| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Letters to the Editor |
,
Mannudeep K. Kalra, MD, DNB,* and
Bernhard Schmidt, PhD
Department of Radiology, Division of Abdominal Imaging and Intervention, Massachusetts General Hospital White 270, 55 Fruit Street, Boston, MA 02114* e-mail: mkalra@partners.org
Department of Radiology, University of Milan, Milan, Italy
Siemens Medical Solutions, Forchheim, Germany
Editor:
We read with interest recent Radiology articles of our esteemed colleagues, which address a crucial issue of radiation dose reduction in computed tomographic (CT) scanning (1–3). Whereas Mastora et al (1) employed automatic exposure control (angular modulation) for adapting radiation dose to patient size and attenuation profile for CT scanning of the thoracic outlet, Sigal-Cinqualbre et al (2) reported use of weight-based selection of fixed tube current values for CT scanning of the chest. We were particularly interested in the comments of Dr Paul and colleagues (3), who stated that automatic exposure control techniques must not mask individual dose adaptation in specific indications.
We agree with the observation that the radiation dose associated with the automatic exposure control technique reported by Mastora et al (1) (140 kVp and 140 mAs) is greater than the dose associated with the low-kilovoltage and fixed–tube current protocol used by Sigal-Cinqualbre et al (2,3). However, we do not agree with their comments on the need for individual adaptation of scanning parameters based on anatomic information and use of automatic exposure control technique as a supplementary technique.
In our institution, we routinely use angular z-axis and combined modulation techniques of automatic exposure control for CT scanning of the chest, abdomen, and pelvis and for CT angiography (4). Mastora et al (1) used an online real-time angular modulation technique (CARE Dose; Siemens Medical Solutions, Forchheim, Germany) for CT scanning of the thoracic outlet (1). This technique adapts tube current to maintain constant image quality with radiation dose lower than or similar to the dose associated with fixed tube current scanning performed with the same scanning parameters (4). To use this technique, radiologists can select any tube current–time product and kilovoltage for CT scanning, including the low settings reported by Sigal-Cinqualbre et al for CT scanning of the chest (2). In fact, with angular modulation, the tube current–time product can be set at any desired level for any kilovoltage, and the associated radiation dose is either lower than or equal to the dose delivered with fixed–tube current scanning at the same tube current–time product and kilovoltage. For instance, radiation dose with angular modulation technique at 80 kVp and 135 or 180 mAs will always be lower than or equal to that associated with fixed–tube current scanning at 135 or 180 mAs and 80 kVp used in the protocol evaluated by Sigal-Cinqualbre et al, if remaining scanning parameters are held constant (2,4). This is due to the fact that an angular modulation technique uses the selected tube current–time product value at a given kilovoltage as the "desired image quality" and involves acquisition of images with the "desired image quality" by modulating tube current at levels identical to or less than that of selected tube current–time product.
Therefore, greater radiation dose associated with the higher kilovoltage and tube current–time product protocol (140 kVp and 140 mAs) evaluated by Mastora et al (1) does not suggest that automatic exposure control technique will be associated with higher radiation dose than an individual dose adaptation protocol at the same scanning parameters. However, the study provides proof of a concept that automatic exposure control technique can be used for imaging of a complex anatomic region, such as the thoracic outlet (1).
Indeed, radiation dose associated with automatic exposure control can be modified in a way similar to individually adapted low-dose protocols with fixed–tube current scanning. We use an automatic exposure control technique for performing CT angiography of the chest and abdomen at 100 kVp. Furthermore, recently introduced combined modulation (CARE Dose 4D; Siemens Medical Solutions) automatically determines whether the patient is "slim" or "obese" from a single localizer radiograph and adapts the dose to the user-specified image quality reference tube current–time product and strength of modulation (weak, average, or strong).
In our opinion, automatic exposure control allows users to select a desired image quality (and eventually a protocol) and focuses their attention on the indication of the study instead of the individual patient size, leaving this evaluation to the scanner itself. However, the selected desired image quality with automatic exposure control can also be adjusted on the basis of study indications or individual patient size to obtain desired dose reduction (4). In contradiction to the fixed tube current protocols for individual dose adaptation, these techniques permit dose modulation at different projection angles in the x-y plane, along the scanning directions or both, and increase radiation dose efficiency of CT scanners (4). Indeed, with constant evolution of multi–detector row CT technology and introduction of newer systems, it is important to understand that a particular set of fixed–tube current scanning parameters for one type of scanner cannot be applied to other scanners (5). The final goal must be to obtain similar "desired image quality" for different scanners, which is the criterion used by automatic exposure control techniques to adapt radiation dose associated with CT scanning.
Although individual dose adaptation research studies can provide valuable information for determining a reference image quality level for automatic exposure control techniques, they must not replace or supercede the latter technique. Further research is needed to explore the remaining frontier of dose reduction with promising automatic dose modulation techniques.
REFERENCES
Radiology Unit, Marie Lannelongue Hospital 133 av de la Résistance, 92350 Plessis Robinson, France e-mail: pauljf@ccml.com
We appreciate the comments of Dr Rizzo and colleagues about both our study, involving the use of low-kilovoltage chest CT protocols (1), and our letter concerning the use of automatic exposure control techniques for individual dose adaptation (2). However, a few points have to be clarified.
We did not claim that automatic dose reduction techniques were not useful, and we never compared them directly with our low-kilovoltage approach, but we pointed out that, in our opinion, the patient’s anatomy is the first parameter to consider, especially for the choice of kilovoltage setting, to optimize radiation dose. Kilovoltage setting is rarely considered, although lowering of the kilovoltage drastically decreases dose, since dose varies with the square of the kilovoltage. We gave evidence of feasibility of 80-kV protocols for thoracic CT in selected slim patients.
We are also convinced that the new automatic dose system mentioned (CARE Dose 4D; Siemens Medical Solutions), which allows automatic correction for a patient’s full anatomy (due to the additional modulation in the z-axis), will be much more efficient than the former one (ie, angular modulation technique alone, reported by Mastora et al [3]). The new automatic dose system was not available at the time of our study. This new approach corresponds exactly to our opinion of the need to consider a patient’s anatomy as the first criterion for dose optimization.
The problem of choice of the optimal kilovoltage, however, is still not addressed with any automatic exposure system at the present time. We are convinced that until automatic systems select optimal kilovoltage automatically, there will still be needs for individual dose reduction to be done by radiologists.
REFERENCES
This article has been cited by other articles:
|
|
 |
|
  U.K. Udayasankar, K. Braithwaite, M. Arvaniti, D. Tudorascu, W.C. Small, S. Little, and S. Palasis Low-Dose Nonenhanced Head CT Protocol for Follow-Up Evaluation of Children with Ventriculoperitoneal Shunt: Reduction of Radiation and Effect on Image Quality AJNR Am. J. Neuroradiol., April 1, 2008; 29(4): 802 - 806. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| RADIOLOGY | RADIOGRAPHICS | RSNA JOURNALS ONLINE |
