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We commend Ling et al. 1 on their work in adding lowdose computed tomography (LDCT) to ventilationperfusion lung scans (V/Q) with single-photon emission computed tomography (SPECT) for the evaluation of patients with suspected pulmonary embolism (PE). Literature in this domain 2,3 largely shares this view, but to date, trials are limited to single-centre studies. Hence, this study is, at the very least, noteworthy.But, would SPECT V/Q (without LDCT) have contributed to similar outcomes? The title and opening paragraph suggest that the article might go some way in answering this important question. SPECT acquisitions are performed routinely for all V/Q studies in our hospital. Would adding LDCT enhance the diagnostic yield? Should we be changing our practice? Unfortunately, our quest for these answers ends in disappointment.Comparing outcomes from SPECT V/Q with and without LDCT would be the most obvious avenue in resolving the dilemma. This would be more useful and attainable than using the composite reference standard chosen. If there is a gain in specificity, surely, this is the simplest way to quantify it.Our interest was also piqued by the population evaluated in this study. The prevalence of PE by Wells criteria (compared with other studies in this domain) is encouraging. It is also conspicuous that the study population is 'young', and the prevalence of lung disease is low. In this light, did the patients genuinely benefit from the addition of LDCT? While it may have been a relatively low dose of radiation (0.9 mSv), 1 could this have been avoided altogether? After all, this would be adding 60% to the radiation dose already incurred from V/Q (1.5 mSv, with or without SPECT acquisition) and an even greater effective breast dose in women. It is clear that patients already receive excessive radiation through various diagnostic processes. 4 Would it be more judicious to apply LDCT selectively for a population above a certain age threshold and/or with known pulmonary parenchymal pathology? References1 Ling IT, Naqvi HA, Siew TK, Loh NK, Ryan GF. SPECT ventilation perfusion scanning with the addition of low-dose CT for the investigation of suspected pulmonary embolism. Intern Med J 2012; 42: 1257-61. 2 Roach PJ, Gradinscak DJ, Schembri GP, Bailey EA, Willowson KP, Bailey DL. SPECT/CT in V/Q scanning. Semin Nucl Med 2010; 40: 455-66. 3 Gutte H, Mortensen J, Jensen CV, Johnbeck CB, von der Recke P, Petersen CL et al. Detection of pulmonary embolism with combined ventilation-perfusion SPECT and low-dose CT: head-to-head comparison with multidetector CT angiography.

We agree wholeheartedly with the conclusion of Martineau-Beaulieu and Lanthier 1 that radiation doses need to be reduced in hospital inpatients. Putting aside the apparent overestimation of the radiation dose associated with a thallium scan -the figure quoted is more than 50% over that cited by most authorities 2 -we also reflect on the potentially strong diagnostic benefits of these procedures.Three of the four procedures associated with the highest radiation doses also have the greatest potential for establishing a life-saving diagnosis and/or significantly impacting on the management algorithm at the outset of hospital admission. Namely, these are coronary angiogram/angioplasty, computed tomography (CT) pulmonary angiogram and positron emission tomography with fluorodexoyglucose.On the one hand, any one of these procedures alone carries a radiation burden exceeding even the mean dose of the surveyed patients -which, in itself, is almost three times the annual background. However, these procedures may also expedite the correct diagnosis and/or instigate an appropriate treatment paradigm. This may, in fact, have greater overall benefit for the patient -and the healthcare system -in terms of time and resources. This is of particular importance given the high incidence of readmission rates among medical inpatients, as highlighted recently. 3 Certainly, we are encouraged by thoughtful application of new diagnostic techniques (where appropriate) to minimise radiation exposure to patients, such as with CT coronary angiography, 4 whose clinical value is supported by increasing volumes of evidence. 5 We also advocate for further analysis of the data as to how often the associated radiation doses directly resulted in optimal patient outcomes. Conversely, would the authors be able to garner more useful information by establishing how much of the radiation exposure of the average hospital inpatient was actually unnecessary?