Gantry Tilt Research Articles

An animal PET scanner using flat-panel position-sensitive PMTs

To design, build, and evaluate an animal PET scanner, which can be used with non-human primates under conscious condition, incorporating flat-panel position-sensitive photomultiplier tubes (PS-PMTs). The system contains 30 detector modules, each having two PS-PMTs and 16×18 lutetium–yttrium oxyortho-silicate scintillation crystal arrays. The system has 17,280 crystals (480 per ring) arranged in 36 rings, with a diameter of 508 mm and axial extent of 108 mm. The gantry tilt mechanism enables PET studies to be performed on a monkey in the sitting position. Data can be acquired in either the 2D or 3D mode, with the slice collimators being retracted in the 3D mode. At the center of the field-of-view, radial resolution is 2.7 mm full width at half maximum (FWHM) and tangential resolution is 2.4 mm FWHM, while axial resolution is 2.5 mm FWHM for direct slices and 2.7 mm FWHM for cross slices. Scatter fraction, count rate capability, and sensitivity were evaluated using a cylindrical phantom 10 cm in diameter. The noise equivalent count rate in the 3D mode is equivalent to that in the 2D mode at a three times higher radioactivity level. Total system sensitivity is 1.3 kcps/(kBq/mL) in 2D mode and 7.4 kcps/(kBq/mL) in the 3D mode. Animal studies with a monkey were performed to evaluate the imaging capabilities of the scanner. The new PET scanner will be a useful research tool with non-human primates for pre-clinical drug development.

Quality assurance for computed-tomography simulator: In home Z-phantom for mechanical tests of the couch and the gantry

Objective The main purpose of this work was to present a Z-phantom manufactured in home (at National Cancer Institute Cairo University) and it’s use in a simple way to check the accuracy of the computed-tomography (CT) table movement and CT gantry tilt, also the other general quality control (QC) tests of the CT simulator used at radiotherapy department.

Effect of pelvic inclination and torsional deformity on canine acetabular morphology with computed tomography

To evaluate how the inclination and torsional deformity of the hemipelvis using extra-rotation as a model affect acetabular angle (AA) and dorsal acetabular rim angle (DARA) assessment with computed tomography (CT). A normal canine hemipelvis positioned in dorsal recumbency was scanned with a 16-slice multidetector CT scanner, with different degrees of torsional deformity, using extra-rotation around the long axis (0-5-10-15°) of the hemipelvis. Each degree of extra-rotation was acquired at 0°, +20° and -20° of gantry tilt, to mimic different pelvic inclinations on its transverse axis. Cranial and central individual acetabular angles (IAA) and central DARA were calculated and correlated with inclination and torsional deformity. A very strong negative correlation was found between cranial and central IAA, pelvic inclination, and torsional deformity. A very strong positive correlation was found between DARA, pelvic inclination, and torsional deformity. Pelvic inclination and torsional deformity affect acetabular angles assessment with CT. The greater the inclination (ilia far from the tabletop in dorsal recumbency) and torsional deformity of the pelvis, the worse the acetabulum appeared. A standardized scanning protocol for acetabular morphology assessment is needed and it should consider pelvic inclination. The actual relevance of pelvic torsional deformity is not well known and it should be investigated more thoroughly.

Evaluation of patient dose and operator dose in swallowing CT studies performed with a 320-detector-row multislice CT scanner

Recently, attempts to develop new types of swallowing function analysis with 320-detector-row multislice CT (320-MDCT) have been reported. The present report addresses (1) patient exposure, (2) operator exposure, and (3) spatial dose distribution. For dose measurement, a human-body phantom in which 303 thermoluminescent dosimeter elements were inserted and a survey meter was used. The patient position was confirmed with a single-volume scan at a tube voltage of 120 kV, a tube current of 10 mA, a rotation speed of 0.35 s/rot., a slice thickness of 0.5 mm, coverage of 160 mm, a scan field of view of 240 mm, a small focal spot size, and a gantry tilt angle of 22° (volume CT dose index displayed on the console 0.8 mGy, dose-length product 12.1 mGy cm). The effective dose for the patient in swallowing CT (SCT) was 3.9 mSv. The conversion factor for obtaining the effective dose was 0.0066 mSv/mGy cm. The effective dose for the operator was 0.002 mSv. In the operator exposure measurement, the ambient dose equivalent H*(10), that would be produced by an expanded and aligned radiation field at a depth 10 mm in the International Commission on Radiation Units and Measurements sphere, was 0.012 mSv. In this report, the safety of SCT, which has become possible with the introduction of 320-MDCT, was evaluated by measurement of the exposure to the patient and operator.

Peak Skin and Eye Lens Radiation Dose From Brain Perfusion CT Based on Monte Carlo Simulation

The purpose of our study was to accurately estimate the radiation dose to skin and the eye lens from clinical CT brain perfusion studies, investigate how well scanner output (expressed as volume CT dose index [CTDI(vol)]) matches these estimated doses, and investigate the efficacy of eye lens dose reduction techniques. Peak skin dose and eye lens dose were estimated using Monte Carlo simulation methods on a voxelized patient model and 64-MDCT scanners from four major manufacturers. A range of clinical protocols was evaluated. CTDI(vol) for each scanner was obtained from the scanner console. Dose reduction to the eye lens was evaluated for various gantry tilt angles as well as scan locations. Peak skin dose and eye lens dose ranged from 81 mGy to 348 mGy, depending on the scanner and protocol used. Peak skin dose and eye lens dose were observed to be 66-79% and 59-63%, respectively, of the CTDI(vol) values reported by the scanners. The eye lens dose was significantly reduced when the eye lenses were not directly irradiated. CTDI(vol) should not be interpreted as patient dose; this study has shown it to overestimate dose to the skin or eye lens. These results may be used to provide more accurate estimates of actual dose to ensure that protocols are operated safely below thresholds. Tilting the gantry or moving the scanning region further away from the eyes are effective for reducing lens dose in clinical practice. These actions should be considered when they are consistent with the clinical task and patient anatomy.

Geometric Calibration of Novel Cone-Beam CT Developed For Dedicated Stereotactic Radiotherapy Unit

A novel Cone-Beam CT system has been under development for a dedicated stereotactic radiosurgery/radiotherapy unit (Gamma Knife PerfeXion) to be equipped with image guidance. The stability and the reproducibility of the imaging system were investigated using the full geometric calibration. The imaging system consists of a rotational arm (Parker system) holding X-ray tube (Dunlee PX 1483), flat panel detector (Varian 4030 CB) and in-house system control and reconstruction software (Pilot 1.0). The arm travels in the vertical direction through the linear guide system (Parker 406xr) from its home position to the imaging position by 1250 mm. The accuracy and precision of the vertical motion is measured using dial gauge each time before geometric calibration. Geometric calibrations were repeated over several days with 1) the fixed calibration phantom position and 2) repositioning the imaging system in order to measure its stability and reproducibility. The calibration phantom consists of 16 ball bearings precisely located in two circular trajectories in a cylindrical plastic phantom. All the geometric parameters including source position, detector position and detector tilt angles with respect to gantry angle were determined. The calibration algorithm previously developed and verified at the cone-beam CT systems on optical bench and clinical linear accelerator was applied. Positioning accuracy of the vertical linear guide measured with dial gauge is better than 0.02mm (average absolute error). More than 200 projection images of calibration phantom were acquired each time over 210 degrees of gantry rotation for geometric calibration. Piercing points was reliably reproduced and its position was always in radius of 0.2mm or half pixel radius. In the current design, the reproducibility of vertical position, V, is limited to the precision of mechanical pin structure which allows three different X-ray tube positions. Gantry angles are noticed to have the maximum error of 0.2 degrees. Detector tilt angles are all within 0.5 degrees. Considering the uncertainty of the calibration algorithm of 0.4o (roll and pitch) and 0.01o (yaw) in detector tilt angles, the yaw is found to be the only angle that is significantly tilted. Adjusting this angle can impact the image quality. Source and detector trajectories show interesting behavior of gantry tilt and wobbling motion although the deviation from the ideal trajectory is less than 0.2 mm, which has negligible impact on image quality. Geometric calibration showed excellent stability and reproducibility of the prototype imaging system. More extensive work on quality assurance including full geometric calibration will be done after the imaging system is installed on the treatment unit.

Diagnostic accuracy of state-of-the-art MDCT scanners without gantry tilt in patients with oral and oropharyngeal cancer

PurposeCurrent CT-protocols for staging oral and oropharyngeal cancer include primary transversal slices and secondary tilted slices to avoid artifact-producing regions of dental metalwork. Some of the latest MDCT scanners do not allow gantry tilt. Hence, we assessed the relevance of secondary tilted slices in tumor staging. Materials and methodsScans of a tiltable 64-row MDCT-scanner of 82 patients with oral or oropharyngeal cancer were retrospectively and independently evaluated twice by three readers: once using the primary transversal scans only, and once taking the transversal and secondary tilted scans into account. Tumor diameters and TN-stage were determined with both methods. Artifacts on transversal scans were analyzed using a 3-point-ranking-scale. ResultsImage quality was impaired by severe artifacts in 24% of transversal slices of the oral cavity and in 12% of the oropharynx. The three readers detected an average of 57.7±2.1 of 82 tumors (70%) on transversal CT slices. An average of 6.3±0.6 more tumors (8%) were detected when transversal studies were evaluated in conjunction with secondary tilted slices, leading to a significantly (p=0.0156–0.0313) increased average detection rate of 64.0±2.0 tumors (78%). Moreover, secondary tilted slices led to a correction of underestimated tumor stages in up to six patients (7.3%). ConclusionTilted slices that avoid artifact-producing regions of dental metalwork significantly improve the reader's sensitivity and are of incremental value for staging of oral and oropharyngeal cancers.

Monte Carlo simulation of contrast‐enhanced whole brain radiotherapy on a CT scanner

To perform a feasibility study of contrast-enhanced whole brain radiotherapy for treating patients with multiple brain metastasis using a conventional computed tomography (CT) scanner. The treatment dose was optimized to be applied in a single run using a maximum tube power of 5200 kWs at 140 kV. CT scans of a large and a small head were used as reference. Irradiation geometry, shielding, axial beam collimation, radial beam collimation, gantry tilt, and tube current for beam modulation were optimized using a Monte Carlo simulation and a contrast agent concentration of 5 mg/ml iodine in the tumor. The statistical uncertainty of the Monte Carlo simulation was corrected using back convolution. Using a CT tube with a beam collimation of 28.8 mm, a mean tumor dose of 1.76 +/- 0.13 Gy was achieved, while the head bone dose was 2.61 +/- 0.18 Gy with a normal brain dose of 0.98 +/- 0.06 Gy, eye dose of 0.19 +/- 0.05 Gy, and lens dose of 0.15 +/- 0.03 Gy, respectively. Using a CT tube with dose modulation and a beam collimation of 40.0 mm, the mean tumor dose was 2.00 +/- 0.11 Gy with a head bone dose of 1.96 +/- 0.14 Gy, normal brain dose of 1.13 +/- 0.08 Gy, eye dose of 0.21 +/- 0.05 Gy, and lens dose of 0.16 +/- 0.02 Gy, respectively. Thus a standard CT scanner enables an effective tumor dose of 37.0 Gy to be administered in 13 fractions, while exposing healthy brain to an effective dose of 17.2 Gy and head bone to 69.3 Gy. Additional radial collimation implemented in the hardware improves the therapeutic tumor dose by 25.2% in relation to the bone dose. Contrast-enhanced total brain radiotherapy is feasible using a conventional CT tube with optimized dose application.

Radiation Dose for Routine Clinical Adult Brain CT: Variability on Different Scanners at One Institution

The purpose of this study was to determine, using an anthropomorphic phantom, whether patients are subject to variable radiation doses based on scanner assignment for routine CT of the brain. Twenty metal oxide semiconductor field effect transistor dosimeters were placed in the brain of a male anthropomorphic phantom scanned three times with a routine clinical brain CT protocol on four scanners from one manufacturer in four configurations and on one 64-MDCT scanner from another manufacturer. Absorbed organ doses were measured for skin, cranium, brain, lens of the eye, mandible, and thyroid. Effective dose was calculated on the basis of the dose-length product recorded on each scanner. Organ dose ranges were as follows: cranium, 2.57-3.47 cGy; brain, 2.34-3.78 cGy; lens, 2.51-5.03 cGy; mandible 0.17-0.48 cGy; and thyroid, 0.03-0.28 cGy. Statistically significant differences between scanners with respect to dose were recorded for brain and lens (p < 0.05). Absorbed doses were lowest on the single-detector scanner. In the comparison of MDCT scanners, the highest doses were found on the 4-MDCT scanner and the dual-source 64-MDCT scanner not capable of gantry tilt. Effective dose ranged from 1.22 to 1.86 mSv. According to the phantom data, patients are subject to different organ doses in the lens and brain depending on scanner assignment. At our institution with existing protocols, absorbed doses at brain CT are lowest with the single-detector CT scanner, followed by MDCT scanners capable of gantry tilt. On scanners without gantry tilt, CT of the brain should be performed with careful head positioning and shielding of the orbits. These precautions are especially true for patients who need repeated scanning and for pediatric patients.

Percutaneous radiofrequency ablation for lung tumors beneath the rib under CT fluoroscopic guidance with gantry tilt

Radiofrequency (RF) ablation of lung tumors has become a treatment of choice, especially for unresectable cases. However, RF ablation of small lung lesions located just beneath the rib is difficult. To evaluate the efficacy and safety of gantry tilting for the performance of RF ablation of peripheral lesions located beneath the rib. Our study was based on 18 of 293 lesions in the lung for which RF ablation was performed under CT scan fluoroscopic guidance at our institution between October 2004 and March 2009. For these 18 lesions, RF ablation was performed with gantry tilting because a rib blocked visualization of the RF ablation route even after other attempts had been made to change the relationship between the target and the rib. All RF needles, with only one exception, were successfully advanced to hit the tumor. The commonest complication was a pneumothorax, which occurred in seven procedures. No serious complications occurred. The progression-free rates were 82.4% at 6 months, 62.5% at 12 months, and 30% at 24 months. Mean local progression-free duration was 17.6+/-11.6 months (range 4-36 months). RF ablation under CT scan fluoroscopic guidance with gantry tilt is a useful and safe technique for RF ablation of lung nodules located beneath the rib.

SU‐FF‐T‐54: Quantification of CT Artifact Reduction of Second Generation Intracavitary Brachytherapy Applicator

Purpose: A second generation CT‐friendly applicator with movable shields has been developed for cervical carcinoma intracavitary brachytherapy (ICBT). We report on a method to quantify the reduction in CT artifact in a phantom study using this applicator. Methods and Material: A gynecological phantom was fabricated to mimic a realistic patient anatomy and to fit a standard intrauterine tandem (45 degrees) and 20‐mm diameter colpostats. Once the applicator is inserted, packing using gauze was inserted around the colpostats to reduce air cavity. The phantom with the applicator set was scanned using a CT simulator (AcQSimCT, Philips) using axial mode (2.5 mm slice, 120 kVp, 110mAs). To eliminate image artifacts, two non‐overlapping, 30° gantry tilt axial CT scanning series which met at the middle of the ovoid were used. Shields were moved before each series scan to the opposite side of the ovoid. A second CT scan without moving the shields was performed to compare artifacts. The image sets were then rigidly aligned globally using mutual information. A region of interest inside the phantom from the slice with greatest CT artifacts visually was picked for quantitative analysis. The CT number from each voxel was tallied and summary statistics were calculated. Because the images are aligned, we could determine the difference in CT numbers between corresponding voxels from both images. Results: Visually, the CT scan set with shields moved out of the way were almost completely free of artifacts. The average %diff between all the voxels was 0.6%±42% (2σ), with maximum %diff of 1,039% for voxels with the largest artifact. Conclusions: The new applicator provides artifact‐free CT imaging without compromising the shields. We found the CT protocol with tilted gantry is easy to perform using CT simulators available in the radiation oncology department.

Exact image reconstruction for a circle and line trajectory with a gantry tilt

We investigate image reconstruction with a circle and line trajectory with a tilted gantry. We derive new equations for reconstruction from the line data, such as equations of filtering lines, range of filtering lines and range of the line scan. We analyze the detector requirements and show that the line scan does not impose extra requirements on the cylindrical detector size with our algorithm, that is, the axial truncation of the filtering lines does not occur. We discuss full-scan and short-scan versions of the algorithm. Evaluation of our algorithm uses simulated and real 256-slice data.

ガントリに傾斜を付け撮影することで生じる画質への影響と補正‐新しく考案した球体ファントムを用いての画質の補正‐

The influence of gantry inclination on image quality was examined in the Y and Z directions and an increase in the volume of a scan part. FWHM (full width at half maximum) in the Y direction decreased at the rate of 2.53%, which was caused by leaving the center of the gantry, and spatial resolution was improved. FWHM (y) in the Z direction increased at a rate of y=2.4x (1/costheta)+0.91 which was caused by an increase in the gantry angle of inclination, and spatial resolution was declined. The relation between FWHM (y) and pitch (x) was y=0.19x+3.12. The increase in FWHM caused by the increase in the gantry angle of inclination was revised with pitch from two formulae, and it became constant. Noise was corrected in the same way, and it became constant. The increase in the noise that occurs due to the increase in the volume of the body caused by the gantry tilt angle was corrected with the tube current, and applied to AEC (automatic exposure control). The influence of gantry inclination on image quality was adjusted, and image quality was improved.

Clinical evaluation of a newly developed method for avoiding artifacts caused by dental fillings on X-ray CT

To evaluate the clinical usefulness of gantry tilt scanning as an image reconstruction technique for avoiding artifacts caused by metallic dental fillings. Gantry tilt scanning was used with multidetector-row computed tomography for imaging in patients with dental fillings. Using a novel PC-based program, the oblique images obtained were reconstructed to transverse images using nearest neighbor and bilinear interpolation methodologies in order to avoid metallic streak artifacts. Coronal images were reformatted with the reconstructed transverse images, and the continuity of the reconstructed images was evaluated. Gantry tilt scanning was performed in 12 patients with metal artifacts, and the original and reconstructed images were classified into four grades and assessed by two radiologists. Results of the clinical evaluation indicated that the original images with artifacts, only 4% had good image quality in the region around the medial pterygoid muscle, only 8% depicted areas around the internal carotid artery and internal jugular vein, and only 12% could depict the areas around the parotid gland in the clinical evaluation. These values were improved to 60, 96, and 100%, respectively, in the reconstructed transverse images. Gantry tilt scanning as an image reconstruction technique improves image quality and removes most, if not all, artifacts caused by metallic dental fillings. The resulting images can be used in the evaluation of oropharyngeal lesions in patients with dental fillings.

SU-FF-I-10: A New Method to Perform CT Gantry Tilt Angle Quality Control Using Commercially Available Phantom

Purpose: To develop a filmless method to perform CT gantry tilt angle quality control test by using commercialy available phantom to adapt the modern digital radiological environment. Method and Materials: A Catphan® 500 CT performance phantom (The Phantom Laboratory, Inc.) was set up on the CT patient couch according to manufacturer's instructions with the CTP401 module of the phantom centered in the scan plane. The CTP401 module contains four ramps (two vertical and two horizontal) which are angled ±23° from the scan plane when gantry tilt is 0°. Axial scans through the ramps were acquired over a range of prescribed slice thicknesses to determine slice sensitivity profile width, or true slice thickness (Tslice), by averaging the slice sensitivity profile width derived from all four ramps. Following this, gantry tilt (θ) was measured over the entire range of tilt (−30° to +30°) by scanning the CTP401 module through the vertical ramp of the same sign (−23° or +23°) and measuring the FWHM length (LFWHM,θ) of the ramp image. Then, gantry tilt (θ) was calculated as for gantry tilts less than 23°, and as for gantry tilts greater than 23°. Calculated gantry angles were validated by comparison to gantry angles determined using the conventional film measurement method described in AAPM Report No. 39. Measurements were obtained at nine gantry tilt angles on 16— slice CT systems. Results: Different slice thicknesses obtained at 9 different angles will be presented. On average, gantry tilt measured using the Catphan agreed with the conventional film measurement method within 1°. Conclusion: This new method removes the requirement to use film for gantry tilt measurements. It is easy to calculate and can be performed without purchasing additional phantoms or equipment. It could be applied to clinical CT gantry angle QC in either on diagnostic or simulation CT.

The effect of computed tomographical gantry angle on the measurement of the canine intercondylar notch

This study was conducted to evaluate the clinical application of computed tomography of the canine femoral intercondylar notch. The canine femoral intercondylar notch is angled 12 degrees from the dorsal plane and obliqued 7 degrees proximolateral to distomedial in the sagittal plane. Measurements of the notch were performed with eight, 12, and 16 degrees of gantry tilt. With the exception of proximal opening notch angle, significant differences were not detected in measurements referenced to 12 degrees of gantry tilt. Evidence from this study indicated that a +/- 4 degree variation in gantry tilt angle from a desired angle of 12 degrees did not significantly affect clinical interpretations of intercondylar notch measurements or notch width index ratios.

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We present a cone-beam image reconstruction algorithm for helical CT scanning with a tilted gantry and N-PI data acquisition. When the gantry is tilted, the effective source trajectory in the patient's reference frame lies on an elliptical cylinder, rather than on a circular cylinder as in the standard helical scanning mode. The aim of this work is to provide a means of reconstructing an image object directly from cone-beam projection data without transforming the image object into a virtual object and without rebinning projection data acquired for a real object into the projection data of the virtual object. This task has been accomplished by the application of an exact reconstruction algorithm, which utilizes an important geometrical property of the elliptical helical trajectory: the existence of generalized N-PI lines for a given image point. Based on this property, a mathematically exact image reconstruction scheme via filtering the backprojection image of differentiated projection data (FBPD) is applied to solve the reconstruction problem. Due to the gantry tilt, the required detector size is different from that of the standard helical trajectory (nontilted). A systematic analysis of the required detector size is presented. For an N-PI data acquisition scheme, an image may be reconstructed using data from an N-PI window, an (N?2)-PI window, and so on. Although the images reconstructed using an N-PI (N>1) window are noisier than the images reconstructed from a 1-PI window, a weighted-average scheme over reconstructed images is presented to generate a final image with significantly lower noise variance than that in the 1-PI data acquisition scheme. The image reconstruction algorithm was numerically validated using a mathematical phantom.

放射線治療計画における義歯アーチファクトの影響と改善策―ガントリ傾斜撮影から再構成したＣＴ水平断像の利用―

Intensity-modulated radiation therapy (IMRT) radiation treatment planning (RTP) requires accuracy. Metal artifacts are one of the factors that influence RTP. The metal artifacts from dental structures are problems at the level of the oropharynx, since these artifacts impair visualization of tumors or lymph nodes and change CT (computed tomography) values. We simulated RTP at the level of the oropharynx using CT images with and without artifacts from dental structures. Gantry tilt scanning was performed to avoid artifacts from dental structures and transverse images reconstructed from oblique images by gantry tilt scanning using a technique of multiplanar reconstruction (MPR) . The reconstructed transverse images were used for the RTP. The reconstructed transverse images were useful to obtain accurate target volumes and the RTP of two opposed equally weighted beams by correct CT values. As dose distribution was changed slightly by the metal artifacts, the use of CT images without artifact is recommended in RTP.

A new, easy, fast, and safe method for CT-guided sacroplasty

Sacral insufficiency fractures constitute clinical challenges because no effective surgical techniques can be applied and only a conservative treatment is currently performed. Sacroplasty is increasingly used to treat sacral insufficiency fractures. A computed tomography (CT)-guided technique concerning the placement of the sacroplasty needles within the sacral wings by using a laser alignment light guidance associated with a CT gantry tilt in a plane parallel to the sacral bone is presented. This method allowed a fast and precise placement of the needle in and along the sacral wings, thus preventing the use of multiple needles to reach the fracture sites.

Tilted cone-beam reconstruction with row-wise fan-to-parallel rebinning

Reconstruction algorithms for cone-beam CT have been the focus of many studies. Several exact and approximate reconstruction algorithms were proposed for step-and-shoot and helical scanning trajectories to combat cone-beam related artefacts. In this paper, we present a new closed-form cone-beam reconstruction formula for tilted gantry data acquisition. Although several algorithms were proposed in the past to combat errors induced by the gantry tilt, none of the algorithms addresses the scenario in which the cone-beam geometry is first rebinned to a set of parallel beams prior to the filtered backprojection. We show that the image quality advantages of the rebinned parallel-beam reconstruction are significant, which makes the development of such an algorithm necessary. Because of the rebinning process, the reconstruction algorithm becomes more complex and the amount of iso-centre adjustment depends not only on the projection and tilt angles, but also on the reconstructed pixel location. In this paper, we first demonstrate the advantages of the row-wise fan-to-parallel rebinning and derive a closed-form solution for the reconstruction algorithm for the step-and-shoot and constant-pitch helical scans. The proposed algorithm requires the ‘warping’ of the reconstruction matrix on a view-by-view basis prior to the backprojection step. We further extend the algorithm to the variable-pitch helical scans in which the patient table travels at non-constant speeds. The algorithm was tested extensively on both the 16- and 64-slice CT scanners. The efficacy of the algorithm is clearly demonstrated by multiple experiments.