Automatic Tube Current Modulation Systems Research Articles

Impact of the automatic tube current modulation (ATCM) system on virtual monoenergetic image quality for dual-source CT: A phantom study

PurposeTo assess the impact of the automatic tube current modulation (ATCM) on virtual monoenergetic images (VMIs) quality in dual-source CT(DSCT). Materials and methodsAcquisitions were performed on DSCT using the Mercury phantom. The acquisition parameters for an abdomen-pelvic examination with single-energy CT(SECT) and dual-energy CT(DECT) imaging were used. Acquisitions were performed for each imaging mode using fixed mAs and ATCM. The mAs value was set to obtain a volume CT dose index of 11 mGy in fixed mAs acquisitions. This value was used as the reference mAs in ATCM acquisitions. The noise power spectrum and task-based transfer function at 40,50,60 and 70 keV levels were computed on VMIs and SECT images. The detectability index (d′) was calculated for a lesion with an iodine concentration of 10 mg/mL. ResultsThe noise magnitude on VMIs was higher with the ATCM system than with fixed mAs for all energy levels and section diameters of 21,26 and 31 cm. The noise texture and spatial resolution were similar between the fixed mAs and ATCM acquisitions for both imaging modes. The d’ values were lower for all energy levels with ATCM than with fixed mAs acquisitions for 21 and 26 cm diameters by −39.82 ± 9.32%, similar at 31 cm diameter −4.13 ± 0.24% and higher at 36 cm diameter 10.40 ± 6.69%. It was higher on VMIs at all energy levels compared to SECT images. ConclusionsThe ATCM system could be used with DECT imaging to optimize patient exposure without changing the noise texture and spatial resolution of VMIs compared to fixed mAs and SECT.

Comparison of image quality and radiation dose of different scanning methods used for computed tomography of the unilateral shoulder.

The effect of different computed tomography (CT) scanning methods of the shoulder on image quality is uncertain. To compare the effect of different methods of CT scanning of the right shoulder on image quality and radiation dose. A total of 30 adults were divided into five groups. Group A received scans centered on the body's long axis, a scout direction of 0° + 90°, and automatic tube current modulation (ATCM). The other four groups (B, C, D, E) received isocenter scans centered on the shoulder with different scout directions (B and C: 0° + 90°, D: 0°, E: 0° + 270°) and tube currents (B: 420 mA; C, D, E: ATCM). The volume CT dose index (CTDIvol), dose-length product (DLP), image objective noise, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) were compared. Three subjective measures were also compared (noise, stripe artifacts, diagnostic confidence). The five groups differed significantly in all subjective and objective indexes. The CTDIvol and DLP decreased in the order of groups C, A, B, E, and D; the differences between groups A and B were not significant (P > 0.05). Groups B, C, and E had better SNR and CNR than groups A and D (P < 0.01). Subjective evaluations indicated group D was worse than groups B, C, and E (P < 0.05). In the ATCM system that uses the last scout view, CT of the shoulder should use isocenter scanning with the lateral scout view when the tube is away from the long axis of the body as the last execution direction.

Evaluation of automatic tube current modulation of CT scanners using a dedicated and the CTDI dosimetry phantoms.

PurposeTo investigate the operation principles of the automatic tube current modulation (ATCM) of a CT scanner, using a dedicated phantom and the CT dosimetry index (CTDI) phantom.Material and methodsThe Mercury 4.0 phantom and three different configurations of the CTDI dosimetry phantom were employed. A frequently used clinical scanning protocol was employed as a basis for the acquisitions performed with all phantoms, using both scanning directions. Additional acquisitions with different pitch and examination protocols were performed with Mercury phantom, to further explore their effect on ATCM and the resulting image quality. Different software named DICOM Info Extractor, ImageJ, and imQuest, were used to derive CTDIvol and table position, image noise, and water equivalent diameter (WED) of each phantom CT image, respectively. ImQuest was also used to derive the detectability index (d’) of five different materials (air, solid water, polystyrene, iodine, and bone) embedded in the Mercury phantom.ResultsIt was exhibited with all four phantoms that the scanning direction greatly affects the modulation curves. The fitting of the dose modulations curves suggested that for each table position what determines the CTDIvol value is the WED values of the phantom structures laying ahead towards the scanning direction, for a length equal to the effective width of the X‐ray beam. Furthermore, it was also exhibited that ATCM does not fully compensate for larger thicknesses, since images of larger WED phantom sections present more noise (larger SD) in all four phantoms and in Mercury 4.0 phantom smaller detectability (d’).ConclusionMercury 4.0 is a dedicated phantom for a complete and in‐depth evaluation of the ATCM operation and the resulting image quality. However, in its absence, different CTDI configurations can be used as an alternative to investigate and comprehend some basic operation principles of the CT scanners’ ATCM systems.

A PHANTOM EVALUATION OF THE USE OF CT AUTOMATIC TUBE CURRENT MODULATION WITH LOW TUBE POTENTIALS FOR IODINATED CONTRAST STUDIES.

This paper aimed to investigate effects of different tube voltage and image quality settings on radiation dose and image quality for patients undergoing computed tomography iodinated contrast studies using automatic tube current modulation system and to recommend settings to achieve improved radiation dose and image quality values. A Pagoda phantom with an additional rod of iodine contrast was scanned using different tube voltages and noise index (NI) settings. Size-specific dose estimate (SSDE) and image quality (noise, contrast, contrast-to-noise ratio (CNR) and figure of merit (FOM)) were analysed. Values of SSDE were maintained with similar NI settings. Contrast and CNR were higher for lower tube voltage settings. Better FOM values can be achieved with higher NI settings with the lower kVs. To achieve better CNR and SSDE compared with the standard setting of 120kV, a 80kV with an NI setting of 15 was recommended.

Comparison of entrance surface air kerma to eye lens in head computed tomography protocols between 32-MDCT and 64-MDCT on an anthropomorphic phantom

Background: Brain multi-detector computed tomography (MDCT) is commonly performed for diagnosis of traumatic or non-traumatic brain injury cases. During brain CT scan, the eye lens is highly sensitive to radiation and may cause radiation-induced cataracts irradiated by CT primary beam. Objectives: This study aimed to determine and compare entrance surface air kerma (ESAK) to the eye lens in clinical routine head protocols between 32-MDCT and 64-MDCT using an anthropomorphic phantom. Materials and methods: A PBU-60 head phantom was scanned by 32-MDCT and 64-MDCT in helical, axial, and tilted axial modes used in clinical routine head protocols with tube voltage of 120 kVp, tube current of 108-150 mAs for 32-MDCT, and 200-310 mAs for 64-MDCT. The NanodotTM optically stimulated luminescent dosimeters (OSLDs) was used to measure ESAK to eye lens. Dose length product (DLP), normalized volume CT dose index (nCTDIvol), and normalized mean ESAK were compared between two CT scanners. Results: The ranges of mean normalized ESAK to the eye lens in each scanning mode was found from 0.41±0.01 to 0.51±0.01 mGy/100 mAs for 32-MDCT and 0.30±0.01 to 0.40±0.01 mGy/100 mAs for 64-MDCT. The normalized ESAKs obtained from 64-MDCT were lower than 32-MDCT by 21.57-37.50%. The lowest normalized ESAK of 0.30±0.01 mGy/100 mAs was obtained in tilted axial scanning mode in 64-MDCT with the difference of 37.50% compared to 32-MDCT of using identical scanning mode. Conclusion: This study demonstrated that normalized mean ESAK to the eye lens for 64-MDCT in all brain scanning protocols was lower compared to 32-MDCT. In addition, using tilting gantry in axial scanning mode as well as using an automatic tube current modulation system could be beneficial for reducing radiation dose to eye lens during brain CT in clinical routine

Effects of tube potential selection together with computed tomography automatic tube current modulation on CT imaging performance

The effects of tube potential selection with a computed tomography (CT) automatic tube current modulation (ATCM) system on radiation dose and image quality have been investigated on a Canon CT scanner. The use of different values of tube voltage for imaging, and the appropriate settings of the ATCM system, were evaluated. The custom-made phantom consisted of three sections of different sizes with inserts of various materials. It was scanned using tube potentials of 80–140 kV and different image quality ATCM settings. CTDIvol and image quality in terms of noise, contrast, and contrast-to-noise ratio (CNR) for air, polyethylene (PE), acrylic, polyoxymethylene (POM) and polyvinylchloride (PVC) were analysed. A figure of merit (FOM) was estimated by combining CNR and CTDIvol. CTDIvol values were similar for all values of tube voltage and individual image quality ATCM settings when tube current was not restricted by the maximum value. The contrasts were independent of ATCM image quality setting, but CNR increased at the higher image quality level as image noise decreased. Both contrast and CNR decreased with increasing tube voltage for PVC and PE, but increased for POM and acrylic. PVC was the only insert material for which there was a significant improvement in contrast at lower tube potentials. FOM indicated that standard (SD = 10) and low dose (SD = 12.5) ATCM settings might be appropriate. The optimum tube voltage settings for imaging the PVC was 80−100 kV, but not for the lower contrast POM and acrylic, for which the standard tube voltage setting of 120 kV was better. The tube potential should be carefully set to gain radiological protection optimisation and keep the radiation dose as low as possible. Results indicate that 100 kV is likely to be appropriate for imaging small and medium-sized Thai patients when iodine contrast is used.

Design and use of a phantom for testing and comparing the performance of computed tomography automatic tube current modulation systems

Automatic tube current modulation (ATCM) is now available on all computed tomography (CT) scanners, but there is no standard phantom for testing its operation. For this study, a phantom comprising five elliptical sections of varying diameters in the shape of a pagoda has been made to represent the range of sizes for patients in Thailand and the Far East. ATCM performance can be evaluated in terms of how tube current and image noise vary with changes in the sizes of the sections. Additional rods of different materials with similar properties to human tissues have been inserted to allow the measurement of contrast-to-noise ratios (CNRs) for assessment of image quality. The phantom has been used to test and compare the performance of CT ATCM systems for the major four CT scanner vendors. The results showed that the ATCM systems of Toshiba and GE CT scanners maintained image noise and CNR within narrower ranges by varying tube current aggressively along the scan length, and commencing modulation before the high attenuation sections are reached. In contrast, the ATCM systems of Philips and Siemens scanners adjusted the tube current within narrower ranges, allowed larger variations in image noise and CNR, and commenced modulation at the start of sections with higher attenuation. The phantom can be used to confirm functionality of the system for acceptance testing, as well as providing information on the tradeoff between radiation dose and image quality when setting up clinical protocols during commissioning of new CT scanners. The phantom can also be used on a routine basis to check that performance is maintained, and could be used in the future for selecting protocol settings to give required values of CNR or adjusting protocol settings on different CT scanners to provide similar levels of clinical performance.

Effect of CT Localizer Radiographs on Radiation Dose Associated With Automatic Tube Current Modulation: A Multivendor Study

To assess the influence of the CT localizer radiograph on the automatic tube current modulation system of 7 CT scanners produced by 4 different CT manufacturers. The influence of the localizer orientation, table height, tube current and tube potential values on the radiation dose of the related CT scan were evaluated. Images were acquired by using an anthropomorphic phantom positioned in the CT gantry isocenter as well as from -6 cm to +6 cm vertically to the isocenter. Vertical movement of the CT table height affected the radiation dose in all scanners using anterior-posterior or a posterior-anterior localizer orientation albeit differently, depending on the manufacturer; only in 1/7 scanner no influence was observed. The latero-lateral localizer orientation proved to be more effective in limiting the influence of the vertical miscentering in all scanners. Changing localizer's tube voltage influenced the scan radiation dose in scanners produced by two manufacturers, while no significant effect was observed in scanners produced by the other two manufacturers. No significant dose variation was observed in 6/7 scanners when changing the localizer's tube current. Localizer radiograph shows a significant influence on the radiation exposure but with different outcomes depending on the manufacturer of the CT scanner. Radiologists and radiographers should have a thorough understanding of these differences to assure patients the best examination in terms of radiation dose and image quality.

Automatic calculation of patient size metrics in computed tomography: What level of computational accuracy do we need?

ObjectivesTo compare the effectiveness of two different patient size metrics based on water equivalent diameter (D w), the mid‐scan water equivalent diameter D w_c, and the mean (average) water equivalent diameter in the imaged region, D w_ave, for automatic detection of accidental changes in computed tomography (CT) acquisition protocols.MethodsPatient biometric data (height and weight) were available from a previous survey for 80 adult chest examinations, and 119 adult single‐acquisition chest–abdomen–pelvis (CAP) examinations for two 16 slice scanners (GE LightSpeed and Toshiba Aquilion RXL) equipped with automatic tube current modulation (ATCM). D w_c and D w_ave were calculated from the archived CT images. Size‐specific dose estimates (SSDE) were obtained from volume CT dose index (CTDI vol), using the conversion factors for a patient diameter of D w_c.Results CTDI vol and SSDE correlate better with D w_ave than with D w_c. R‐squared values of linear fits to CTDI vol of CAP examinations were 0.81–0.89 for D w_c and 0.93–0.94 for D w_ave (SSDE: 0.69–080 for D w_c, 0.87–0.92 for D w_ave). Percentage differences between D w_c and D w_ave were −4 ± 4% for chest and +5 ± 4% for CAP examinations (in % of D w_ave). However, small D w variations translated as larger variations in CTDI vol for these ATCM systems (e.g., a 24% increase in D w doubled CTDI vol). The dependence of CTDI vol on D w_ave was similar for chest and CAP examinations performed with similar ATCM parameters, while use of D w_c resulted in a clear separation of the same data according to examination type. Maximum D w variation in the imaged region was 5.6 ± 1.6 cm for chest and 6.5 ± 1.4 cm for CAP examinations.Conclusions D w_ave is a better metric than D w_c for binning similar‐sized patients in dose comparison studies, despite the additional computational effort required for its calculation Therefore, when implementing automatic determination of D w for SSDE calculations, automatic calculation of D w_ave should be considered.

Practical experiences in the transfer of clinical protocols between CT scanners with different ATCM systems

Automatic tube current modulation (ATCM) systems to aid in optimizing dose and image noise have become standard on computed tomography (CT) scanners over the last decade. ATCM systems of the main vendors modulate tube current in slightly different ways, with some using a control parameter related to image noise (e.g. Toshiba, GE) while others use a quality reference image mAs (e.g. Siemens). The translation of clinical protocols including ATCM operation between CT scanners from different manufacturers in order to obtain similar levels of image quality with optimized exposure variables has become an important issue. In this study, cylindrical phantoms of different sizes representing small, average and large patients, have been combined into one phantom, which has been scanned on Siemens, Toshiba and GE CT scanners with the full ranges of ATCM image quality settings. The volume weighted CT dose index (CTDIvol) and image noise over each section of the phantom were recorded for every setting. Relationships between the image quality level settings, and CTDIvol and measured image noise were analysed in order to investigate ATCM performance. Equations were developed from fits of the data to enable CTDIvol and image noise to be expressed in terms of the image quality parameters for different size phantoms on each scanner. The Siemens scanner protocol was chosen as the reference, as it avoided high doses for large patients, while allowing full modulation of tube current for patients of all sizes, and so was considered to provide optimized performance. The equations derived were used to equate the noise parameters on Toshiba and GE scanners to the quality reference mAs on the Siemens scanner, so that clinical protocols incorporating similar levels of optimization could be obtained on the three CT scanners.

Signal-to-noise ratio and dose to the lens of the eye for computed tomography examination of the brain using an automatic tube current modulation system.

The study aimed to evaluate the image quality in terms of signal-to-noise ratio (SNR) and dose to the lens of the eye and the other nearby organs from the CT brain scan using an automatic tube current modulation (ATCM) system with or without CT gantry tilt is needed. An anthropomorphic phantom was scanned with different settings including use of different ATCM, fixed tube current time product (mAs) settings and degree angles of gantry tilt. Gafchromic film XR-QA2 was used to measure absorbed dose of the organs. Relative doses and SNR for the various scan settings were compared with the reference setting of the fixed 330 mAs. Average absorbed dose for the lens of the eyes varied from 8.7 to 21.7mGy. The use of the ATCM system with the gantry tilt resulted in up to 60% decrease in the dose to the lens of the eye. SNR significantly decreased while tilting the gantry using the fixed mAs techniques, compared to that of the reference setting. However, there were no statistical significant differences for SNRs between the reference setting and all ATCM settings. Compared to the reference setting of the fixed effective mAs, using the ATCM system and appropriate tilting, the gantry resulted in a substantial decrease in the dose to the lens of the eye while preserving signal-to-noise ratio. CT brain examination should be carefully controlled to optimize dose for lens of the eye and image quality of the examination.

Dose optimization in computed tomography comparing automatic tube current modulation and fixed tube current techniques

Introduction High radiation doses received, especially in young patients, have a great importance with the known risk cancer development. Computed Tomography examinations correspond to 70% of all radiation doses in diagnostic imaging modalities. With advances in CT scanners, some features such as automatic tube current modulation (ATCM) permits acquiring high quality images with low radiation doses. However, without an optimization process, the protocols used in CT routine induce low-quality images that compromise the reliable medical diagnosis or induce radiation doses questioning the ALARA principle. Purpose The purpose of this study was to evaluate the radiation dose of fixed mA techniques and tube current modulation techniques in CT the abdomen-pelvis protocol. Material and methods We fixed three current protocols (300 mA, 250 mA and 200 mA) and one ATCM protocol (SD 10.0) performed in a 16-slice Toshiba CT scanner with “SureExposure3D” ATCM system. Protocols were applied in an anthropomorphic phantom (Alderson-Rando Phantom) for a dosimetric evaluation and to determine organ absorbed doses. The effective doses, E , were also calculated according to ICRP 103. Results The ATCM technique SD 10.0 has the lowest amount of absorbed dose. The larger E was found in the protocol with 300 mA. The difference to the SD protocol 10.0, which showed lower E , was of 79.49%. Conclusion ATCM protocols can be an excellent alternative to dose reduction in CT scans, since it does not impair the diagnostic image quality. Disclosure The authors declare that there is no conflict of interests.

Assessing the feasibility of simulating different tube current limits in noise oriented ATCM systems

Introduction Automatic tube current modulation (ATCM) allows automatic dose tailoring in Computed Tomography (CT), according to patient size and anatomical region. In noise-oriented ATCM systems, it is advisable to set minimum and maximum values of tube current (Imin and Imax). Large-scale databases are changing the way medical physicists compare data. Therefore, it is important to understand how an incorrect value of Imin or Imax may be detected from large scale dose distributions. Purpose To characterize noise oriented ATCM systems from two different vendors, and assess the feasibility of simulating different values of Imin and Imax, for a patient population examined with optimized settings. Materials and methods A RS-330 Lung/Chest Phantom (RSD, USA) was imaged with different acquisition settings, using scanners GE Lightspeed and Toshiba Aquilion RXL. The tube current as a function of couch position, I(z), was extracted from DICOM headers. For each Imin and Imax, I(z) was compared with the full range curve thresholded to the desired values. Results The I(z) curves obtained suggest a series of equally-spaced modulation points. Between successive modulation points, I(z) varies almost linearly. The settings of Imin and Imax change the values of I attained at each modulation point, and therefore simulated and real I(z) curves are slightly different. This difference decreases when modulation points are closer together (finer sampling). The sampling depends on equipment and collimation settings. Conclusion With detailed sampling, differences between simulated and real I(z) curves are below 5%. This difference increases when sampling is less detailed, particularly for lower Imax. Disclosure The authors have no relevant financial or non-financial relationships to disclose.

Patient-specific dosimetry in CT

Radiation originated from CT examinations has been identified as the major contributor of radiation dose to patient population from medical purposes in the United States. Therefore, there is a great concern regarding the risk associated with radiation exposure from these examinations in both children and adults. Although effective dose has been the most commonly used dose metric when reporting the risk related to a given type of procedure, it has been recently suggested that the radiation dose delivered to specific radiosensitive organs should be determined. Physical anthropomorphic phantoms and mathematical models have been widely used for this purpose. However, the variability of human anatomy may be significant, especially for pediatric patients. In addition, modern CT scanners are equipped with automatic tube current modulation systems (TCM), which allow for images at reduced patient radiation dose. These systems modulate the tube current depending on the patient body size. Consequently, the dose reduction due to the activation of the TCM will considerably vary for patients with different body habitus. Methods based on voxelized patient models have been developed to individualize patient radiation dose. Size-specific, scanner-independent ‘CTDIvol to organ’ dose conversion factors have been produced to estimate organ doses from various CT examinations. Patient’s sex, body status and TCM settings should be taken into account in patient specific CT-dosimetry. In this presentation methods developed to determine individual-specific organ doses from CT examinations, based on patient specific models, will be reviewed.

Investigation of the influence of image reconstruction filter and scan parameters on operation of automatic tube current modulation systems for different CT scanners.

Variation in the user selected CT scanning parameters under automatic tube current modulation (ATCM) between hospitals has a substantial influence on the radiation doses and image quality for patients. The aim of this study was to investigate the effect of changing image reconstruction filter and scan parameter settings on tube current, dose and image quality for various CT scanners operating under ATCM. The scan parameters varied were pitch factor, rotation time, collimator configuration, kVp, image thickness and image filter convolution (FC) used for reconstruction. The Toshiba scanner varies the tube current to achieve a set target noise. Changes in the FC setting and image thickness for the first reconstruction were the major factors affecting patient dose. A two-step change in FC from smoother to sharper filters doubles the dose, but is counterbalanced by an improvement in spatial resolution. In contrast, Philips and Siemens scanners maintained tube current values similar to those for a reference image and patient, and the tube current only varied slightly for changes in individual CT scan parameters. The selection of a sharp filter increased the image noise, while use of iDose iterative reconstruction reduced the noise. Since the principles used by CT manufacturers for ATCM vary, it is important that parameters which affect patient dose and image quality for each scanner are made clear to operator to aid in optimisation.

Automatic exposure control in CT: the effect of patient size, anatomical region and prescribed modulation strength on tube current and image quality.

To study the effect of patient size, body region and modulation strength on tube current and image quality on CT examinations that use automatic tube current modulation (ATCM). Ten physical anthropomorphic phantoms that simulate an individual as neonate, 1-, 5-, 10-year-old and adult at various body habitus were employed. CT acquisition of head, neck, thorax and abdomen/pelvis was performed with ATCM activated at weak, average and strong modulation strength. The mean modulated mAs (mAsmod) values were recorded. Image noise was measured at selected anatomical sites. The mAsmod recorded for neonate compared to 10-year-old increased by 30%, 14%, 6% and 53% for head, neck, thorax and abdomen/pelvis, respectively, (P < 0.05). The mAsmod was lower than the preselected mAs with the exception of the 10-year-old phantom. In paediatric and adult phantoms, the mAsmod ranged from 44 and 53 for weak to 117 and 93 for strong modulation strength, respectively. At the same exposure parameters image noise increased with body size (P < 0.05). The ATCM system studied here may affect dose differently for different patient habitus. Dose may decrease for overweight adults but increase for children older than 5years old. Care should be taken when implementing ATCM protocols to ensure that image quality is maintained. • ATCM efficiency is related to the size of the patient's body. • ATCM should be activated without caution in overweight adult individuals. • ATCM may increase radiation dose in children older than 5years old. • ATCM efficiency depends on the protocol selected for a specific anatomical region. • Modulation strength may be appropriately tuned to enhance ATCM efficiency.

Relationships between patient size, dose and image noise under automatic tube current modulation systems

Automatic tube current modulation (ATCM) systems are now used for the majority of CT scans. The principles of ATCM operation are different in CT scanners from different manufacturers. Toshiba and GE scanners base the current modulation on a target noise setting, while Philips and Siemens scanners use reference image and reference mAs concepts respectively. Knowledge of the relationships between patient size, dose and image noise are important for CT patient dose optimisation. In this study, the CT patient doses were surveyed for 14 CT scanners from four different CT scanner manufacturers. The patient cross sectional area, the tube current modulation and the image noise from the CT images were analysed using in-house software. The Toshiba and GE scanner results showed that noise levels are relatively constant but tube currents are dependent on patient size. As a result of this there is a wide range in tube current values across different patient sizes, and doses for large patients are significantly higher in these scanners. In contrast, in the Philips and Siemens scanners, tube currents are less dependent on patient size, the range in tube current is narrower, and the doses for larger patients are not as high. Image noise is more dependent on the patient size.

大腸3D-CTにおける被ばく線量と画質の比較研究 -自動露出機構 vs. 一定線量撮影-

大腸3D-CTにおける被ばく線量と画質の比較研究 -自動露出機構 vs. 一定線量撮影-

Comparison of different phantom designs for CT scanner automatic tube current modulation system tests

Modern CT scanners modulate tube current during scans according to patient size, shape and attenuation. However, the ATCM (automatic tube current modulation) systems for different CT manufacturers work on different principles. Although the systems are used for the majority of patients and examinations, there is no standard phantom for routine quality control of CT scanner ATCM operation. The ideal phantom for testing these systems should be capable of evaluating how tube current and image quality as well as dose vary according to changes in patient size and shape. For this study, a conical phantom designed by ImPACT has been compared with two phantoms made from elliptical sections with varying dimensions. The concept of the designs is to reflect the ATCM performance for the varying shapes and dimensions along the length of the human body. The first phantom comprises five elliptical sections with a wide range of different dimensions and the second has three sections that are more similar in size. The phantoms have been used to test ATCM systems for Philips, Siemens, GE and Toshiba scanners. Although the results of the tube current modulation patterns were similar for all CT scanners, the abrupt changes in attenuation for the first sectional phantom provoked an abnormal ATCM response for the GE and Toshiba scanners. The second sectional phantom was developed from the results of the first, and was more effective for ATCM system testing and could be used for dose and image quality assessment in standard positions. However, the ImPACT conical phantom provided the best overall assessment of performance in terms of tube current modulations and noise pattern.

Considerations on an automatic computed tomography tube current modulation system

The scope of this study was to evaluate the effects on radiation output and image noise varying the acquisition parameters with an automatic tube current modulation (ATCM) system in computed tomography (CT). Chest CT examinations of an anthropomorphic phantom were acquired using a GE LightSpeed VCT 64-slice tomograph. Acquisitions were performed using different pitch, slice thickness and noise index (NI) values and varying the orientation of the scanned projection radiograph (SPR). The radiation output was determined by the CT dose index (CTDIvol). Image noise was evaluated measuring the standard deviation of CT numbers in several regions of interest. The radiation output was lower if the SPR was acquired in the anterior-posterior projection. The radiation dose with the posterior-anterior SPR was higher, because the divergence of the X-ray beam magnifies the anatomical structures closest to the tube, especially the spinal column, and this leads the ATCM system to estimate higher patient attenuation values and, therefore, to select higher tube current values. The NI was inversely proportional to the square root of the CTDIvol and, with fixed NI, the CTDIvol increased as the slice thickness decreased. This study suggests some important issues to use the GE ATCM system efficiently.