Heiko Wolf

Dose reduction in CT by anatomically adapted tube current modulation. II. Phantom measurements

Theoretical considerations and simulation studies have led to the expectation that patient dose in CT (computed tomography) can be reduced significantly without a concomitant loss in image quality if tube current is modulated according to rotation angle-dependent x-ray attenuation. In this study, the simulation results presented in Part I were validated with phantoms. We used one cylindrical, two oval, and one elliptical phantom, available both as mathematical descriptions and in physical form, to mimic different parts of the human anatomy. Prototype hardware was available to control tube current on a commercial clinical CT scanner. The potential for dose reduction was evaluated for sinusoidal and attenuation-based current modulation for variable modulation amplitudes. Agreement between simulations and measured results was better than within 10%. Dose reduction values of 8%-56% were found depending on the phantom geometry and tube current modulation function. Attenuation-based tube current modulation consistently yielded higher reduction than fixed-shape sinusoidal modulation functions. For the shoulder phantom and 70% modulation amplitude, 44.6% dose reduction was measured as compared to 34.1% for sinusoidal modulation. A maximum of 56% was measured for the shoulder phantom including inserts. Specifying mAs reduction as an estimate for dose reduction proved to be a valid and conservative estimate; measured dose is reduced more strongly than the total mAs product both centrally and on average. First patient studies confirm the results of simulation and phantom studies. We conclude that attenuation-based online tube current control has great potential for reducing patient dose in CT and that it should be made generally available for clinical use.

Dose reduction in CT by anatomically adapted tube current modulation. I. Simulation studies

Tube current modulation governed by x-ray attenuation during CT (computed tomography) acquisition can lead to noise reduction which in turn can be used to achieve patient dose reduction without loss in image quality. The potential of this technique was investigated in simulation studies calculating both noise amplitude levels and noise distribution in CT images. The dependence of noise on the inodulation function, amplitude of modulation, shape and size of the object, and possible phase shift between attenuation and modulation function were examined. Both sinusoidal and attenuation-based control functions were used to modulate tube current. Noise reduction was calculated for both ideal systems and for real systems with limited modulation amplitude. Dose reductions up to 50% can be achieved depending on the phantom geometry and tube current modulation function. Attenuation-based tube current modulation yields substantially higher reduction than fixed-shape modulation functions. Optimal results are obtained when the current is modulated as a function of the square root of attenuation. A modulation amplitude of at least 90% should be available to exploit the potential of these techniques.

Dose reduction in CT by on-line tube current control: principles and validation on phantoms and cadavers.

Abstract. We investigated approaches to reducing the dose in CT without impairing image quality. Dose can be reduced for non-circular object cross-sections without a significant increase in noise if X-ray tube current is reduced at angular tube positions where the X-ray attenuation by the patients is small. We investigated different schemes of current modulation during tube rotation by simulation and phantom measurements. Both pre-programmed sinusoidal modulation functions and attenuation-based on-line control of the tube current were evaluated. All relevant scan parameters were varied, including constraints such as the maximum modulation amplitude. A circular, an elliptical and two oval water phantoms were used. Results were validated on six cadavers. Dose reduction of 10–45 % was obtained both in simulations and in measurements for the different non-circular phantom geometries and current modulation algorithms without an increase in pixel noise values. On-line attenuation-based control yielded higher reductions than modulation by a sinusoidal curve. The maximal dose reduction predicted by simulations could not be achieved due to limits in the modulation amplitude. In cadaver studies, a reduction of typically 20–40 % was achieved for the body and about 10 % for the head. Variations of our technique are possible; a slight increase in nominal tube current for high-attenuation projections combined with attenuation-based current modulation still yields significant dose reduction, but also a reduction in the structured noise that may obscure diagnostic details. We conclude that a significant reduction in dose can be achieved by tube current modulation without compromising image quality. Attenuation-based on-line control and a modulation amplitude of at least 90 % should be employed.

Dose reduction in computed tomography by attenuation-based on-line modulation of tube current: evaluation of six anatomical regions.

Abstract. This study investigated the potential of attenuation-based on-line modulation of tube current to reduce the dose of computed tomography (in milliamperes) without loss in image quality. The dose can be reduced for non-circular patient cross-sections by reducing the tube current at the angular positions at which the diameter through the patient diameter is smallest. We investigated a new technical approach with attenuation-based on-line modulation of tube current. Computed tomographic projection data were analyzed to determine the optimal milliampere values for each projection angle in real time, instead of performing prior measurements with localizer radiographs. We compared image quality, noise pattern, and dose for standard scans and for scans with attenuation-based on-line modulation of tube current in a group of 30 radiation therapy patients. Six different anatomical regions were examined: head, shoulder, thorax, abdomen, pelvis, and extremities (knee). Image quality was evaluated by four radiologists in a blinded fashion. We found the dose to be reduced typically by 15–50 %. In general, no deterioration in image quality was observed. Thus the dose in computed tomography be reduced substantially by technical measures without sacrificing image quality. Attenuation-based on-line modulation of tube current is an efficient and practical means for this.

Dose reduction in CT examination of children by an attenuation-based on-line modulation of tube current (CARE Dose)

Abstract. In a controlled patient study we investigated the potential of attenuation-based on-line modulation of the tube current to reduce milliampere values (mAs) in CT examinations of children without loss of image quality. mAs can be reduced for non-circular patient cross sections without an increase in noise if tube current is reduced at those angular positions where the patient diameter and, consequently, attenuation are small. We investigated a technical approach with an attenuation-based on-line control for the tube current realised as a work-in-progress implementation. The CT projection data are analysed in real time to determine optimal mAs values for each projection angle. We evaluated mAs reduction for 100 spiral CT examinations with attenuation-based on-line modulation of the tube current in a group of children. Two radiologists evaluated image quality by visual interpretation in consensus. We compared the mAs values read from the CT scanner with preset mAs of a standard protocol. Four different scan regions were examined in spiral technique (neck, thorax, abdomen, thorax and abdomen). We found the mAs product to be reduced typically by 10–60% depending on patient geometry and anatomical regions. The mean reduction was 22.3% (neck 20%, thorax 23%, abdomen 23%, thorax and abdomen 22%). In general, no deterioration of image quality was observed. There was no correlation between the age and the mean mAs reduction in the different anatomical regions. By classifying the children respectively to their weight, there is a positive trend between increasing weight and mAs reduction. We conclude that mAs in spiral CT examinations of children can be reduced substantially by attenuation-based on-line modulation of the tube current without deterioration of image quality. Attenuation-based on-line modulation of tube current is efficient and practical for reducing dose exposure to children.

Mehrzeilen-Spiral-CT in der Diagnostik von Pankreastumoren

ZusammenfassungZiel unserer Untersuchungen war es, die Möglichkeiten der Mehrzeilen-Detektor-Spiral-CT und ihre Bedeutung für das Staging von Pankreastumoren zu evaluieren. Bei insgesamt 50 Patienten, bei denen der Verdacht auf ein Pankreaskarzinom bestand, wurde im Rahmen der Tumorstagings ein biphasisches hochaufgelöstes Mehrzeilen-Spiral-CT mit einer Schichtkollimation von 4×1 mm, einem Pitch von 3,5–4, 120 ml Kontrastmittel, 50 ml 0,9%NaCl-Bolus, 3,0 ml/s Fluß und einem Startdelay von durchschnittlich 40 s (Pankreasparenchymphase) und 80 s (portalvenöse Phase) durchgeführt. Die Mehrzeilen-Spiral-CT ist in der Lage die gesamte Pankreasloge und auch die angrenzenden Organe mit hoher Ortsauflösung in allen Raumebenen abzubilden. Die nahezu isotrope multiplanare Bildgebung erlaubt die vollständige Erfassung der Tumorausdehnung in allen Raumebenen und eine bessere Abgrenzung der Tumoren gegenüber dem angrenzenden Fettgewebe, den benachbarten Organen (Gefäße, Duodenum, Magen) und einen sichereren Nachweis von peripankreatischen Lymphknoten. Die Mehrzeilen-Spiral-CT und der Einsatz von interaktiven multiplanaren Rekonstruktionen verbessern nachhaltig die Bestimmung der Ausdehnung von Pankreaskarzinomen.SummaryPurpose. Investigation of the capabilities of MSCT and its value for the staging of pancreatic carcinomas. Methods. 50 Patients with suspected pancreatic carcinoma were examined with a biphasic multislice-spiral-CT protocol: slice collimation 4×1 mm, Pitch 3.5–4 mm. After administation of 120 ml contrast medium and 50 ml NaCl with a flow rate of 3.0 ml/s the examination was started with a delay of 40 s (pancreatic phase) and 80 s (portalvenous phase). Results. Multislice spiral CT allows the examination of the whole upper abdomen with nearly isotropic data sets. This is the premise for the optimal assessment of the tumor extent in all planes, excellent demarcation of the tumor against the adjacent vessels and organs and the demarcation of small peripancreatic lymph nodes. Conclusions. Multislice spiral CT and the use of interactive multiplanar reconstructions improve the staging of pancreatic cancer.