Reduction of motion artefacts in on-board cone beam CT by warping of projection images

Objective

We describe the development and testing of a motion correction method for flat panel imager-based cone beam CT (CBCT) based on warping of projection images.

Methods

Markers within or on the surface of the patient were tracked and their mean three-dimensional (3D) position calculated. The two-dimensional (2D) cone beam projection images were then warped before reconstruction to place each marker at the projection from its mean 3D position. The motion correction method was tested using simulated cone beam projection images of a deforming virtual phantom, real CBCT images of a moving breast phantom and clinical CBCT images of a patient with breast cancer and another with pancreatic cancer undergoing radiotherapy.

Results

In phantom studies, the method was shown to greatly reduce motion artefacts in the locality of the radiotherapy target and allowed the true surface shape to be accurately recovered. The breast phantom motion-compensated surface was within 1 mm of the true surface shape for 90% of surface points and greater than 2 mm from the true surface at only 2% of points. Clinical CBCT images showed improved image quality in the locality of the radiotherapy target after motion correction.

Conclusion

The proposed method is effective in reducing motion artefacts in CBCT images.Motion artefacts in cone beam CT (CBCT) occur as a result of movement of the patient during scan acquisition leading to inconsistent data for three-dimensional (3D) reconstruction. This is a particular problem for flat panel imager-based CBCT systems used in image-guided radiotherapy, in which the scanner rotation speed is limited, leading to acquisition times of 1–2 min. Physiological motions such as breathing or internal gas movements lead to anisotropic disturbance, with consequential blurring and streak artefacts in the reconstructed CBCT images [1, 2]. These impede the accurate local delineation of tumours, organs and body surface, which is important for image-guided radiation therapy (IGRT). In IGRT, it is common to acquire a CBCT image immediately prior to treatment delivery for the purpose of verifying that the patient and the relevant internal structures are positioned as intended. If motion artefacts can be removed, making the position of these important objects easier to identify, then the value of the images for geometric verification in radiotherapy may be greatly enhanced.A number of methods have been used to reduce the severity of motion artefacts in CBCT. Elimination of the motion at source, for example through use of breath-hold during acquisition [3], is effective but not applicable in all situations. Sorting of projections into different breathing phases to produce respiratory-correlated CBCT reconstructions has also been reported [4-6]. This requires acquisition of additional projections (with correspondingly increased patient dose), and is applicable only to periodic motions such as breathing.Methods to compensate motion effects during the reconstruction can be applied in 3D, in which the attenuation distribution to be reconstructed is treated as a function of time and the motion path of each voxel is derived from a prior motion model [7-10]. However, a prior motion model is not always available or may be inaccurate.Alternatively, corrections can be applied to the projection data before back projection. Lu and Mackie [11] described a motion correction for fan beam CT, tracking in-plane motion of nodal points in the sinogram and using this to derive patient motion according to a simple model. The sinogram data were then adjusted to correct for this motion.A method of shifting CBCT projection images based on the position of markers attached to a moving rigid phantom has been shown to reduce motion artefacts [12]. Perrenot et al [13] and Schäfer et al [14] used two markers attached to a coronary stent to define an affine transformation of each projection image in order to match the marker positions to the forward projection of their position at a chosen reference time. A more complex projection-based motion correction by warping projection images was described by Hansis et al [15]. Projection images from 3D coronary angiography were warped to reduce discrepancies between measured vessel positions and forward projected vessel positions from an initial echocardiogram (ECG)-gated reconstruction.These two-dimensional (2D) corrections, applied in the projection image domain, are more approximate in nature than the 3D methods. Overlying structures in a projection image, which do not have identical motions, cannot be corrected by manipulating data in the projection domain. However, the corrections can be valid in a local region or where motion may be considered to be approximately rigid. Correction of motion artefacts in a local region can be particularly useful for CBCT images used for target position verification in radiotherapy. Here, sufficient image quality in the region of the radiotherapy target is necessary to allow assessment of its position in relation to the applied radiation beams. Corrections in the projection domain can be simpler to apply than full 3D corrections and do not require a prior motion model.In this paper we describe a motion compensation method for CBCT using a limited number of radio-opaque markers tracked in the projection data. This allows the mean 3D position of each marker to be determined [16]. Projection images are then warped to place each marker at the forward projected mean position for that marker. The method is demonstrated for both markers placed on the surface of the patient and implanted fiducial markers within the patient, as are widely used in radiotherapy [17, 18].We apply the proposed motion compensation method to images from a wide-angle CBCT scanner integrated with a radiotherapy linear accelerator. Improvements in image quality are demonstrated for both phantom and clinical images.     

Accidental external rotation of distal interlock jig in retrograde femoral nailing can lead to more prominent screws

Objective

Symptomatic distal interlocking screws in retrograde femoral nailing are common due the difficulties of imaging the trapezoidal femur. Screws appearing to have appropriate length on imaging may possibly be prominent, creating symptoms. Screw trajectory may influence the degree of this radiographic error. We hypothesize that external rotation of screw trajectory will increase measurement error of screw length.

Effect of the sample matrix on the determination of indinavir in human urine by HPLC with turbo ion spray tandem mass spectrometric detection

The HPLC/tandem mass spectrometric (LC/MS/MS) behavior of indinavir, an HIV protease inhibitor, in human urine is presented as an example of a case where endogenous matrix components were found to interfere with the ionization of the target analyte. The MS/MS system used for these experiments was equipped with a turbo ion spray LC interface. Results from two sample preparation procedures (direct dilution of urine vs urine extraction) and two chromatographic systems (low vs. high capacity factor (k')) for the analytes were compared. Additionally, the precision of the analysis that was achieved while using a stable isotope labeled internal standard is contrasted with the results obtained using an analog of indinavir as internal standard. The results obtained indicated that during development and validation of LC/MS/MS based assays the potential effect of co-eluting 'unseen' endogenous species should be evaluated to ensure that sample preparation and chromatography is adequate to overcome the matrix effect problems.     

Ejaculatory Disorders in Men With Urethral Stricture and Impact of Urethroplasty on the Ejaculatory Function: A Systematic Review

Background

Urethral stricture may disturb both micturition and semen emission. Urethroplasty, despite the restoration of a proper urethral patency, may not eliminate the accompanying ejaculatory dysfunction (EjD).

Clinical and Economic Outcomes of Coronary Angioplasty Alone or in Combination with Stents in Academic Health Centers: A Retrospective Database Analysis

OBJECTIVES: Indications for the use of stents are evolving and their optimal place in therapy remains to be defined. The objective was to compare the 1-year clinical and economic outcomes of percutaneous transluminal coronary agioplasty (PTCA) with and without the use of stents. METHODS: This retrospective database analysis was conducted using data from 60 US academic medical centers in 1996 and 1997. Outcomes of interest included in-hospital mortality (both on sentinel and readmission visits), readmission rates, revascularization procedures, length of stay, and the cost of sentinel and readmission hospitalization. RESULTS: A total of 27,020 patients either did (n = 13,254) or did not (n = 13,766) receive coronary stents in conjunction with PTCA. The median cost of hospitalization for the stent group was about dollar 1,409 higher (mean, dollar 1,200) than the no-stent group and the length of stay was similar (4.3 days vs. 4.5 days, respectively, P = .2628). Mortality rates on readmission (0.9% stent vs. 0.8% no-stent, P = NS) did not differ. However, patients in the stent group had better mortality rates compared to the no-stent group during the sentinel visit (1.7% vs. 2.7%, P < .001). Stent use was not associated with a significantly lower risk of a revascularization procedure on readmission to the same institution (OR 0.95, 95% CI 0.87-1.04, P = .28). CONCLUSIONS: Stents were associated with a 1% decrease of inpatient mortality during the sentinel visit without impacting length of stay or readmission rates. This advantage was associated with a dollar 1,409 higher median cost of hospitalization in the stent group.     

Temperature-dependent appearance of forensically useful flies on carcasses

Flies are frequently used for postmortem interval (PMI) estimations. These estimates are usually based on the age of larval or pupal specimens. However, the age defines only the minimum PMI. In order to move forensic entomology further, a method useful for the estimation of an interval preceding insect appearance on a corpse called the pre-appearance interval (PAI) is needed. Recently, it was demonstrated that the PAI of several carrion beetles is closely related to the temperature prevailing throughout this interval. Hence, it was postulated to estimate PAI from temperature. In order to check premises for using this approach with flies, a test of the relationship between adult or oviposition PAI and temperature was made for nine species of European flies. Data on PAI originated from pig carcasses decomposing under various temperatures. Adult PAI of Hydrotaea dentipes, Hydrotaea ignava, Hydrotaea similis, Phormia regina, and Stearibia nigriceps and oviposition PAI of S. nigriceps were exponentially related to temperature. Only S. nigriceps revealed a close relationship, demonstrating solid premises for PAI estimation from temperature alone. Adult and oviposition PAI of Calliphora vomitoria and adult PAI of Hydrotaea pilipes were not related to temperature. Adult and oviposition PAI of Lucilia sericata and Lucilia caesar responded similarly, with an abrupt and large increase in a narrow range of low temperatures and no response in a broad range of high temperatures. Probably, different mechanisms form the basis for the response of PAI to temperature in flies colonizing carcasses shortly after death and flies colonizing carcasses later in the decomposition process.     

Development and implementation of an electrochemiluminescence immunoassay for the determination of an angiogenic polypeptide in dog and rat plasma

A quantitative method based on electrochemiluminescence immunoassay for the determination of the angiogenic agent aFGF-S117 has been developed and validated. Two polyclonal antibodies specific to aFGF-S117 and a wild-type aFGF antibody were selected for the analysis. The assay was based on the non-competitive sandwich immunoassay principle in which the drug is trapped with a biotinylated antibody that is immobilized on a streptavidin magnetic particle. The drug is then sandwiched with a ruthenium chelated second antibody. The assay demonstrates good accuracy and reproducibility at plasma concentration of 0.5 ng/ml.