Vibration Response Imaging: evaluation of rater agreement in healthy subjects and subjects with pneumonia

The study population consisted of 43 subjects: 23 patients who were diagnosed with community acquired pneumonia and 20 healthy subjects who participated in the study as control cases. Patients were recruited by consecutive sampling from patients hospitalized in the Respiratory Medicine Department of the University of Thessaly between September 2007 and November 2007 and healthy subjects were recruited from the medical personnel of the hospital during the same period.

Diagnosis of pneumonia was determined by the treating physician, based on medical history, physical examination and radiographic findings. Patients with chest cage or spine deformity, skin lesions, excessive hirsutism on the back and any patient deemed unable to be lifted to a near-sitting position with assistance were excluded. Diagnostic and treatment decisions for all cases described in this study were according to accepted criteria for diagnosing pneumonia [10, 11]. The study protocol was approved by the Institutional Review Board and informed consent was obtained from each participant prior to the inclusion in the study.

Vibration Response Imaging device (Deep Breeze Ltd, Or Akiva, Israel) includes a hardware board for sampling, amplification, processing and A/D data conversion, a PC platform for generation of images and 40 active piezoelectric contact sensors (Meditron, Oslo, Norway) to record lung vibrations. The sensors have a linear frequency response of ± 2 db in the frequency range 50-400 Hz assembled on two planar arrays with a linear frequency response of ± 2 decibel (db) in the frequency range of 50 Hz to 400 Hz and two inactive contact sensors (left and right peripheries of the first row). The arrays were attached to the posterior chest by an open system with a PC-controlled low vacuum to maintain a constant mechanical load on the sensors (Figure 1). The sensors were coupled to the subject's back by a computer controlled low-suction vacuum.

All subjects were examined with the VRI at the bedside during their hospitalization and recordings were performed by their attending physician as previously described [12]. Subjects were instructed to breath deeper than normal through an open mouth during a 12-second recording (3 or 4 respiratory cycles). No forced exhalation or other breathing manoeuvres were performed. During air movement in and out of the lungs, vibrations propagate through the lung tissue. Vibrations were recorded by the surface skin sensors, which were attached to the patient's back. The vibration energy was transmitted to the VRI device, and thereafter, a dynamic digital image was created by means of specifically designed software. Lung sound signals were then transferred to a computer and were analyzed by the software. The signals were processed by band-pass filtering with different frequency ranges: 100-250 Hz for capturing breath sounds while filtering heart sounds and chest-wall movement. The VRI dynamic image was created from a series of gray-scale still images similar to ventilation scanning images of the lung or frames which represent 0.17 seconds of vibration energy recording. In addition, a graph is produced that represents the average vibration energy as a function of time throughout the respiratory cycle. High data values, in which lung vibration energy is greater, are depicted as dark colours (black) and low data values are shown as light colours (light grey); the minimum is defined as 'white'. The maximal energy frame (MEF) is the frame producing the maximal geographical area of lung vibrations in the selected range of frames.

VRI images were evaluated separately by six raters who were qualified physicians and had undergone a basic training in image interpretation by analyzing healthy learning sample images. The training was conducted by showing to the readers the dynamic images sequentially in order to enhance their ability to distinguish basic characteristics of the VRI image including inspiration, expiration, left right synchronization and peak intensity of inspiration [13].

The raters blindly analyzed VRI findings of the study population in a random order, without any previous knowledge of the participants' medical history or the number of images obtained from them. The raters were blind to one another and evaluated twelve features related to i) VRI graphical quality (very good to excellent or not), ii) synchronization of the progression of breath sound distribution between the left and right lung images, iii-x) localization of abnormally increased or decreased intensity in VRI image in right and left, upper-middle and lower lung zones, xi) presence of artefacts, defined as VRI signal with no or poor progression during inhalation-exhalation and no detection of relevant abnormalities in the Chest X-ray, xii) presence of abnormally high or low intensity on VRI-MEF corresponding to the areas of pulmonary consolidation on chest radiographs. After evaluation of i to x features, patients' and healthy controls' chest radiography was demonstrated to raters and features xi and xii were then evaluated. The participants recorded their interpretation of each VRI on a structured questionnaire and these data were used to evaluate inter-rater agreement. In order to measure intra-observer agreement the evaluation was repeated for all VRI images (thus, each reviewer assessed 86 images in total) using the methodology described above.

For qualitative assessment of VRI images, the rater's evaluations were analyzed by degree of reliability and agreement.

Basic characteristics of patients who participated in the study are shown in Table 1. Among the patients with pneumonia 20 had Chronic Obstructive Pulmonary Disease (COPD), 8 arterial hypertension, 11 stable cardiac disease (congestive heart failure 5 cases, ischemic heart disease 6 cases) and 2 diabetes mellitus. There were eighteen cases with radiographically right lung consolidation (13 in the lower and 5 in the upper lung field) and 5 cases with left lung consolidation (3 in the upper lung field and 2 in the lower lung field). A representative case is shown in Figure 2 and 3.

The mean (SE) time for completing a VRI recording in patients and controls was 8(2) and 6(2) minutes respectively (T-test, p = 0.2). Raters identified small artefacts (0.36 artefacts per image by each rater) in VRI images in both healthy (0.27 artefacts/image/rater) and pneumonia cases (0.41 artefacts/image/rater).