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Letters to the Editor |
Department of Radiology, Section of Vascular and Intervention, University of Iowa Hospitals and Clinics, 200 Hawkins Drive, 3JPP, Iowa City, IA 52242
e-mail: hicham-abada{at}uiowa.edu
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Furthermore, we would like to highlight one technical point. Dr Iezzi and colleagues used an injection protocol that corresponded to a loading dose of iodine less than 1 g/sec. We know, from our experience and from the cardiac literature, the expected vascular enhancement based on contrast media concentration and injection rate parameters. Becker et al (3) demonstrated that, with the same concentration of contrast material as used by Dr Iezzi and colleagues, but with a higher injection rate (4 mL/sec) along with a higher amount of contrast material (140 mL) resulting in a loading dose of 1.2 g/sec, aortic attenuation in mean is equal to 273 HU. The CT attenuation in the study by Dr Iezzi and colleagues is then below that threshold and is sufficient to discriminate calcification (>350 HU) from contrast enhancement during the arterial phase.
In addition, the delayed phase CT acquisition would demonstrate a drop in aortic enhancement that increases this level of discrimination. In the event that this distinction remains not possible, even a single extra CT section obtained at 1 minute after the delayed phase acquisition can easily remove all doubt.
Delayed acquisition also has the additional advantage of allowing a comprehensive understanding of inflow and outflow vessels, which is of utmost importance in planning therapy when an embolization procedure is indicated.
Once the importance of radiation dose is mentioned, a global effective approach should be initiated either by including automatic techniques (4) or by lowering voltage settings (5). However, the authors did not mention any attempt toward that aim. Our current CT angiography protocol for imaging the aorta includes the use of automatic dose reduction (CARE Dose 4D; Siemens, Forchheim, Germany) combined with a 100-kV protocol when patient weight is less than 80 kg. Wintersperger et al (5) demonstrated the clinically relevant use of a 100-kV protocol for imaging aortoiliac vessels.
Follow-up of patients with CT angiography is a real-time investigation; close monitoring by the radiologist is mandatory.
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* Department of Clinical Science and Bioimaging, Section of Radiology, University of Chieti, via dei Vestini, Chieti 66013, Italy
e-mail: r.iezzi{at}rad.unich.it
Department of Bioimaging and Radiological Sciences, Catholic University of Rome, Italy
We are very pleased with Drs Abada and Golzarian for their interest in our study (1), and we appreciate their comments. Our suggested acquisition protocol (unenhanced and arterial phase CT at 1-month follow-up and only arterial phase CT at subsequent follow-up) seems to ensure adequate assessment in patients treated with endovascular abdominal aortic aneurysm repair (EVAR) and reduces unnecessary radiation exposure.
We affirmed that unenhanced CT scans are required mainly to help the evaluation of contrast material–enhanced data sets to differentiate small endoleaks from other entities, but we never affirmed that it is impossible to distinguish them by using delayed phase images alone. Furthermore, we share the opinion that delayed phase CT should be able to depict endoleak (2) and that arterial phase CT should be sufficient to discriminate calcification and endoleak in almost all cases, owing to the expected vascular enhancement.
However, instead of routinely acquiring delayed phase CT scans, the same purpose can be reached by acquiring initial unenhanced CT scans at 1-month follow-up only and by using these images as a guide for interpretation of arterial phase images acquired at subsequent follow-up CT evaluations. Furthermore, we think that "even a single extra CT section obtained at 1 minute after the delayed phase acquisition" cannot be implemented in practice. Few centers have the staff or processes in place to interpret ongoing studies and compare arterial and delayed phase scans while the patient is lying in the CT scanner.
On the other hand, Drs Abada and Golzarian also underline the potential additional advantage of delayed phase CT in the understanding of inflow and outflow vessels. Although we share their opinion, we think that it is not convenient to routinely acquire a delayed phase CT scan with only this purpose. Few patients require embolization therapy, as confirmed by the experience of Dr Golzarian and a colleague (3) (three patients of more than 300 treated). Furthermore, as also reported by Dr Golzarian and colleague (3), selective diagnostic angiography performed just before the planned embolization is superior to CT angiography for the detection of outflow vessels.
About the last commentary, we completely agree with the authors that any radiation dose should be minimized. We started by reducing the number of imaging phases routinely performed, by assuming that a reduction should lead to a reduction in unnecessary radiation exposure. We also read the article by Wintersperger et al (4) with interest. It would be interesting to investigate the possibility of reducing x-ray exposure during multidetector CT angiography follow-up after EVAR and to define the real influence of radiation dose on CT image quality, and we are trying to do this.
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