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British Journal of Radiology (2005) 78, 46-50
© 2005 British Institute of Radiology
doi: 10.1259/bjr/53924376
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Full Paper

Effect of the introduction of helical CT on radiation dose in the investigation of pulmonary embolism

J O'Neill, MB, BCh, FRCR, MSc 1 J T Murchison, MBChB, FRCR, FRCP 2 L Wright, BSc, MSc 2 and J Williams, MSc, FIPEM 2

1 Radiology Department, St Josephs Hospital, 50 Charlton Ave East, Hamilton, Ontario, Canada and 2 Department of Radiology, Royal Infirmary of Edinburgh, Little France Crescent, Edinburgh EH16 4SA, UK

Correspondence: John M O'Neill, Radiology Department, St Josephs Hospital, 50 Charlton Ave East, Hamilton, Ontario, Canada


   Abstract
Top
 Abstract
Introduction
 Materials and method
 Patient dosimetry
 Results
 Discussion
 References
 
The aim of this study was to assess the change in patient radiation dose in the radiological investigation of pulmonary embolism since the introduction of helical CT pulmonary angiography (CTPA) in a large teaching hospital. All radiological investigations performed as an integral part of the imaging protocol in the investigation of clinically suspected pulmonary embolism (PE) were retrospectively reviewed. The protocol for the investigation of PE changed in our institution after the introduction of CTPA. Protocols 1 and 2 were the protocols in place before and after the introduction of CTPA, respectively. An in-depth evaluation was made of the imaging records and radiation dose for 30 consecutive patients investigated for clinically suspected PE in 1995 (protocol 1) and 2002 (protocol 2). Radiation doses were then extrapolated for the total number of patients investigated in each year. The number of radiological investigations performed per patient decreased from a mean of 1.17 in protocol 1 to 1.06 in protocol 2. There was a 44% increase in the total number of patients investigated. The effective dose per patient increased from 1.30 mSv to 1.35 mSv with the introduction of CTPA into the imaging protocol, an increase of only 4%. First line investigations showed a significant decrease in indeterminate examinations from 25.7% to 8.5%. Two different imaging protocols are reviewed with respect to type and number of procedures required for the investigation of PE and the resulting patient effective dose incurred. Results demonstrate an increase in the number of patients being investigated for suspected PE and a small increase in effective dose per patient since the introduction of helical CTPA. Although CTPA in itself incurs a higher effective dose, this is offset by the significant decrease in the number of non-diagnostic and total number of investigations per patient. In addition the ventilation component of lung scintigraphy was not required in protocol 2, thus reducing the dose further. We believe this small increase in effective dose is justified by the decrease in non-diagnostic studies and the reduction in total number of investigations per patient. We hope this paper will serve as a stimulus for the radiology community to examine current protocols in all areas of diagnostic imaging. We stress the importance of assessing new and established imaging investigative protocols to maximize the benefit and reduce any risk to patients.


   Introduction
Top
 Abstract
 Introduction
Materials and method
 Patient dosimetry
 Results
 Discussion
 References
 
The introduction of a new diagnostic imaging test in the investigative protocol for the diagnosis and management of patient pathology requires the fulfilment of several criteria. The predominant criterion is that the benefit of the test outweighs any incurred risks and that this benefit/risk ratio is equal to or greater than for the imaging tests already in place. In the investigation of pulmonary embolism (PE), helical computed tomographic pulmonary angiography (CTPA) and the newer multislice CTPA require both the use of ionizing radiation and intravenous contrast material (IVCM) which can be considered risk factors. The risk of adverse reactions from IVCM is established but to our knowledge there has been no review of the radiation dose incurred in this new investigation as part of a diagnostic imaging protocol with direct comparison with those imaging tests previously available. The purpose of our study is to assess what impact the introduction of helical CTPA in the investigation protocol of PE has had on patient radiation dose. In addition we review the measurement of this radiation dose and if there has been a change, the significance of this to the patient.

Pulmonary embolism is a common condition with significant morbidity and mortality, the latter reaching 30% in some series in untreated cases [1, 2]. Appropriate use of anticoagulation or thrombolytic therapy improves outcome with a reduction in mortality to as little as 2.5% [3]. Therapy however is not risk free and serious complications may occur [4]. Identifying the correct patients to treat is therefore essential. Unfortunately the clinical manifestations of PE are variable and lack specificity to diagnose reliably or exclude clinically significant PE [5]. Imaging is mandatory to confirm the diagnosis but conventional strategies have been imprecise [6].

Multiple studies have reviewed CTPA against both formal pulmonary angiography and radionuclide ventilation/perfusion (V/Q) imaging and have consistently demonstrated a high sensitivity and specificity [7–12]. It is only minimally invasive, allows direct visualization of the thrombus and thrombus load. However demonstration of PE beyond segmental artery level is limited at present for single slice CT. Whether subsegmental emboli contribute significantly to morbidity and mortality is debatable [13].

The dosimetric measurement that provides a direct relationship to the radiation hazard is the effective dose (ED). This was introduced in 1990 by the International Commission on Radiological Protection and is defined by the sum of the products of the equivalent organ dose and its relevant organ weighting factor for all tissues and organs within the body. It is therefore not measured directly. Expressing the dose in the form of an effective dose allows the relative biological effect and therefore the relative risk of different radiation doses to be compared directly. It is this latter point that makes it the most appropriate measurement for use in this study due to the use of different imaging modalities.


   Materials and method
Top
 Abstract
 Introduction
 Materials and method
Patient dosimetry
 Results
 Discussion
 References
 
A retrospective review of imaging protocols for the investigation of PE was performed. Protocol 1 and 2 were assigned to the imaging protocols in place in our centre before and after the introduction of CTPA, respectively (Figures 1 and 2GoGo). Imaging records and radiation dose for 30 consecutive patients investigated for clinically suspected PE in 1995 were reviewed. Plain radiographs were a necessary part of both protocols and therefore were excluded from further calculations assessing the change, if any, in effective dose. Radionuclide lung scintigrams (V/Q) and where appropriate lower limb venography and conventional pulmonary angiograms were assessed. The process was repeated in 2002 using the same parameters with the addition of CTPA. Due to the small number of conventional pulmonary angiograms, each of these studies was separately assessed for ED. The same applies to the seven cases of lower limb venography in protocol 2, with 20 cases assessed for protocol 1. Both years were then assessed with respect to the total number of radiological investigations performed for clinically suspected PE and the total effective doses involved.



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  		Figure 1. Protocol 1, radiological investigation clinically suspected pulmonary embolism (PE) pre CT pulmonary angiography (CTPA). CXR, chest radiograph; U/S, ultraround.

 


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  		Figure 2. Protocol 2, radiological investigation clinically suspected pulmonary embolism (PE) post CT pulmonary angiography (CTPA). CXR, chest radiograph; U/S, ultraround.

 
Patients in protocol 2 had a perfusion scan performed if the chest radiograph was normal and there was no history of chronic pulmonary disease. If the chest radiograph provided an alternative diagnosis, no further imaging was performed with respect to PE unless there was a high clinical suspicion of concurrent PE. Otherwise the remaining patients proceeded to CTPA.

Radionuclide lung scintigrams were obtained using a GE MaxiCamera 400 (Single Head; GE, Milwaukee, WI) gamma camera. 80 MBq technetium-99m macro-aggregated albumin (MAA) was injected via a peripheral intravenous cannula with saline flush. Images were obtained in anterior, posterior, right and left anterior and posterior obliques, with 400 000 counts per image. Those patients requiring ventilation as part of this study, protocol 1, received 160 MBq of xenon-133 via aerosol. Reports were divided into three groups, normal, indeterminate and high probability and assessed in conjunction with the clinical presentation.

CTPA was performed on a GE HiSpeed Advance Single Slice CT machine (Milwaukee, WI, USA). 100 ml non-ionic contrast media (Niopam 300 mg ml–1), was diluted with 40 ml saline and injected via a 16G or 18G needle into the antecubital fossa at a rate of 4 ml s–1. Images were obtained with a delay of between 10–15 s, depending on the patient's cardiac status, from diaphragm to aortic arch using a pitch of 1.7, 3 mm slice thickness with image reconstruction at 1.5 mm intervals. Tube potential, tube current and the number of slices per examination were noted.

Venography and Doppler examinations are well recognized investigations for deep venous thromboembolic disease and were performed using standard protocols. Conventional pulmonary angiography was performed using a femoral approach and 100 cm 5 F pigtail catheter. Selective right and left main pulmonary artery runs with AP and 30° anterior oblique views were performed. Coned segmental runs were acquired when indicated.


   Patient dosimetry
Top
 Abstract
 Introduction
 Materials and method
 Patient dosimetry
Results
 Discussion
 References
 
Doses for CT scanning were calculated from the CT dose index measured in air (CTDIair) using the matched scanner technique developed by ImPACT [14]. Patient organ doses and thus effective dose are calculated using the NRPB Monte Carlo dose data for CT scanners [18]. The scanning protocol on the single slice scanner used 120 kV and 200 mA with a slice thickness of 3 mm and a pitch of 1.7.

Doses for radionuclide studies were derived from the administered activity using data in the ARSAC Notes for Guidance [15].

For conventional pulmonary angiography and for venography, effective dose was calculated from dose–area product (DAP) using conversion factors taken from Hart et al [16].

All dose calculations assume that the patient has an average build.


   Results
Top
 Abstract
 Introduction
 Materials and method
 Patient dosimetry
 Results
Discussion
 References
 
The dose for CTPA was calculated for 16 patients. The average ED was 1.6 mSv. The standard scanning protocol was used for all patients with the only variant being the scanned length varying from 78 mm to 108 mm leading to a relatively small range of ED (1.4–1.9 mSv). V/Q (protocol 1) and Q (protocol 2) imaging were associated with doses of 1.2 mSv and 0.8 mSv, respectively, with no variation since the same activity was used for all patients. Doses for lower limb and pelvic venography were assessed for 12 patients. For these patients the average DAP was 3.1 Gy cm2 (range 1.7 to 7.2) from which it was estimated that the average ED was 0.3 mSv. Doses for conventional pulmonary angiography were assessed for three patients. The average DAP was 21.3 Gy cm2 from which ED equal to 3.2 mSv was calculated (range 2.3 to 4.1 mSv).

Both years were then reviewed with respect to total investigations and results. In the first year 526 patients were investigated for PE with 617 examinations performed, equivalent to 1.17 per patient. 526 V/Q studies demonstrated a combined positive or normal result in 74.3% with the remaining 25.7% been indeterminate. Additional investigations in the latter group were performed in 63% (Table 1Go). The average dose was 1.30 mSv per patient with 92% of this average being attributed to the V/Q studies (Table 2Go).


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  		Table 1. Results Protocol 1 investigations

 

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  		Table 2. Effective dose (ED) (mSv) per examination (Protocol 1)

 
In the final year of the study 760 patients were investigated for clinically suspected PE, with 805 investigations, equivalent to 1.06 investigations per patient. CTPA was diagnostic in 97.8%, been the primary investigation in 59.3% of patients with Q lung scanning in the remaining 40.7%. The latter examination was diagnostic in 79.3% with 21% indeterminate. Further investigations included CTPA in 48% of the indeterminate group with 26% being positive (Table 3Go). 2.2% of CTPA examinations were indeterminate. In 8.9% of patients in the second protocol further investigations were performed due to discrepancy between the clinical and radiological diagnosis and to assess the presence and extent of peripheral thrombus. The average effective dose was 1.35 mSv per patient with 24% and 75% of this being attributed to Q studies and CTPA, respectively (Table 4Go).


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  		Table 3. Results Protocol 2 investigations

 

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  		Table 4. Effective dose (ED) (mSv) per examination (Protocol 2)

 

   Discussion
Top
 Abstract
 Introduction
 Materials and method
 Patient dosimetry
 Results
 Discussion
References
 
Radiological examinations are required to conform to two principles: justification to ensure that the benefit outweighs any risk, and optimization to ensure that dose is kept as low as reasonably achievable (ALARA) consistent with the clinical imperative for accurate diagnosis. In radiology using ionizing radiations, risk is assumed to be proportional to effective dose. In evaluating any new investigation protocol, it is not only necessary to consider the benefits in terms of diagnostic accuracy and patient acceptability, the risks, including the radiation risks, must also be assessed. Radiation risks are assessed in terms of effective dose and for the general population, the fatal cancer risk has been estimated to be 5% per sievert [17]. In this study we have considered the radiation risks for two protocols. In protocol 1 all patients for whom PE was not excluded by a preliminary chest X-ray, had V/Q scans with an average of 0.16 additional follow up radiology investigations, the average patient dose was 1.3 mSv. More recently (in protocol 2) the chest X-ray has been used to determine whether a perfusion scan or a CT pulmonary angiogram is the most appropriate investigation. For this protocol the number of follow up radiological investigations was reduced (to an average of 0.07 per patient) and the average effective dose per patient (1.35 mSv) was not significantly different. The overall radiological risk for the two protocols is therefore in the range of 6 to 7 fatal cancers per 100 000.

There are significant uncertainties in the estimation of effective dose from the measurable dosimetry quantities, i.e. activity, DAP and CTDI. In each case the standard conversion factors from the measured quantity to ED are taken from calculations based on a mathematical phantom representing an average sized person. The inherent uncertainties in the calculation depend on whether it is an internal radiation source, as in radionuclide imaging, or whether it is an external source, CT or conventional angiography. There are additional uncertainties introduced into the CT dose calculation due to the use of the scanner matching method used in the ImPACT dose calculator [14]. There are also patient-to-patient dose variations, the effect of which may have influenced the results of the study due to the relatively low proportion of the full patient cohort for whom doses were assessed. However, the effect of these variations was minimized by standardization of the three principal investigations. In protocol 1, 92% of the average patient dose was from the V/Q scan for which a standard activity was administered. In protocol 2, 24% of the dose was from perfusion scans with a standard activity and 75% of the dose from CTPA in which the maximum to minimum dose ratio assessed for 16 patients was 1.36. The much bigger uncertainties in the doses for other examinations (venography and conventional angiography) therefore have little significance in the overall conclusions.

The results are for a particular model of single slice helical scanner used for CTPA in this Hospital at the time of this study. The variation in dose between different models of scanner and between different institutes is known to be very large, e.g. by a factor of 8.4 for routine chest CT in an early CT dose survey [19]. It should also be noted that it is increasingly common to use multislice scanners for CTPA studies for which dose might be expected to be higher than reported here due to the use of a lower effective pitch. Notwithstanding these reservations regarding the dose which might be given elsewhere, it has been demonstrated that it is possible to use CTPA in protocol 2 without a significant increase in effective dose and thus in radiation risk.

The new diagnostic protocol that we present has a minimal increase in effective dose. The ED for CTPA is higher than for perfusion or V/Q scans and the role and position of CTPA in other diagnostic protocols must be carefully examined so that the patient receives the optimum benefit while minimizing risk.

In conclusion, our study has reviewed the total effective dose of the diagnostic imaging protocols as outlined above pre and post introduction of CTPA. Although in itself CTPA has a higher ED than the previously used V/Q imaging, the reduction in the number of additional investigations, and performing perfusion only for lung scintigraphy in protocol 2, has limited its impact from an ED viewpoint. With the introduction of CTPA there has been a significant increase in the total number of patients investigated, which is likely due to a multitude of factors including increased awareness of a disease with the advent of a new diagnostic tool, a reduction in the number of non-diagnostic results and the possibility of identifying alternative diagnosis. This has led to a corresponding increase in the total population radiation dose but only a minimal increase in the ED per patient investigated. This must be balanced against the social, economic and patient benefits due to the reduced number of examinations performed per patient with a significant reduction in the number of non-diagnostic examinations of a potentially fatal medical condition. This study will serve as a valuable baseline for the assessment of multislice CT with respect to ED. We hope this paper will serve as a stimulus for the radiology community to examine current protocols in all areas of diagnostic imaging. We stress the importance of assessing new and established imaging investigative protocols to maximize the benefit and reduce any risk to patients.

Received for publication October 7, 2003. Revision received August 23, 2004. Accepted for publication September 17, 2004.


   References
Top
 Abstract
 Introduction
 Materials and method
 Patient dosimetry
 Results
 Discussion
 References
 

  1. Dalen JE, Alpert JS. Natural history of pulmonary embolism. Prog Cardiovasc Dis 1975;17:259–70.[Medline]
  2. Moser KM. State of the art: venous thromboembolism. Am Rev Respir Dis 1990;414:235–49.
  3. Carson JL, Kelley MA, Duff A, Weg JG, Fulkerson WJ, Palevesky M, et al. The clinical course of pulmonary embolism. N Engl J Med 1992;326:1240–5.[Abstract]
  4. Ginsberg JS. Management of venous thromboembolism. N Engl J Med 1996;335:1817–28.
  5. British Thoracic Society. Suspected acute pulmonary embolism: a practical approach. Thorax 1997;52:s1–24.
  6. Murchison JT, Gavan DR, Reid JH. Clinical utilization of the non-diagnostic lung scintigram. Clin Radiol 1997;52:295–8.[CrossRef][Medline]
  7. Safriel Y, Zinn H. CT pulmonary angiography in the detection of pulmonary emboli. A meta-analysis of sensitivities and specificities. J Clin Imaging 2002;26:101–5.
  8. Hansell DM. Spiral computed tomography and pulmonary embolism: current state. Clin Radiol 1997;52:575–81.[CrossRef][Medline]
  9. Remy-Jardin M, Remy J, Deschildre F, Artaud D, Borgi JP, Hossein-Foucher C, et al. Diagnosis of pulmonary embolism with spiral CT: comparison with pulmonary angiography and scintigraphy. Radiology 1996;200:699–706.[Abstract/Free Full Text]
  10. Goodman LR, Lipchik RJ, Kuzo RS. Acute pulmonary embolism: the role of computed tomographic imaging. J Thorac Imag 1997;12:83–6.[Medline]
  11. Fennerty T. The diagnosis of pulmonary embolism. BMJ 1997;314:425–9.[Free Full Text]
  12. Teigen CL, Maus TP, Sheedy PF, Stanson AW, Johnson CM, Brear J, et al. Pulmonary embolism: diagnosis with contrast-enhanced electron-beam CT and comparison with pulmonary angiography. Radiology 1995;194:313–9.[Abstract/Free Full Text]
  13. Remy-Jardin M, Remy J, Wattinne L, Giraud F. Central pulmonary thromboembolism: diagnosis with spiral volumetric CT with the single breath-hold technique—comparison with pulmonary angiography. Radiology 1992;185:381–7.[Abstract/Free Full Text]
  14. http://www.impactscan.org/ctdosimetry.htm
  15. Administration of Radioactive Substances Advisory Committee. Notes for guidance on the clinical administration of radiopharmaceuticals and use of sealed radioactive sources. Chilton: NRPB, 1998.
  16. Hart D, Jones DG, Wall BF. Estimation of effective dose in diagnostic radiology from entrance surface dose and dose-area product measurements. NRPB-R262. Chilton: NRPB, 1995.
  17. International Commission on Radiological Protection. 1990 Recommendations of the International Commission on Radiological Protection. ICRP pub 60. Oxford: Pergamon, 1991.
  18. Jones DG, Shrimpton PC. Normalised organ doses for X-ray computed tomography calculated using Monte Carlo techniques. NRPB software report SR250. Chilton: NRPB, 1993.
  19. Shrimpton PC, Jones DG, Hillier MC, Wall BF, Le Heron JC, Faulkner K. Survey of CT practice in the UK Part 2: Dosimetric aspects. NRPB-R249. Chilton: NRPB, 1991.



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