w4 Flashcards
What conditions may impose limitations on the quality and information content of medical images?
Accessibility of the organ of interest Variability of information Physiological artifacts and interference Energy limitations Patient safety
Features of the brain are difficult to image because it is protected by the skull (the brain is not accessible). What is one way someone could image the arteries in the brain?
requires the injection of an X-ray contrast agent and the subtraction of a reference image.
what are image artifacts and why do they occur
any feature which appears in an image which is not present in the original imaged object.
An image artifact is sometime the result of improper operation of the imager, and other times a consequence of natural processes or properties of the human body.
similar in meaning to interference and noise.
The distinction between a normal pattern and an abnormal pattern is often clouded by significant overlap between the ranges of the features or variables that are used to characterize the two categories. The problem is compounded when multiple abnormalities need to be considered. Imaging conditions and parameters could cause further ambiguities due to the effects of subject positioning and projection.
How may this variability of information affect the imaging of a cancerous breast tissue? (3 ways)
- most are irregular and spiculated in shape, whereas benign masses are smooth and round or oval. However, some may present smooth shapes, and some benign masses may have rough shapes.
- A tumor may present a rough appearance in one view or projection, but a smoother profile in another. Furthermore, the notion of shape roughness is nonspecific and open-ended.
- Overlapping patterns caused by ligaments, ducts, and breast tissue that lie in other planes, but are superimposed to a single image plane in imaging, could also affect the appearance of tumors in images. Can use multiple views and spot magnification, but at the cost of additional radiation dose to the subject.
what is the measure used to represent the “energy” of an x-ray beam
The kVp, standing for kilo-volt-peak, is a commonly used indicator of penetrating capability, and often referred to as the “energy” of the X-ray beam.
how may energy limitations affect the acquisition of an X-ray mammography
low-energy X-ray photons are absorbed more readily than high-energy photons by the skin and breast tissues, meaning more energy is required to penetrate through the body. However, breast tissue is mainly soft tissue, meaning a lower kVp would be desired in order to maximize image contrast. A compromise is required.
how is contrast defined in medical imaging
the difference between the image parameter in a region of interest (ROI) and that in a suitably defined background.
e.g. If the image parameter is expressed in optical density (OD), contrast is defined as: C_OD = forground_OD - background_OD.
Contrast needs to be normalized, what are the two ways of normalizing
normal contrast: Cn = (f – b) / (f + b)
simultaneous contrast: Cs = (f – b) / b
what is simultaneous contrast, and why is its distinction from normal contrast important
Describes the way color is perceived when two image objects are placed next to each other e.g. if two images share color values but have different contrasts, and they are placed next to each other, the color values will not appear the same. Compared to normal contrast, simultaneous contrast better emphasizes this effect. Used for calculating Just-Noticeable Difference.
for a greyscale range of 0 to 255, what color is 0 and what color is 255
0 - black
255 - white
what is Just-Noticeable Difference and Weber’s Law
an experimentally derived threshold indicating at which simultaneous constrast value is the difference between forground and background noticible. Important in analyzing contrast, visibility, and the quality of medical images.
Experiments have shown that the JND is almost constant, at approximately 2% over a wide range of background intensities; this is known as Weber’s law.
how is Histogram defined in medical imaging
a characterization of image quality. A histogram of the intensity profile (probability density function) of an image.
h(rk) = nk
where rk is the k-th gray level/partition and nk is the number of pixels in the image having gray level rk.
How are histograms used to characterize image quality?
lets the user see the distribution of contrast throughout the image. low contrast imaging, such as MR Angiography, have narrow pixel distributions. This data can be used in medical image enhancement algorithms to increase the contrast based on the pixel’s bin.
what are some sources of noise
sources of noise could be physiological, the instrumentation used, or the environment of the experiment.
what is Digital subtraction angiography / Temporal Subtraction
a fluoroscopy technique used in interventional radiology to clearly visualize blood vessels in a bony or dense soft tissue environment. Images are produced by subtracting a “pre-contrast image” from an image taken when a contrast medium/agent has been introduced into a structure. Pixels are subtracted in a pixel-wise way.
what are the three common types of noise
Salt-and-pepper noise: random occurrences of black and white pixels
Impulsive noise: random occurrences of white pixels
Gaussian noise: variations in intensity drawn from a
Gaussian normal distribution
What are the two major categories of Digital Image Processing
spatial domain processing techniques: based on direct manipulation of pixels in an image
frequency (transform) domain processing techniques are based on modifying the Fourier (or others) transform of an image.
