fixed departmental overhead at our institution is based on the yearly total departmental volume of all types of examinations. The indirect costs per CT examination were calculated by summing the aggregate radiology department fixed costs (excluding the capital equipment and service contracts) and dividing it by the total number of radiology examinations performed in 1998.

Indirect costs also include hospital space costs, which differed between the two scanners given their different space requirements. Space costs include building depreciation, repairs, maintenance, housekeeping, and utilities. These are charged to departments on a square footage basis at our hospital, and the rate is the same for different departments. Hospital space costs were calculated by multiplying the scanner area, as specified in the following text, by the hospital rate per square foot. Other indirect costs, such as hospital administration, were excluded from this analysis.

Calculation of Break-Even Volume and Sensitivity Analysis
Because fixed costs are relatively high at low examination volumes for both types of scanners, it is useful to know the break-even volume for operating these scanners. This can assist a department in determining whether the purchase of such a device is economical given their projected volume. Break-even volume was determined by calculating the contribution to fixed costs (ie, revenue per examination minus variable costs per examination). Total fixed costs per year equal the fixed direct costs (equipment and service contract costs) and the indirect fixed costs (departmental overhead and hospital overhead for space). Break-even volume equals fixed costs per year divided by the contribution per examination.

A sensitivity analysis was performed to determine the relative effects of equipment costs (price of scanner plus service contract) and personnel costs on the overall cost of the scanner. This was performed by increasing the aggregate equipment cost by 20% and determining this effect on the aggregate cost at both high- and low-volume models for each scanner. Aggregate equipment costs were then decreased by 20%, and the overall effect on examination cost was calculated for each scanner at high- and low-volume models. A similar calculation was performed by increasing and decreasing personnel costs by 20% and determining the effect on overall examination cost for both scanners with the high- and low-volume models. We also determined a break-even cost for the transportable scanner so that its cost per scan was equal to that of the fixed platform scanner at high and low examination volumes. This calculation was performed by keeping the examination volume constant (1,820 for low volume and 5,460 for high volume) and using our numbers for personnel cost, radiology department overhead, and hospital space costs presented in the Results section. The calculation was performed as follows: transportable equipment costs + transportable personnel costs + transportable overhead costs + transportable space costs = fixed equipment costs + fixed personnel costs + fixed overhead costs + fixed space costs. This equation can be solved for transportable equipment costs since all other variables are known.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
The total costs of both the transportable and fixed platform scanners are summarized in Table 1. Personnel costs are summarized in Table 2.

Fixed Costs
The equipment purchase price was $549,000 for the transportable scanner and $400,000 for the fixed platform scanner. With use of a 7-year depreciation model, the annual equipment cost was thus $78,429 for the transportable scanner and $57,143 for the fixed platform scanner. By using the low-volume model for the transportable scanner of 1,820 examinations per year, the equipment depreciation per examination would be $43.09. With the high-volume model of 5,460 examinations per year for the transportable scanner, the equipment depreciation per examination would be $14.36. For the low-volume model of the fixed platform scanner (3,153 examinations per year), the equipment depreciation would be $18.12 per examination. By using the high-volume model for the fixed platform scanner (14,130 examinations per year), the equipment depreciation would be $4.04 per examination.

Annual service contract costs were $80,000 for the transportable scanner and $90,000 for the fixed platform scanner. Thus, with use of the volume statistics listed in the preceding paragraph, the service contract costs per examination for the transportable scanner would be $43.96 and $14.65 for the low- and high-volume models, respectively. For the fixed platform scanner, the service contract costs per examination would be $28.54 and $6.37 for the low- and high-volume models, respectively.

Variable Costs
Aggregate personnel costs were significantly higher (P = .001) for the transportable CT scanner ($70.73) than for the fixed platform CT scanner ($55.69). This was largely attributed to the longer radiology technologist time required for transportable CT. The fixed platform CT scanner required significantly (P < .001) more nurse and respiratory therapist time than did the transportable CT scanner, mainly owing to patient preparation time, transportation time, and nursing time spent in the radiology suite where the examination was being performed. The mean personnel costs per examination for transportable studies performed by month for the 6 months of the study were $84.53, $71.79, $66.15, $83.77, $76.64, and $62.21, respectively. There was a slight trend toward decreasing costs over the course of the study (P = .03). Thus, there did appear to be a learning curve associated with use of the transportable scanner.

Indirect Costs
In 1998, the radiology department overhead was $3,017,733 excluding capital equipment and service contracts. In the same year, the department volume was 322,691 examinations (37,378 CT) resulting in an overhead of $9.35 per examination. If a transportable scanner were added in 1998, aggregate departmental volume would have increased to 324,511 using the low-volume model or to 328,151 using the high-volume model. Assuming the departmental overhead remained stable, the indirect costs per examination for the transportable scanner would be $9.30 and $9.20 using the low- and high-volume models, respectively. Adding a fixed platform scanner to the department would have increased the volumes to 325,844 and 336,821 and the indirect costs per examination to $9.26 and $8.96 for the low- and high-volume models, respectively.

Space costs were based on the square footage occupied by the scanners. The transportable scanner occupied 72 square feet in the corridor adjacent to the neurosurgery ICU. The fixed platform scanner occupied 850 square feet in the radiology department. In 1998, the hospital overhead burden for space was $2.91 per square foot. Thus, the annual space costs were $209.52 for the transportable scanner and $2,473.50 for the fixed platform scanner. On the basis of the low- and high-volume models, the space costs per examination for the transportable scanner were $0.12 and $0.04, respectively. The space costs for the fixed platform scanner using the low- and high-volume models were $0.78 and $0.18 per examination, respectively.

Calculation of Break-Even Volume and Sensitivity Analysis
If a hospital is reimbursed based on fee for service, a break-even volume can be calculated for each scanner. To calculate a break-even volume for purposes of comparison of the scanners, a proxy for the amount of reimbursement received was assumed to be the Medicare technical reimbursement for a nonenhanced brain CT examination, which was $191 in 1998. This reimbursement is the same for transportable and fixed platform scanners. The contribution to fixed costs per examination is revenue minus variable cost per examination. These values were $120 ($191 - $71) for the transportable scanner and $135 ($191 - $56) for the fixed platform scanner. Annual CT division fixed costs per year can be calculated by summing the annual cost of the scanner ($78,429 transportable, $57,142 fixed), annual service contracts ($80,000 transportable, $90,000 fixed), annual departmental overhead ($349,550 for each), and annual hospital space costs ($210 transportable, $2,474 fixed). Thus, the annual fixed costs were $508,189 for the transportable scanner and $499,166 for the fixed platform scanner. For the transportable scanner, the break-even volume was 508,189/120 or 4,235 examinations per year. For the fixed platform scanner, the break-even volume was 499,166/135 or 3,698 examinations per year.

Results of the sensitivity analysis are presented in Table 3. These results show that the transportable examinations remain more costly than the fixed platform examinations if the equipment and personnel costs are increased or decreased by 20%. Equipment costs had a greater effect on aggregate cost differences because they represent the single largest component of examination cost. With use of the low-volume model, the break-even transportable equipment cost per examination was $67.05 or 23% ([$87.05 - $67.05]/$87.05) lower than the prices in 1998. With the high-volume model, the break-even transportable equipment costs were $12.32 per examination or 58% ([$29.02 - $12.32]/$29.02) lower than the prices in 1998.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

Analysis of direct variable costs demonstrated higher personnel costs for the transportable CT scanner ($70.73) than for the fixed platform scanner ($55.69). Closer analysis of the personnel costs revealed that the major reason for the increased cost of the transportable scanner was the increased technologist cost ($43.00 transportable, $9.07 fixed platform). This increased technologist cost more than offset the reduced nursing time required for the transportable scanner ($25.42 transportable, $37.30 fixed). Although the salary of nurses is higher than that of CT technologists at our institution, the technologist time outweighed the aggregate cost of nursing, respiratory therapist, and transport personnel required to transport the patient to the radiology department.

The break-even examination volume was 4,235 examinations per year for the transportable scanner and 3,698 examinations per year for the fixed platform scanner. Thus, from a purely economic standpoint, a radiology department should not consider purchasing a transportable scanner if their projected volume is going to be less than 4,235 examinations per year using this device. With a separate strategy, a department could lobby for higher reimbursement for a portable CT device, citing increased cost as a justification. Alternatively, the price of the transportable equipment and/or service contract could be reduced to make the cost per examination more comparable to that of the fixed platform scanner. Our calculations show that this reduction in equipment cost would have to be substantial (23%–58%) to make it comparable. Note that our data represent the results of an economic analysis, and the decision to purchase this device should also be based on medical necessity.

In this study, the personnel costs to the radiology department increased with the use of a transportable CT scanner since the main source of increased personnel cost is CT technologist time. Conversely, expenses to other hospital departments were reduced by using a transportable CT scanner because less nursing, respiratory therapist, and transporter time is required with this device. Thus, the cost of using a transportable CT scanner will vary depending on whether the data are analyzed with a departmental cost perspective or a hospital-wide cost perspective. In our study, we reported on aggregate costs that can be interpreted by using either method. Hospital personnel salaries vary depending on the geographic region; thus, the costs we report may not be directly applicable to all institutions. We report personnel times so that institutions can extrapolate their expected costs on the basis of their salary and benefit structure. Also, because each institution has different distances from the ICU to the radiology department, transport times will vary. However, our sensitivity analysis revealed that the difference in costs between the two scanners persists in spite of a 20% increase or decrease in the personnel costs. Because of this and the large effect of examination volume and equipment costs, we would not expect regional differences in salary or distance from the ICU to the radiology department to have a major effect on aggregate costs.

The costs of performing a CT examination have been investigated by others (3,5). These authors did not analyze the use of a transportable CT scanner, but they did examine the costs of fixed platform scanners in academic radiology departments. The technical cost of a nonenhanced CT examination reported by Saini and colleagues (3) was $150. Our range of $75 to $112 for a fixed platform CT examination was lower than their estimates, but they included a hospital overhead estimate of 85% in the radiology costs. If we added their estimate of hospital overhead of 85% to our calculations, our costs would be $139–$208, comparing favorably with their results. Nissenbaum and colleagues (5) performed a detailed cost analysis of all components of all CT examinations (including professional reimbursement) at an academic institution. They broke down their analysis by type of examination. When the professional and film costs were deleted from their analysis of nonenhanced brain CT examinations (making their results comparable to ours), their cost was $117 per examination. This value is slightly higher but comparable to our results. Our estimates of hospital overhead are lower than those of other published estimates because we used hospital charges for space and not other hospital administrations such as hospital support personnel, billing, administrators, and so forth. Geographic variations in administrative and overhead costs as well as salaries can affect total technical costs.

There are several limitations to this study. Estimating potential examination volume is difficult to perform with the transportable scanner because no institution has published a large experience with this device. To account for this, we calculated low- and high-volume models for both scanners. For the fixed platform scanner, we used true examination volume of two scanners operating at our institution during the time of the study. For the transportable scanner, we used a low-volume model of seven examinations per day and a high-volume model of 15 examinations per day. This high estimate of 15 examinations per day may overestimate potential demand. The transportable scanner could be used to image other inpatients or perform other types of examinations to generate this volume, but given the expense of the device and personnel, this may not prove practical. A second potential limitation of our study is the personnel used to staff the transportable scanner. In our model, two technologists performed most transportable examinations. If only one technologist was used with a transporter (or technologist aide) to help assemble the scanner, the variable cost of performing a transportable examination could have been reduced. Transportable in-hospital scanning represents a relatively new application of CT technology. Because the ease of use and design of the equipment may change over time and thus affect its costs, the current analysis may have to be reevaluated in light of future developments. Despite these potential limitations, we believe this study is important, as it attempts to estimate costs of a new modality.

The overall utility of transportable imaging technology depends on multiple factors, including image quality, examination cost, and value to the patient of being imaged in the ICU rather than the radiology department. When analyzing the last component, it is clear that the utility of a transportable CT scanner will depend on the severity of illness of the patient. Thus, if a medically stable patient cohort that does not require nurse monitoring is imaged, the transportable scanner probably will not be cost-effective. Conversely, if the patient population is very ill such that leaving the ICU would place them at great medical risk, the transportable scanner will be medically and economically more valuable. Thus, the true value of transportable imaging technology is partially dependent on the severity of illness of the patients being imaged. These parameters are difficult to measure reliably (13).

We believe it is important to quantify the costs of a new transportable imaging modality. In the current era of fiscal responsibility, careful analysis of the costs for new imaging devices should be performed to determine their true value to a department. Effective cost analysis can lead to improved efficiency of radiology departments faced with limited resources.

	FOOTNOTES

 
Abbreviation: ICU = intensive care unit

Author contributions: Guarantor of integrity of entire study, W.W.M.S.; study concepts, all authors; study design, W.W.M.S., G.S.D., I.F.G., J.T.R.; literature research, W.W.M.S., J.T.R., C.M.C.; clinical studies, W.W.M.S., W.J.S., C.M.C.; data acquisition, W.J.S., C.M.C., I.F.G.; data analysis/interpretation, W.W.M.S., J.T.R., I.F.G.; statistical analysis, I.F.G.; manuscript preparation, definition of intellectual content, editing, revision/review, and final version approval, all authors.

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This Article Abstract Full Text (PDF) All Versions of this Article: 2261012047v1 226/1/63    most recent Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Mayo-Smith, W. W. Articles by Dorfman, G. S. Search for Related Content PubMed PubMed Citation Articles by Mayo-Smith, W. W. Articles by Dorfman, G. S.