Technical Note: Validation of two methods to determine contact area between breast and compression paddle in mammography.

PURPOSE: To assess the accuracy of two methods of determining the contact area between the compression paddle and the breast in mammography. An accurate method to determine the contact area is essential to accurately calculate the average compression pressure applied by the paddle. METHODS: For a set of 300 breast compressions, we measured the contact areas between breast and paddle, both capacitively using a transparent foil with indium-tin-oxide (ITO) coating attached to the paddle, and retrospectively from the obtained mammograms using image processing software (Volpara Enterprise, algorithm version 1.5.2). A gold standard was obtained from video images of the compressed breast. During each compression, the breast was illuminated from the sides in order to create a dark shadow on the video image where the breast was in contact with the compression paddle. We manually segmented the shadows captured at the time of x-ray exposure and measured their areas. RESULTS: We found a strong correlation between the manual segmentations and the capacitive measurements [r = 0.989, 95% CI (0.987, 0.992)] and between the manual segmentations and the image processing software [r = 0.978, 95% CI (0.972, 0.982)]. Bland-Altman analysis showed a bias of -0.0038 dm2 for the capacitive measurement (SD 0.0658, 95% limits of agreement [-0.1329, 0.1252]) and -0.0035 dm2 for the image processing software [SD 0.0962, 95% limits of agreement (-0.1921, 0.1850)]. CONCLUSIONS: The size of the contact area between the paddle and the breast can be determined accurately and precisely, both in real-time using the capacitive method, and retrospectively using image processing software. This result is beneficial for scientific research, data analysis and quality control systems that depend on one of these two methods for determining the average pressure on the breast during mammographic compression.

Pain-preventing strategies in mammography: an observational study of simultaneously recorded pain and breast mechanics throughout the entire breast compression cycle.

BACKGROUND: Many women consider mammography painful. Existing studies on pain-preventing strategies only mention pain scores reported before and after breast compression. Studying the pain dynamics during the entire compression cycle may provide new insights for effective pain-preventing strategies. METHODS: This observational study included 117 women who consented to use a custom turning knob to indicate their pain experience during standard mammographic breast compressions in the Academic Medical Center in Amsterdam, The Netherlands. The breast thickness, compression force, contact area, contact pressure and pain experience were recorded continuously. Breast volume was calculated retrospectively from the mammograms. We visualized the progression of pain in relation to breast mechanics for five groups of breast volumes and we performed multivariable regressions to identify factors that significantly predict pain experience. RESULTS: Breast compressions consisted of a deformation phase for flattening, and a clamping phase for immobilization. The clamping phase lasted 12.8 ± 3.6 seconds (average ± standard deviation), 1.7 times longer than the 7.5 ± 2.6 seconds deformation phase. During the clamping phase, the average pain score increased from 4.75 to 5.88 (+24 %) on a 0 - 10 Numerical Rating Scale (NRS), and the proportion of women who reached severe pain (NRS ≥ 7) increased from 23 % to 50 % (more than doubled). Moderate pain (NRS ≥ 4) was reported up to four days after the mammogram. Multivariable analysis showed that pain recollection of the previous mammogram and breast pain before the compression, are significant predictors for pain. Women with smallest breasts experienced most pain: They received highest contact pressures (force divided by contact area) and the pressure increased at the highest rate. CONCLUSION: We suggest further research on two pain-preventing strategies: 1) using a personalized compression protocol by applying to all breasts the same target pressure at the same, slow rate, and 2) shortening the phase during which the breast is clamped.

Towards personalized compression in mammography: a comparison study between pressure- and force-standardization.

OBJECTIVE: To compare a conventional 14 decanewton (daN) force-standardized compression protocol with a personalized 10kilopascal (kPa) pressure-standardized protocol. METHODS: A new add-on contact area detector, which enables pressure-standardized compression, is validated in a double-blinded intra-individual comparison study. Breast screening participants (433) received one craniocaudal (CC) and one mediolateral oblique (MLO) compression for both breasts. Three of these compressions were force-standardized, and one, blinded and randomly assigned, was pressure-standardized. Participants scored their pain experience on an 11-point numerical rating scale (NRS). Three experienced breast-screening radiologists, blinded for compression protocol, indicated which images required retakes. RESULTS: An unanticipated under-compression issue that occurred at forces below 5daN was effectively solved with minimal extra radiographer training during the study. For pressure-standardized compressions obtained at 5daN or more, the compressed breasts thickness increased on average 4.2% (MLO)-6.3% (CC), average pain scores were reduced by 10% (MLO)-17% (CC) and the proportion of women experiencing severe pain (NRS≥7) was reduced by 27% (MLO)-32% (CC), compared with force-standardized compressions (all p-values <0.05). Average glandular dose (AGD) and proportions of retakes were similar for both protocols. CONCLUSION: Pressure-standardized compressions resulted in AGD values and a retake proportion similar to force-standardized compressions, while pain was significantly reduced.

Mammographic compression--a need for mechanical standardization.

BACKGROUND: A lack of consistent guidelines regarding mammographic compression has led to wide variation in its technical execution. Breast compression is accomplished by means of a compression paddle, resulting in a certain contact area between the paddle and the breast. This procedure is associated with varying levels of discomfort or pain. On current mammography systems, the only mechanical parameter available in estimating the degree of compression is the physical entity of force (daN). Recently, researchers have suggested that pressure (kPa), resulting from a specific force divided by contact area on a breast, might be a more appropriate parameter for standardization. Software has now become available which enables device-independent cross-comparisons of key mammographic metrics, such as applied compression pressure (force divided by contact area), breast density and radiation dose, between patient populations. PURPOSE: To compare the current compression practice in mammography between different imaging sites in the Netherlands and the United States from a mechanical point of view, and to investigate whether the compression protocols in these countries can be improved by standardization of pressure (kPa) as an objective mechanical parameter. MATERIALS AND METHODS: We retrospectively studied the available parameters of a set of 37,518 mammographic compressions (9188 women) from the Dutch national breast cancer screening programme (NL data set) and of another set of 7171 compressions (1851 women) from a breast imaging centre in Pittsburgh, PA (US data set). Both sets were processed using VolparaAnalytics and VolparaDensity to obtain the applied average force, pressure, breast thickness, breast volume, breast density and average glandular dose (AGD) as a function of the size of the contact area between the breast and the paddle. RESULTS: On average, the forces and pressures applied in the NL data set were significantly higher than in the US data set. The relative standard deviation was larger in the US data set than in the NL data set. Breasts were compressed with a force in the high range of >15 daN for 31.1% and >20 kPa for 12.3% of the NL data set versus, respectively, 1.5% and 1.7% of the US data set. In the low range we encountered compressions with a pressure of <5 daN for 21.1% and <5 kPa for 21.7% of the US data set versus, respectively, 0.05% and 0.6% in the NL data set. Both the average and the standard deviation of the AGD were higher in the US data set. CONCLUSION: (1) Current mammographic breast compression policies lead to a wide range of applied forces and pressures, with large variations both within and between clinical sites. (2) Pressure standardization could decrease variation, improve reproducibility, and reduce the risk of unnecessary pain, unnecessary high radiation doses and inadequate image quality.

Groupwise rigid registration of wrist bones.

We present an extension of the symmetric ICP algorithm that is unbiased for an arbitrary number (N > or = 2) of shapes, using rigid transformations and scaling. The method does not require the selection of a reference shape or registration order and hence it is unbiased towards any of the registered shapes. The functional to be minimized is non-linear in the transformation parameters and thus computationally complex. We therefore propose a first order approximation that estimates the transformation parameters in a closed form, with computational complexity (see text for symbol)(N2). Using a set of wrist bones, we show that the least-squares minimization and the proposed approximation converge to the same solution. Experiments also show that the proposed algorithms lead to smaller registration errors than algorithms that select a reference shape or register to an evolving mean shape. The low computational cost and trivial parallelization enable the alignment of large numbers of bones.

An efficient and robust algorithm for parallel groupwise registration of bone surfaces.

In this paper a novel groupwise registration algorithm is proposed for the unbiased registration of a large number of densely sampled point clouds. The method fits an evolving mean shape to each of the example point clouds thereby minimizing the total deformation. The registration algorithm alternates between a computationally expensive, but parallelizable, deformation step of the mean shape to each example shape and a very inexpensive step updating the mean shape. The algorithm is evaluated by comparing it to a state of the art registration algorithm. Bone surfaces of wrists, segmented from CT data with a voxel size of 0.3 x 0.3 x 0.3 mm3, serve as an example test set. The negligible bias and registration error of about 0.12 mm for the proposed algorithm are similar to those in. However, current point cloud registration algorithms usually have computational and memory costs that increase quadratically with the number of point clouds, whereas the proposed algorithm has linearly increasing costs, allowing the registration of a much larger number of shapes: 48 versus 8, on the hardware used.

A 4D statistical model of wrist bone motion patterns.

Direct imaging of ligament damage in the wrist remains a challenge. Still, such damage can be assessed indirectly through the analysis of changes in wrist pose and motion pattern. For this purpose we built a statistical reference model that describes healthy motion patterns. We show that such a model can also be used to detect and quantify pathologies. A model that only describes the global translations and rotations of the carpal bones is insufficiently accurate due to size and shape variations of the bones. We present a local statistical motion model that minimizes the influence of size and shape differences by analyzing the coordinate differences of pairs of points on adjacent bone surfaces. These differences are determined in a set of 14 healthy example wrists imaged in a range of poses by means of 4D-RX imaging. The distribution of the differences as a function of the pose form the local statistical motion model (LSMM). Translations of 2 mm and rotations of 20° with respect to the healthy example wrists are detected as outliers in the point pair distributions. An evaluation involving wrists with a damaged ligament between scaphoid and lunate shows that not only joint space widenings can be detected, but also shifts of congruent bone surfaces. The LSMM is also used to perform a virtual reconstruction of the most likely healthy wrist after a simulated perturbation of bones. The reconstruction precision is shown to be about 1 mm. Therefore, the presented 4D statistical model of wrist bone movement may become a valuable clinical tool for diagnosis and surgical planning.

How to objectively classify residents based on their psychomotor laparoscopic skills?

In minimally invasive surgery (MIS), a surgeon needs to acquire a certain level of basic psychomotor MIS skills to perform surgery safely. Evaluation of those skills is a major impediment. Although various assessment methods have been introduced, none of them came as a superior. Three aspects of assessing psychomotor MIS skills are discussed here: (i) advantages and disadvantages of currently available assessment methods, (ii) methods to objectively classify residents according to their level of psychomotor skills, and (iii) factors that influence psychomotor MIS skills. Motion analysis has a potential to be the means to deal with assessment of psychomotor skills. Together with classification methods (e.g. linear discriminant analysis), motion analysis provides an aid in deciding whether a resident is ready to move to the next level of training. Presence of factors that influence psychomotor MIS skills results in a high need for standardisation of valid tasks and setups used for the assessment of MIS skills.

Choosing surgical lighting in the LED era.

BACKGROUND: The aim of this study is to evaluate the illumination characteristics of LED lights objectively to ease the selection of surgical lighting. METHODS: The illuminance distributions of 5 main and 4 auxiliary lights were measured in 8 clinically relevant scenarios. For each light and scenario, the maximum illuminance E(c) (klux) and the size of the light field d(10) (mm) were computed. RESULTS: The results showed: that large variations for both E(c) (25-160 klux) and d(10) (109-300 mm) existed; that using auxiliary lights reduced both E(c) and d(10) by up to 80% and 30%; that with segmented lights, uneven light distributions occurred; and that with colored LED lights shadow edges on the surgical field became colored. CONCLUSIONS: Objective illuminance measurements show a wide variation between lights and a superiority of main over auxiliary lights. Uneven light distributions and colored shadows indicate that LED lights still need to converge to an optimal design.

Interobserver variability in the detection of cerebral venous thrombosis using CT venography with matched mask bone elimination.

OBJECTIVES: Computed tomography venography (CTV) has proven to be a reliable imaging method in the evaluation of cerebral venous thrombosis with good correlation to magnetic resonance (MR) imaging and digital subtraction angiography (DSA). It is fast and widely accessible, especially in the emergency setting. For better visualization of vascular structures bone is often removed from the images. The purpose of this study was to evaluate the quality of a fully automatic bone removal method, matched mask bone elimination (MMBE), and to assess the interobserver variability of the CTV technique. PATIENTS AND METHODS: Fifty patients with clinical suspicion of cerebral venous thrombosis underwent multislice CTV with MMBE post-processing. Axial source images and maximum intensity projections were retrospectively evaluated by two neuroradiologists for quality of bone removal and for the presence or absence of thrombosis in nine dural sinuses and five deep cerebral veins. A per sinus/vein and a per patient analysis (thrombosis in at least one sinus or vein) was performed and interobserver agreement was assessed. RESULTS: Both observers considered bone removal good in all patients (100%). Interobserver agreement per patient was excellent (kappa=0.83), with a full agreement in 47 of 50 patients (94%). The interobserver agreement per sinus or vein was good (kappa=0.76), with a full agreement in 679 of 700 sinuses or veins (97%). CONCLUSION: CTV aided with MMBE is a robust technique for visualization of the intracranial venous circulation, removing bone effectively. CTV has high interobserver agreement for presence or absence of cerebral venous thrombosis.

Detection of in vivo dynamic 3-D motion patterns in the wrist joint.

We present a method for measurement dynamic in vivo carpal motion patterns. The method consists of a 4-D rotational X-ray (RX) with improved image quality and image processing for accurate detection in vivo wrist motion measurements. Dynamic 3-D imaging yields a number of volume reconstructions of the wrist at different phases of its cyclic motion. Next, the carpal reconstructions are registered to their static acquired and segmented counterpart in all phases. With this information, the relation between the applied motion and carpal kinematic behavior is acquired, i.e., the motion patterns. We investigated the precision of the image acquisition and processing and tested it on three healthy subjects. The precision of the image acquisition and image processing is in the range of submillimeters and subdegrees, respectively, which is better than existing systems and sufficient for clinical investigations. Reproducibility measurements show some more deviation ( > 1 degrees). This method was tested on four human volunteers and agrees for the greater part with previously done invasive and nondynamic measurements. In vivo motion pattern measurement with 4-D-RX imaging and processing is accurate and noninvasive. The motion patterns can reveal disorders that could not have been detected in either video fluoroscopy, computed tomography, or MRI.

Retracting and seeking movements during laparoscopic goal-oriented movements. Is the shortest path length optimal?

AIMS: Minimally invasive surgery (MIS) requires a high degree of eye-hand coordination from the surgeon. To facilitate the learning process, objective assessment systems based on analysis of the instruments' motion are being developed. To investigate the influence of performance on motion characteristics, we examined goal-oriented movements in a box trainer. In general, goal-oriented movements consist of a retracting and a seeking phase, and are, however, not performed via the shortest path length. Therefore, we hypothesized that the shortest path is not an optimal concept in MIS. METHODS: Participants were divided into three groups (experts, residents, and novices). Each participant performed a number of one-hand positioning tasks in a box trainer. Movements of the instrument were recorded with the TrEndo tracking system. The movement from point A to B was divided into two phases: A-M (retracting) and M-B (seeking). Normalized path lengths (given in %) of the two phases were compared. RESULTS: Thirty eight participants contributed. For the retracting phase, we found no significant difference between experts [median (range) %: 152 (129-178)], residents [164 (126-250)], and novices [168 (136-268)]. In the seeking phase, we find a significant difference (<0.001) between experts [180 (172-247)], residents [201 (163-287)], and novices [290 (244-469)]. Moreover, within each group, a significant difference between retracting and seeking phases was observed. CONCLUSIONS: Goal-oriented movements in MIS can be split into two phases: retracting and seeking. Novices are less effective than experts and residents in the seeking phase. Therefore, the seeking phase is characteristic of performance differences. Furthermore, the retracting phase is essential, because it improves safety by avoiding intermediate tissue contact. Therefore, the shortest path length, as presently used during the assessment of basic MIS skills, may be not a proper concept for analyzing optimal movements and, therefore, needs to be revised.

Ultrasound imaging and contrast agents: a safe alternative to MRI?

Microbubble contrast media are used to enhance ultrasound images. Because ultrasound is a real-time investigation, contrast-enhanced ultrasound offers possibilities for perfusion imaging. This review is conducted to evaluate the safety of contrast-enhanced ultrasound and its possible role in medical imaging. The safety of diagnostic ultrasound is still an important field of research. The wanted and unwanted effects of ultrasound and microbubble contrast media as well as the effects of ultrasound on these microbubbles are described. Furthermore, some of the possible applications and indications of contrast-enhanced ultrasound will be discussed. The shared advantages of MRI and ultrasound are the use of non-ionizing radiation and non-nephrotoxic contrast media. From this review it can be concluded that, for certain indications, contrast enhanced ultrasound could be a safe alternative to MRI and a valuable addition to medical imaging.

4D rotational x-ray imaging of wrist joint dynamic motion.

Current methods for imaging joint motion are limited to either two-dimensional (2D) video fluoroscopy, or to animated motions from a series of static three-dimensional (3D) images. 3D movement patterns can be detected from biplane fluoroscopy images matched with computed tomography images. This involves several x-ray modalities and sophisticated 2D to 3D matching for the complex wrist joint. We present a method for the acquisition of dynamic 3D images of a moving joint. In our method a 3D-rotational x-ray (3D-RX) system is used to image a cyclically moving joint. The cyclic motion is synchronized to the x-ray acquisition to yield multiple sets of projection images, which are reconstructed to a series of time resolved 3D images, i.e., four-dimensional rotational x ray (4D-RX). To investigate the obtained image quality parameters the full width at half maximum (FWHM) of the point spread function (PSF) via the edge spread function and the contrast to noise ratio between air and phantom were determined on reconstructions of a bullet and rod phantom, using 4D-RX as well as stationary 3D-RX images. The CNR in volume reconstructions based on 251 projection images in the static situation and on 41 and 34 projection images of a moving phantom were 6.9, 3.0, and 2.9, respectively. The average FWHM of the PSF of these same images was, respectively, 1.1, 1.7, and 2.2 mm orthogonal to the motion and parallel to direction of motion 0.6, 0.7, and 1.0 mm. The main deterioration of 4D-RX images compared to 3D-RX images is due to the low number of projection images used and not to the motion of the object. Using 41 projection images seems the best setting for the current system. Experiments on a postmortem wrist show the feasibility of the method for imaging 3D dynamic joint motion. We expect that 4D-RX will pave the way to improved assessment of joint disorders by detection of 3D dynamic motion patterns in joints.

Objective computerized determination of the minimum cross-sectional area of the nasal passage on computed tomography.

OBJECTIVES/HYPOTHESIS: Current methods that measure cross-sectional areas of the nasal passage on computed tomography (CT) do not determine the minimum cross-sectional area that may be an important factor in nasal airway resistance. Objective measurement of the dimensions of the nasal passage may help in the diagnosis, as well as the choice and evaluation of surgical treatment for upper airway insufficiencies. STUDY DESIGN: Retrospective and clinical study. METHODS: Software was developed that automatically calculates the minimum cross-sectional area of the nasal passage on CT. RESULTS: Evaluation shows that the minimization algorithm in the software reliably calculates the position and orientation of the oblique plane on which the minimum cross-section lies. CONCLUSION: The developed method may be used for objective and observer-independent evaluation of surgical treatment options.

Investigation of training needs for functional endoscopic sinus surgery (FESS).

The use of simulators for training FESS may in the future offer substantial advantages like increased exposure to difficult scenarios, reduced learning curves, and reduced costs. Training simulators may range from very simple, involving only visual simulation, to more complex, involving haptic simulation or force feedback. To effectively employ these training means, insight is needed into the training needs for FESS procedure. A study was carried out to investigate which subtasks of FESS are hardest to perform and have the longest learning curve. A questionnaire was distributed among two groups of Ear, Nose and Throat (ENT) surgeons participating in a basic, as well as in an advanced sinus surgery course. Results showed that tasks related to spatial orientation are judged as hardest, whereas manual tasks are considered less difficult. These results suggest that simulators will not necessarily need haptic feedback to train the most important knowledge and skills needed for FESS.

Evaluation of voice control, touch panel control and assistant control during steering of an endoscope.

The increasing amount of equipment used in the Operating Room (OR) asks for ergonomical user interfaces. The aim of this study was to investigate in a pelvi-trainer setting the efficiency, reliability and user satisfaction of voice control, touch panel control and conventional manual control by an assistant. Ten subjects had to control the zoom and light intensity of an endoscope, using voice, a touch panel or an assistant. For each interface, the subject received nine tasks to control to a certain level, light, zoom or both. The experiment was repeated three times (three cycles) and the sequences of interfaces were varied per cycle. Experiments were recorded on video and off-line time needed per task and the number of wrongly interpreted tasks were measured. A questionnaire was used to investigate user satisfaction. Voice control was slower than assistant control and touch panel control (92.5 s, 80.2 s and 76.0 s, respectively, p<0.02). There was no significant difference between touch panel control and assistant control. With voice control, 3.1% of the commands were not interpreted and 1.7% were wrongly interpreted. 40% of the subjects experienced voice control as the quickest, 30% touch panel control and 30% assistant control. 48% of the subjects preferred voice control, 28% the touch panel and 24% assistant control. Voice control was less efficient than touch panel control and manual control by an assistant. The subjects experienced voice control as more efficient, however. In the future, voice control should be improved to overcome wrongly interpreted commands. Furthermore, experiments should be performed in a clinical setting in which the surgeon has to perform two-handed tasks to evaluate the effects on the surgeon's performance.

Reproducibility of multi-slice spiral computed tomography scans: an experimental study.

In multi-slice spiral computed tomography (CT) images interpolation artifacts are present. The relationship between the x-ray tube rotation angle and these artifacts is demonstrated. A head phantom was repeatedly scanned with a four-slice CT scanner at different pitch values. Two scans, made with identical scan parameters, nearly always have different x-ray tube starting angles. Consequently, artifacts appeared differently and residual artifacts appeared when subtractions were made. We conclude that only if the x-ray tube starting angle is equal for both scans or if a very low pitch is used, images are highly reproducible.