Exam 3 Ch 21- Medical Image Descriptors Flashcards
Medical image quality-
refers to fidelity with which anatomy under investigation is rendered
spatial resolution-
-ability to image small objects that have high subject contrast
-improves with smaller pixel size
contrast resolution-
ability to distinguish anatomical structures of similar subject contrast such as liver-spleen & gray matter-white matter
pixel-
element of the picture
Dell
detector element
noise-
random fluctuation of x-ray interaction on the IR
(simplified version- signal not caused by the subject)
quantum mottle-
reduced with the use of high mAs & low kVp
speed medical imaging quality rules-
-fast IR- high noise & low contrast resolution
-slow IR- low noise & high contrast resolution
-spatial resolution is limited to pixel size
preferred technique-
high kVp & low mAs to reduce pt. dose
3 types of radiographic quality-
-IR Factors
-Geometric Factors
-Subject Factors
IR Factors-
pixel size, dynamic range, intensity response, signal to noise, & post-processing
Geometric Factors-
magnification, distortion, & focal-spot blur
Subject Factors-
contrast, thickness, density, atomic #, & motion
magnification-
ability of system to draw/render image at appropriate size
What is the magnification factor (MF) formula?
image size / object size
When OID increases, what also increases?
magnification & distortion increase
Which exams does magnification occur on?
all exams
What is the formula used when the actual measurement of an object size may be impossible?
MF = SID / SOD
When minimizing MF, what does a large SID do?
when SID increases, magnification decreases
When minimizing MF, what does a small OID do?
when OID increases, it also increases magnification & distortion
What is shape distortion caused by?
positioning the tube/part, or IR is angled
distortion is concerned with-
with the part being imaged being unequal shape & size
3 things distortion depends on-
-object thickness
-shape
-position
foreshortening-
angled IR against a part
elongation-
angled IR against the tube
Teleradiology
sending images over a phone line
Telerinkenology
long range radiology (x-rays at 72”)
What does magnification distortion reduce?
image quality
thick objects are more ____ than thin objects
distorted
proper positioning is important because-
irregular shapes appear more distorted when outside the center of the CR
Objects that are further from the IR or Inclined (tilted back) objects aren’t centered correctly & can present with-
distortion
2 types of blur-
focal spot & motion
focal-spot blur has to do with-
the shape of FS & the location on the IR relative to FS
because of effective spot, the way it works is due to-
line focus principle
What do you do to get a small affected FS?
angle the anode face
the beam diversion of 1 source would cause FS itself to-
diverge
presentation of projected FS will have-
different shapes depending where it lands on a rectangular IR
____ will have the least amount of blur because of FS location relative to the IR-
anode side & center
____ will have the most amount of blur because of FS location relative to the IR-
cathode side & not center
helps reduce blurring on edges of anatomy-
short OID
an increased OID is equal to-
a decreased SOD
reduce FS blur-
short OID & centering
subject factors- (5)
-subject contrast
-part thickness
-tissue mass density
-effective atomic #
-object shape
the key to radiographic contrast
subject contrast
helps display subj. contrast-
contrast capabilities of the IR, system, & monitor
where is subject contrast inherent?
in the subject
as part thickness increases-
-scatter & technique increase
-transmission decreases
tissue mass density-
photoelectric effect affected by atomic #
when the effective atomic # is high, contrast is-
contrast is high
transmitted x-rays demonstrated on a radiograph are-
black
relative to cortical bone, on a radiograph soft tissue will show up-
black
photoelectric effect is demonstrated on a radiograph as-
white
job of contrast media-
artificially influence subject contrast
the base of all imaging is-
subject contrast
iodine Z#-
53
barium Z#-
56
Iodine & barium alter-
subject contrast to increase contrast
when the Z# is high & increases, photoelectric effect-
increases rapidly
2 types of contrast-
positive & negative
site contrast-
iodine & barium
iodine & barium shows up _____ on a radiograph
white
as photoelectric effect increases, transmission ____.
decreases
object shape-
the more the shape coincides with the x-ray beam, the greater the contrast
the closer the shape of the object is closer to beam divergence at the level you’re imaging at, you’re going to get the least amount of-
least amount of distortion & blur demonstrated
2 types of motion blur-
voluntary & involuntary
voluntary motion-
movement from the patient such as breathing
how can you reduce voluntary motion in a radiograph?
give good patient instructions
involuntary motion-
peristalsis or the heart beating
how can you reduce involuntary motion in a radiograph?
short exposure times
reduce the likely hood of motion- (5)
-large SID
-reduced OID
-short exposure time
-good patient instructions
-immobilization devices
tools for improving image quality- (5)
-proper patient positioning
-AP v. PA
-AEC selection
-good patient instructions
-breathing instructions
method to improve image quality-
proper positioning of the entire patient so the part being imaged is displayed properly
principle of imaging-
want the part of interest as close to IR as possible
image receptor-
pixel size affects the patients relative dose
small pixels require-
higher dose
large pixel requires-
lower dose
good technical factors selection-
high kVp aids in short exposure times & overall reduces patient dose & improves image quality
