EUROCON 2005 - Ruđer Bošković Institute



(

Quality of chest X-ray images compressed using JPEG AND JPEG2000

Matko Dodig1, Sonja Grgić2 and Slavica Dodig3

Abstract — The aim of this paper is to compare the quality of digitized chest X-ray images with digitally captured chest X-ray images in children with lung tuberculosis (TB), compressed using popular JPEG and JPEG2000 compression algorithms, for further electronic correspondence. All images were compressed using JPEG and JPEG2000 compression algorithms with six compression degrees (3 bpp, 1.5 bpp, 1 bpp, 0.7 bpp, 0.5 bpp, 0.3 bpp). Picture quality was evaluated by means of usual objective measures of picture quality such as MSE, SNR and PSNR. All compressed images were shown to physicians, who were asked to evaluate diagnostic accuracy of each image (by poor, good or excellent). According to physician's subjective evaluation both JPEG and JPEG2000 compression for all X-ray digitized images (all TB-attributes) were accepted as excellent.

Keywords — Chest radiograph, image analysis, image compression, teleradiology.

Introduction

I

maging techniques, such as X-ray imaging, ultrasound, computorized tomography, magnetic resonance, look "through" the body. They play an important role in medical diagnosis. Some of them (ultrasound) have a signal resolution and noise comparable to standard TV. They can be transmitted without significant loss of quality for primary diagnosis, teleradiology or for video conference presentation. X-ray film has much resolution than television [1]. Compressed imaging can lose its high-resolution content. The fundamental difficulty in testing quality of compressed image is how to decide which test images to use for the evaluations [2]. For teleradiology routine X-ray images must be digitized with very high resolution and sent as purely digital files with no quality loss during transmission [2]. For primary diagnosis X-ray images are transmitted usually as digital camera captured images. Recently, digital radiography has demonstrated significant improvements in image quality. Since digital radiography store-and-forward telemedicine in Croatia is available mostly in clinical hospitals, digitized X-ray images still offer a low-cost alternative for physicians in distant medical centers to obtain second opinion from consultant specialists. The aim of this study was to compare the quality of digitized chest X-ray with digitally captured chest X-ray images in children with lung tuberculosis (TB).

Test images

Eight upright posteroanterior chest X-ray images were selected for study: four images were photographed using digital camera (Fig. 1), and four images were digitaly captured. All radiographs were selected by pediatrician and in detail described by radiologist. Photographed X-ray images were placed on 12 mm thin negatoscope (lighted X-ray viewer) and photographed in the same darkened room. The digital camera (Canon Power Shot S45) was mounted on a tripod placed approximately 75 cm from the negatoscope (this distance assures that most chest X-rays filled the camera's LCD monitor; the camera's optical zoom set to the midpoint). Digitaly captured images were obtained using Shimadzu equipment. All images were stored in PC, converted to 24-bit BMP and compressed. To produce test chest X-ray images, two different image compression algorithms were used: JPEG (3] and JPEG2000 (4]. JPEG image compression is designed to accommodate limitations of the human eye [5]. Each X-ray image was compressed with six different compression degrees (3 bpp, 1.5 bpp, 1 bpp, 0.7 bpp, 0.5 bpp, 0.3 bpp).

|[pic] |[pic] |

|(a) Normal finding |(b)TB: Infiltration, cavities |

|[pic] |[pic] |

|(c) TB: Infiltration, cavities,|(d) TB: Infiltration |

|limphadenopathy | |

Fig. 1. Chest X-rays of one healthy child (a) and three children with tuberculosis (b - d)

Picture quality measures

Among many numerical measures of picture quality, we have chosen next measures: mean square error (MSE), optimized quality factor (OQF), signal-to-noise ratio (SNR) and peak-signal-to-noise ratio (PSNR) [2].

MSE and PSNR are the most common measures of picture quality [6]. All compressed images were shown to physicians, who were asked to evaluate diagnostic accuracy of each image, by poor, good or excellent.

Statistical significance was tested by Wilcoxon test and by Student t-test, while correlation between two variables was expressed by coefficient of correlation (r) [7].

Results

Results of objective quality measures of JPEG and JPEG2000 compressed images of each chest X-ray image are summarized in Tables 1. to 4. PSNR and SNR values in digitaly captured images were lower than PSNR and SNR values in photographed/digitized images in both JPEG and JPEG2000 compression, respectively.

Table 1: Quality measures of JPEG compression for each digitized image

|X-ray |GIVEN BITRATE: 3.0 bpp |

| |Optimized Quality |MSE |SNR (dB)|PSNR (dB) |

| |Factor | | | |

|(a) |99 |0.2 |45.5 |55.5 |

|(b) |99 |0.2 |44.4 |55.5 |

|(c) |99 |0.2 |46.2 |55.6 |

|(d) |99 |0.2 |44.6 |55.6 |

|  |GIVEN BITRATE: 1.5 bpp |

|(a) |97 |0.2 |45.9 |55.9 |

|(b) |97 |0.2 |44.8 |55.9 |

|(c) |98 |0.2 |46.0 |55.5 |

|(d) |97 |0.2 |45.0 |56.0 |

|  |GIVEN BITRATE: 1.0 bpp |

|(a) |93 |0.3 |43.9 |53.9 |

|(b) |92 |0.3 |41.7 |52.8 |

|(c) |93 |0.3 |44.7 |54.1 |

|(d) |93 |0.3 |43.1 |54.1 |

|  |GIVEN BITRATE: 0.7 bpp |

|(a) |85 |0.67 |37.8 |47.8 |

|(b) |85 |1.1 |36.5 |47.5 |

|(c) |85 |1.0 |38.7 |48.2 |

|(d) |85 |1.1 |36.8 |47.8 |

|  |GIVEN BITRATE: 0.5 bpp |

|(a) |81 |1.3 |37.0 |47.1 |

|(b) |80 |1.4 |35.5 |46.6 |

|(c) |81 |1.2 |38.0 |47.5 |

|(d) |82 |1.2 |36.2 |47.2 |

| |GIVEN BITRATE: 0.3 bpp |

|(a) |68 |1.9 |35.3 |45.3 |

|(b) |66 |2.2 |33.7 |44.8 |

|(c) |67 |1.9 |36.0 |45.5 |

|(d) |68 |1.9 |34.4 |45.4 |

Data in Table 1. and Table 3., as well as Fig. 2. (JPEG), show that PSNR values are > 50.0 dB in compression degrees 3.0 to 1.0 bpp and < 50.0 dB in compression degrees 0.7 to 0.3 bpp (two tailed probability ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download