computed tomography equipment techniques 12.pdf

Middle Technical University (MTU) College of Health and Medical Techniques -Baghdad Radiological Techniques Department Computed Tomography EquipmentsTechniques Second stage/ 2nd coarse Title: CT image quality Image noise Name of the instructor: Lec. Dr. Lamyaa Fadhil Abdul Hussein Target population:...

Middle Technical University (MTU) College of Health and Medical Techniques -Baghdad Radiological Techniques Department Computed Tomography EquipmentsTechniques Second stage/ 2nd coarse Title: CT image quality Image noise Name of the instructor: Lec. Dr. Lamyaa Fadhil Abdul Hussein Target population: Students of second class 89 Image noise Noise Noise in computed tomography is an unwanted change in pixel values in an otherwise homogeneous image. Often noise is defined loosely as the grainy appearance on cross-sectional imaging. Noise in CT is measured via the signal to noise ratio (SNR); comparing the level of desired signal (photons) to the level of background noise (pixels deviating from normal). The higher the ratio, the less noise is present in the image. Noise in a cross-sectional image will equal a decrease in the picture quality and inadvertently will hinder the contrast resolution. CT numbers of a particular substance such as water are not uniform but rather fluctuate. For water, the CT numbers will fluctuate around an average of 0 (by convention). These random fluctuations in the CT number of a uniform material appear as graininess on a CT image. The degree of random fluctuations depends on the number of x ray photons that contribute to the formation of a CT image. CT noise, therefore, is associated with the number of x rays contributing to each detector measurement. The more x rays used to generate an image, the smaller the amount of image noise. Thus, to understand how each CT technique factor affects image noise, one must imagine the affect of this technique on the number of x rays reaching detector to form the image. Fators affect image noise In radiography, image noise is related to the numbers of x-ray photons contributing to each small area of the image (e.g., to each pixel of a digital radiograph). In CT, xrays contribute to detector measurements and not to individual pixels. CT image noise is thus associated with the number of x-rays contributing to each detector measurement. 90 To understand how CT technique affects noise, one should imagine how each factor in the technique affects the number of detected x-rays. Examples are as follows and its illustrates in table (1): tube current, scan time, slice thickness, tube voltage patient size. Tube current in mA is directly proportional to the number of x rays reaching the detector. Therefore, increasing mA will decrease image noise. Scan time is also directly proportional to the x ray number and thus as scan time increases, image noise decreases. Scan time and tube current are considered together and measured as mAs (milli- amperes-sec). Table (1) :The affect of various CT conditions on image noise Slice thickness changes the beam width entering the detectors. Thus increasing slice thickness results in increasing the beam width which in turn increases the number of x rays proportionately. Increased slice thickness decreases image noise. 91 Increasing tube voltage (keV) increases the energy of the generated x rays and thus, more x rays will penetrate the patient and reach the detectors. Increasing KeV decreases image noise. Decreasing KeV will increase image noise (less penetrated x rays) and will brighten the contrast. Patient factors may also contribute to image noise. That is, the larger the patient, the more attenuation of x rays and thus, less of these x rays will reach the detectors. Increasing patient size results in more image noise due to less x rays reaching the detectors to form the CT image. Figure (1) shows examples of noise in scans of uniform phantoms using standard and higher-resolution (bone) filters and with standard and very low values for mAs. Figure (1): CT image noise depends on number of x-ray photons contributing to image. (A and B) Comparison of noise from scans using 270 mAs (typical clinical value) and 100 mAs. (C) Appearance of image noise is strongly affected by reconstruction filter; sharp filter such as bone also sharpens (enhances) appearance of noise. 92