Bản chất của hình ảnh y sinh học (Phần 2)

percep-2.1 Diculties in Image Acquisition and Analysis In Chapter 1, we studied several imaging systems and procedures for theacquisition of many dierent types of biomedical images.. Cha

Image Quality and Information Content

Several factors aect the quality and information content of biomedical imagesacquired with the modalities described in Chapter 1 A few considerations

in biomedical image acquisition and analysis that could have a bearing onimage quality are described in Section 2.1 A good understanding of suchfactors, as well as appropriate characterization of the concomitant loss inimage quality, are essential in order to design image processing techniques toremove the degradation and/or improve the quality of biomedical images Thecharacterization of information content is important for the same purposes asabove, as well as in the analysis and design of image transmission and archivalsystems

An inherent problem in characterizing quality lies in the fact that imagequality is typically judged by human observers in a subjective manner Toquantify the notion of image quality is a dicult proposition Similarly, thenature of the information conveyed by an image is dicult to quantify due

to its multifaceted characteristics in terms of statistical, structural, tual, semantic, and diagnostic connotations However, several measures havebeen designed to characterize or quantify a few specic attributes of images,which may in turn be associated with various notions of quality as well asinformation content The numerical values of such measures of a given imagebefore and after certain processes, or the changes in the attributes due to cer-tain phenomena, could then be used to assess variations in image quality andinformation content We shall explore several such measures in this chapter

percep-2.1 Diculties in Image Acquisition and Analysis

In Chapter 1, we studied several imaging systems and procedures for theacquisition of many dierent types of biomedical images The practical appli-cation of these techniques may pose certain diculties: the investigator oftenfaces conditions that may impose limitations on the quality and informationcontent of the images acquired The following paragraphs illustrate a fewpractical diculties that one might encounter in biomedical image acquisitionand analysis

Accessibility of the organ of interest: Several organs of interest inimaging-based investigation are situated well within the body, encased in pro-tective and dicult-to-access regions, for good reason! For example, the brain

is protected by the skull, and the prostate is situated at the base of the der near the pelvic outlet Several limitations are encountered in imagingsuch organs special imaging devices and image processing techniques are re-quired to facilitate their visualization Visualization of the arteries in thebrain requires the injection of an X-ray contrast agent and the subtraction

blad-of a reference image see Section 4.1 Special transrectal probes have beendesigned for 3D ultrasonic imaging of the prostate 92] Despite the use ofsuch special devices and techniques, images obtained in applications as abovetend to be aected by severe artifacts

Variability of information: Biological systems exhibit great ranges of herent variability within their dierent categories The intrinsic and naturalvariability presented by biological entities within a given class far exceeds thevariability that we may observe in engineering, physical, and manufacturedsamples The distinction between a normal pattern and an abnormal pat-tern is often clouded by signicant overlap between the ranges of the features

in-or variables that are used to characterize the two categin-ories the problem

is compounded when multiple abnormalities need to be considered ing conditions and parameters could cause further ambiguities due to theeects of subject positioning and projection For example, most malignantbreast tumors are irregular and spiculated in shape, whereas benign massesare smooth and round or oval However, some malignant tumors may presentsmooth shapes, and some benign masses may have rough shapes A tumormay present a rough appearance in one view or projection, but a smootherprole in another Furthermore, the notion of shape roughness is nonspe-cic and open-ended Overlapping patterns caused by ligaments, ducts, andbreast tissue that may lie in other planes, but are integrated on to a singleimage plane in the process of mammographic imaging, could also aect theappearance of tumors and masses in images The use of multiple views andspot magnication imaging could help resolve some of these ambiguities, but

Imag-at the cost of additional radiImag-ation dose to the subject

Physiological artifacts and interference: Physiological systems aredynamic and active Some activities, such as breathing, may be suspendedvoluntarily by an adult subject (in a reasonable state of health and well-being) for brief periods of time to permit improved imaging However, car-diac activity, blood circulation, and peristaltic movement are not under one'svolitional control The rhythmic contractile activity of the heart poses chal-lenges in imaging of the heart The pulsatile movement of blood through thebrain causes slight movements of the brain that could cause artifacts in an-giographic imaging see Section 4.1 Dark shadows may appear in ultrasoundimages next to bony regions due to signicant attenuation of the investigatingbeam, and hence the lack of echoes from tissues beyond the bony regions along

Image Quality and Information Content 63the path of beam propagation An analyst should pay attention to potentialphysiological artifacts when interpreting biomedical images.

Special techniques have been developed to overcome some of the limitationsmentioned above in cardiac imaging Electronic steering of the X-ray beamhas been employed to reduce the scanning time required for CT projectiondata acquisition in order to permit imaging of the heart see Figure 1.21

State-of-the-art multislice and helical-scan CT scanners acquire the requireddata in intervals much shorter than the time taken by the initial models of

CT scanners Cardiac nuclear medicine imaging is performed by gating thephoton-counting process to a certain specic phase of the cardiac cycle byusing the electrocardiogram (ECG) as a reference see Figure 1.27 and Sec-tion 3.10 Although nuclear medicine imaging procedures take several min-utes, the almost-periodic activity of the heart permits the cumulative imaging

of its musculature or chambers at particular positions repeatedly over severalcardiac cycles

Energy limitations: In X-ray mammography, considering the fact thatthe organ imaged is mainly composed of soft tissues, a low kV p would bedesired in order to maximize image contrast However, low-energy X-ray pho-tons are absorbed more readily than high-energy photons by the skin andbreast tissues, thereby increasing the radiation dose to the patient A com-promise is required between these two considerations Similarly, in TEM, ahigh-kV electron beam would be desirable in order to minimize damage tothe specimen, but a low-kV beam can provide improved contrast The practi-cal application of imaging techniques often requires the striking of a trade-obetween conicting considerations as above

Patient safety: The protection of the subject or patient in a study fromelectrical shock, radiation hazard, and other potentially dangerous conditions

is an unquestionable requirement of paramount importance Most zations require ethical approval by specialized committees for experimentalprocedures involving human or animal subjects, with the aim of minimizingthe risk and discomfort to the subject and maximizing the benets to both thesubjects and the investigator The relative levels of potential risks involvedshould be assessed when a choice is available between various procedures, andanalyzed against their relative benets Patient safety concerns may precludethe use of a procedure that may yield better images or results than others,

organi-or may require modications to a procedure that may lead to inferiorgani-or ages Further image processing steps would then become essential in order toimprove image quality or otherwise compensate for the initial compromise

im-2.2 Characterization of Image Quality

Biomedical images are typically complex sources of several items of tion Furthermore, the notion of quality cannot be easily characterized with asmall number of features or attributes Because of these reasons, researchershave developed a rather large number of measures to represent quantitativelyseveral attributes of images related to impressions of quality Changes inmeasures related to quality may be analyzed for several purposes, such as:comparison of images generated by dierent medical imaging systems comparison of images obtained using dierent imaging parameter set-tings of a given system

informa-comparison of the results of several image enhancement algorithms assessment of the eect of the passage of an image through a transmis-sion channel or medium