Orthopedic Metal Artifact Reduction Algorithm Research Articles

The effects of the orthopedic metal artifact reduction (O-MAR) algorithm on contouring and dosimetry of head and neck radiotherapy patients

Metallic objects, such as dental fillings, cause artifacts in computed tomography (CT) scans. We quantify the contouring and dosimetric effects of Orthopedic Metal Artifact Reduction (O-MAR), in head and neck radiotherapy.The ease of organ contouring was assessed by having a radiation oncologist identify the CT data set with or without O-MAR for each of 28 patients that was easier to contour. The effect on contouring was quantified further by having the physician recontour parotid glands, previously drawn by him on the O-MAR scans, on uncorrected scans, and calculating the Dice coefficent (a measure of overlap) for the contours.Radiotherapy plans originally generated on scans reconstructed with O-MAR were recalculated on scans without metal artifact correction. The study was done using the Analytical Anisotropic Algorithm (AAA) dose calculation algorithm. The 15 patients with a planning target volume (PTV) extending to the same slice as the artifacts were used for this part of the study.The normal tissue doses were not significantly affected. The PTV mean dose and V95 were not affected, but the cold spots became less severe in the O-MAR corrected plans, with the minimum point dose on average being 4.1% higher. In 79% of the cases, the radiation oncologist identified the O-MAR scan as easier to contour; in 11% he chose the uncorrected scan and in 11% the scans were judged to have equal quality. A total of nine parotid glands (on both scans—18 contours in total) in 5 patients were recontoured. The average Dice coefficient for parotids drawn with and without O-MAR was found to be 0.775 +/− 0.045.The O-MAR algorithm does not produce a significant dosimetric effect in head and neck plans when using the AAA dose calculation algorithm. It can therefore be used for improved contouring accuracy without updating the critical structure tolerance doses and target coverage expectations.

Metal Artifact Reduction of Dental Fillings in Head and Neck CT Images

Introduction: The issue of metal artifact and its reduction is as old as the clinical use of computed tomography itself. When metal objects such as dental fillings, hip prostheses or surgical clips are present in the computed tomography (CT) field of view (FOV), make severe artifacts that reduce the image quality and accuracy of CT numbers. They can lead to unreliable clinical results due to inability in true tumor volume delineation and uncertainty in dose calculation of treatment planning software. So far, no generally accepted solution to this problem, especially for small size dental fillings, has been found. Herein, an approach is presented to reduce the dental filling artifacts in CT images of head and neck patients with multiple dental fillings. The proposed approach is compared with the commercial orthopedic metal artifact reduction (O-MAR) algorithm by Philips company. Materials and Methods: Our algorithm consists of six steps: 1) metallic object segmentation by thresholding, 2) obtaining the prior image by multi thresholding of the initial image, 3) normalization of the original image sinogram by prior image, 4) interpolation (replacing the affected projection data by previous slice unaffected projection data, 5) denormalization of the corrected sinogram and 6) application of a two dimensional adaptive filter to the final image. The forth step can be implemented by substitution of affected projections with opposite view projections. Both methods were tested and previous slice substitution was superior. Quantitative evaluation of the commercial and proposed algorithms was done in terms of peak signal-to-noise ratio (PSNR), signal-to- noise ratio (SNR) and structural similarity index (SSIM) on a CT image of a head and neck patient with four dental fillings. The posterior part of the head in original image was considered as the reference image. Also two uniform regions of interest (ROI) were considered in the image. ROI1 was posterior region of the head away from dental fillings, while ROI2 was restricted to the tongue region near to the dental fillings. Results: The PSNR, SNR, SSIM values obtained by the proposed method and O-MAR algorithm were 37.05, 36.13, 22.29, 21.37, 0.93 and 0.84, respectively. Using our approach, the standard deviation of CT numbers in ROI1 was 27 times less than the original image and 7% better than the commercial competitor. However, for ROI2 the commercial algorithm was 36% more successful in reduction of variations due to metal artifact. Conclusion: The proposed approach can be applied successfully for dental filling artifact reduction in head and neck patients. Although the performance of the commercial method was superior to ours in near dental regions, for the farther regions with more critical organs was vice versa. However, weighting the interpolation and adaptive filtering steps may result in even better results.

Combined application of virtual monoenergetic high keV images and the orthopedic metal artifact reduction algorithm (O-MAR): effect on image quality

To determine whether there is any additional metal artifact reduction when virtual monochromatic images (VMI) and metal artifact reduction for orthopedic implants (O-MAR) are applied together compared to their separate application in both phantom and clinical abdominopelvic CT studies. An agar phantom containing a spinal prosthesis was scanned using a dual-layer, energy CT scanner (IQon, Philips Healthcare), and reconstructed with the filtered back-projection algorithm without O-MAR (FBP), filtered back-projection algorithm with O-MAR (O-MAR), VMI140 without O-MAR (VMI140), and VMI140 with O-MAR (VMI140 + O-MAR). Abdominopelvic CT images of 47 patients with metallic prostheses were also reconstructed in the same manner for clinical study. Noise measured as the standard deviation of CT Hounsfield units was compared between the four reconstruction methods in both phantom and clinical studies. Improvements in metal artifact reduction, image quality, and diagnostic improvement were further analyzed in the clinical study. Noise was significantly decreased when both VMI and O-MAR were applied in conjunction compared to their separate application in both phantom (16.3 HU vs. 25.0 and 26.4 HU) and clinical studies (15.8 HU vs. 19.2 and 26.2 HU). In the clinical study, the qualitative degree of artifacts was also significantly reduced with VMI140 + O-MAR (2.85 and 2.87) compared to VMI140 (2.36 and 2.26) or O-MAR (2.13 and 2.04) alone for both reviewers (P < 0.001) and improvements in image quality were observed in all 47 patients, with actual diagnostic improvements observed in three. Metal artifacts can be additionally reduced by applying O-MAR and VMI in conjunction, compared to their separate application, thereby improving diagnostic performance.

Value and Clinical Application of Orthopedic Metal Artifact Reduction Algorithm in CT Scans after Orthopedic Metal Implantation.

ObjectiveTo evaluate orthopedic metal artifact reduction algorithm (O-MAR) in CT orthopedic metal artifact reduction at different tube voltages, identify an appropriate low tube voltage for clinical practice, and investigate its clinical application.Materials and MethodsThe institutional ethical committee approved all the animal procedures. A stainless-steel plate and four screws were implanted into the femurs of three Japanese white rabbits. Preoperative CT was performed at 120 kVp without O-MAR reconstruction, and postoperative CT was performed at 80–140 kVp with O-MAR. Muscular CT attenuation, artifact index (AI) and signal-to-noise ratio (SNR) were compared between preoperative and postoperative images (unpaired t test), between paired O-MAR and non-O-MAR images (paired Student t test) and among different kVp settings (repeated measures ANOVA). Artifacts' severity, muscular homogeneity, visibility of inter-muscular space and definition of bony structures were subjectively evaluated and compared (Wilcoxon rank-sum test). In the clinical study, 20 patients undertook CT scan at low kVp with O-MAR with informed consent. The diagnostic satisfaction of clinical images was subjectively assessed.ResultsAnimal experiments showed that the use of O-MAR resulted in accurate CT attenuation, lower AI, better SNR, and higher subjective scores (p < 0.010) at all tube voltages. O-MAR images at 100 kVp had almost the same AI and SNR as non-O-MAR images at 140 kVp. All O-MAR images were scored ≥ 3. In addition, 95% of clinical CT images performed at 100 kVp were considered satisfactory.ConclusionO-MAR can effectively reduce orthopedic metal artifacts at different tube voltages, and facilitates low-tube-voltage CT for patients with orthopedic metal implants.

35. Evaluation and validation of the use of Orthopedic Metal Artifact Reduction algorithm (O-MAR) for dose calculation in radiotherapy

Introduction O-MAR algorithm mitigates artifacts caused by metal objects in CT images. Initially developed for diagnostic CT images, O-MAR may also prove useful to address CT images used for dose calculation in radiotherapy. Indeed, metal artifacts severely degrade dose calculation accuracy and are usually manually corrected by assigning electron density to 1 g cm −3 . Nevertheless, this process is long, inaccurate and operator-dependent. Moreover, it only applies to soft tissue area. The aim of this study is to evaluate and validate the use of O-MAR on dosimetric CT images for patients with metallic implants (orthopedic and dental). Method The capability of O-MAR to restore real Hounsfield Units (HU) was evaluated on cylindrical phantom considering various inserts (solid water, lung and cortical bone). CT images (Philips BigBore®) were acquired without metal (reference) and with 2 artificial hip joints, with and without O-MAR correction. Same experiment was made with dental implants. The impact of O-MAR on dose calculation was evaluated on previous images using the treatment planning system (TPS) CMS XIO for a 6MV photon beam and for various clinical ballistics on 3 TPS (CMS XIO, IplanRT et Tomotherapy®). Results obtained with O-MAR correction were also compared with those obtained on CT images manually corrected, which are the standard of practice in our institution. For each case, 3D distribution of local percent difference dose was analyzed excluding doses lower than 10% of the prescribed dose. Finally, O-MAR was applied to patients CT images. Results HU measurements show that artefacts have limited impact on lung insert but significant impact on solid water and cortical bone inserts with a difference of 100HU and 250HU respectively. O-MAR effectively reduces these differences to 40HU in solid water and 90HU in cortical bone. Moreover, O-MAR is more effective than manual correction to reduce impact of artefacts on dose calculation. In the phantom with 2 artificial hip joints, for a single 6MV photon beam, the percentage of pixels with a local percent difference dose >+/−1% (PLPD1%) is 5.2% without correction, 2.3% with manual correction and 1.2% with O-MAR. Similar analysis considering 7 IMRT beams (XIO) gives a PLPD1% of 9.5%, 5.7% and 4.4% respectively. O-MAR effectively corrects patients CT images with artificial hip joints. However, for dental implants, some manual re-working remains necessary after O-MAR correction. Conclusion For orthopedic implants, O-MAR is effective to correct metallic artefacts. Comparing with manual correction, O-MAR results in a considerable saving in time and increases accuracy of dose calculation. For dental implants, although manual correction is still required, time saving remains substantial.

SU-E-I-26: The CT Compatibility of a Novel Direction Modulated Brachytherapy (DMBT) Tandem Applicator for Cervical Cancer

Purpose: To examine CT metal image artifact from a novel direction-modulated brachytherapy (DMBT) tandem applicator (95% tungsten) for cervical cancer using a commercially available orthopedic metal artifact reduction (O-MAR) algorithm. Comparison to a conventional stainless steel applicator is also performed. Methods: Each applicator was placed in a water-filled phantom resembling the female pelvis and scanned in a Philips Brilliance 16-slice CT scanner using two pelvis protocols: a typical clinical protocol (120kVp, 16×0.75mm collimation, 0.692 pitch, 1.0s rotation, 350mm field of view (FOV), 600mAs, 1.5mm slices) and a protocol with a higher kVp and mAs setting useful for larger patients (140kVp, 16×0.75mm collimation, 0.688 pitch, 1.5s rotation, 350mm FOV, 870mAs, 1.5mm slices). Images of each tandem were acquired with and without the application of the O-MAR algorithm. Baseline scans of the phantom (no applicator) were also collected. CT numbers were quantified at distances from 5 to 30 mm away from the applicator's edge (in increments of 5mm) using measurements at eight angles around the applicator, on three consecutive slices. Results: While the presence of both applicators degraded image quality, the DMBT applicator resulted in larger streaking artifacts and dark areas in the image compared to the stainless steel applicator. Application of the O-MAR algorithm improved all acquired images, both visually and quantitatively. The use of low and high kVp and mAs settings (120 kVp/600mAs and 140 kVp/870mAs) in conjunction with the O-MAR algorithm lead to similar CT numbers in the vicinity of the applicator and a similar reduction of the induced metal artifact. Conclusion: This work indicated that metal artifacts induced by the DMBT and the stainless steel applicator are greatly reduced when using the O-MAR algorithm, leading to better quality phantom images. The use of a high dose protocol provided similar improvements in metal artifacts compared to the clinical protocol.

Poster — Thur Eve — 11: Validation of the orthopedic metallic artifact reduction tool for CT simulations at the Ottawa Hospital Cancer Centre

Metallic implants in patients can produce image artifacts in kilovoltage CT simulation images which can introduce noise and inaccuracies in CT number, affecting anatomical segmentation and dose distributions. The commercial orthopedic metal artifact reduction algorithm (O‐MAR) (Philips Healthcare System) was recently made available on CT simulation scanners at our institution. This study validated the clinical use of O‐MAR by investigating its effects on CT number and dose distributions.O‐MAR corrected and uncorrected images were acquired with a Philips Brilliance Big Bore CT simulator of a cylindrical solid water phantom that contained various plugs (including metal) of known density. CT number accuracy was investigated by determining the mean and standard deviation in regions of interest (ROI) within each plug for uncorrected and O‐MAR corrected images and comparing with no‐metal image values. Dose distributions were calculated using the Monaco treatment planning system. Seven open fields were equally spaced about the phantom around a ROI near the center of the phantom. These were compared to a “correct” dose distribution calculated by overriding electron densities a no‐metal phantom image to produce an image containing metal but no artifacts.An overall improvement in CT number and dose distribution accuracy was achieved by applying the O‐MAR correction. Mean CT numbers and standard deviations were found to be generally improved. Exceptions included lung equivalent media, which is consistent with vendor specified contraindications. Dose profiles were found to vary by ±4% between uncorrected or O‐MAR corrected images with O‐MAR producing doses closer to ground truth.

Clinical Evaluation of Normalized Metal Artifact Reduction in kVCT Using MVCT Prior Images (MVCT-NMAR) for Radiation Therapy Treatment Planning

To evaluate the metal artifacts in diagnostic kilovoltage computed tomography (kVCT) images of patients that are corrected by use of a normalized metal artifact reduction (NMAR) method with megavoltage CT (MVCT) prior images: MVCT-NMAR. MVCT-NMAR was applied to images from 5 patients: 3 with dual hip prostheses, 1 with a single hip prosthesis, and 1 with dental fillings. The corrected images were evaluated for visualization of tissue structures and their interfaces and for radiation therapy dose calculations. They were compared against the corresponding images corrected by the commercial orthopedic metal artifact reduction algorithm in a Phillips CT scanner. The use of MVCT images for correcting kVCT images in the MVCT-NMAR technique greatly reduces metal artifacts, avoids secondary artifacts, and makes patient images more useful for correct dose calculation in radiation therapy. These improvements are significant, provided the MVCT and kVCT images are correctly registered. The remaining and the secondary artifacts (soft tissue blurring, eroded bones, false bones or air pockets, CT number cupping within the metal) present in orthopedic metal artifact reduction corrected images are removed in the MVCT-NMAR corrected images. A large dose reduction was possible outside the planning target volume (eg, 59.2 Gy to 52.5 Gy in pubic bone) when these MVCT-NMAR corrected images were used in TomoTherapy treatment plans without directional blocks for a prostate cancer patient. The use of MVCT-NMAR corrected images in radiation therapy treatment planning could improve the treatment plan quality for patients with metallic implants.

Pulmonary Pseudoemboli

Metal artifact reduction algorithms for computed tomographic (CT) image reconstruction have recently become commercially available on modern CT scanners for reducing artifacts from orthopedic hardware. However, we have observed that a commercial orthopedic metal artifact reduction algorithm can produce the appearance of artifactual pulmonary emboli when applied to spinal hardware in contrast-enhanced CT scans of the chest. We provide 4 case examples demonstrating this previously undescribed artifact.