Ch. 15 Instrumentation

Nuclear Medicine Images

image quality

illustrates the closeness that one image that is created represents the imaged object

image quality

as technologists, we aim to have the highest ______, so we can give the radiologists the most accurate picture possible

physical characteristics and human observer performance studies

two methods for characterizing pr evaluating image quality

quantity measured or calculated for the image or imaging system

characterizing or evaluating image quality by the means of physical characteristics that can be:

1. spatial resolution

2. contrast

3. noise

three physical characteristics that are commonly used

spatial resolution

correlates to spatial resolution; detail or sharpness

correlates to spatial resolution; detail or sharpness

spatial resolution (characteristic)

contrast

difference in image density or intensity between areas of the imaged object containing different concentrations of radioactivity

difference in image density or intensity between areas of the imaged object containing different concentrations of radioactivity

contrast

noise

statistical noise caused by random fluctuations in radioactive decay or structured noise

statistical noise caused by random fluctuations in radioactive decay or structured noise

noise

using images obtained with different imaging systems or under different imaging conditions

how is evaluating image quality with the use of human observer performance studies done?

objectively

observer performance can be characterized ______ and is related to the physical measures of image quality in terms of quantifiable metrics

physical measures of image quality

observer performance can be related to:

experience and human visulas

observer performance can be objective depending on:

computer observer performance studies

uses a mathematical model that can predict human observer studies and can speed up the time to quantify image quality

a mathematical model that can predict human observer studies and can speed up the time to quantify image quality

computer observer performance studies uses:

spatial resolution

refers to the sharpness or detail of the image, or the ability of the imaging instrument to provide sharpness or detail

refers to the sharpness or detail of the image, or the ability of the imaging instrument to provide sharpness or detail

spatial resolution

collimator resolution, intrinsic resolution and patient motion

factors that can affect spatial resolution

collimator rsoluton

the principle limitung factor when absorptive collimators are used for spatial localization

large

collimator hole diameters muct be relatively ____ to obtain reasonable collimator efficienvy

large

to increase collimator efficiency, collimator hole diameter must be ____

at least as great as the hole diamters

because the collimator holes must be large, there is blurring of the image by an amount of:

blurring

what can happen to the image because of the size of the collimator hole diameters

source to detector distance

besides collimator hole diameter, what else can affect collimator resolution

photomultiplier tubes

when using the gamma camera, the limitation arises primarily because of the statistical variations in the distribution of light photons among the ______

statistical variations in the distribution of light photons

when using the gamma camera, the limitation arises primarily because of the __ __ ___ among the photomultiplier tubes

intrinsic resolution

a function of gamma-ray energy with the gamma camera, becoming poorer with decreasing gamma ray energy

PET systems

what kind of devices have discrete detector elements?

intrinsic resolution

For imaging devices with discrete detector elements, the size of the individual detector elements largely determines the

image sharpness

patient motion can affect

spatial resolution

intrinsic resolution can affect the

cardiac and respiratory

which types of motions can be troublesome due to the lengthy imaging times required in nuclear medicine and the relatively great excursions in distance

2-3 cm

how far does the chest move during a respiration cycle

gated-imaging techniques

what has been employed to combat patient motion

discrete matrix location

nuclear medicine uses pixels or ____ to create images

pixelation effects

what kind of error can occur with discrete matrix location?

two

it is desirable to have how many pixels per full width at half max of system resolution?

to avoid creating distracting pixelation effects and possible loss of image detail

why do you want to have at least two pixels per full width at half max of system resolution?

phantom test

utilizing phantom that mimic organs or desired camera parameters

bar phantoms

these are constructed of lead or tungsten strips which are generally encased in a plastic holder

lead or tungsten

what are bar phantoms made out of

equal to the space between them

bar phantom strips have widths that are;

bar phantoms and four quadrant bar phantom

what are some methods to evaluate spatial resoltuion

four quadrant bar phantom

these typpically have four different strip widths and spacinngs

false

true or false: the four quadrant phantom is placed NaCl crystal

opaque

To properly evaluate spatial resolution with bar phantoms, one must ensure that the thickness of lead strips is sufficient so that they are virtually ____ to the gamma rays being imaged

poor visualization may occur leading to poor contrast of the test image

what happens if the bar phantom thickness is not sufficient?

poor contrast of the test image

poor visualization due to insufficient bar thickness can lead to

1mm or less

tenth value thickness for gamma cameras (Tc99m 140 keV) that use lead are approx ____ in thickness

1cm

tenth value thickness for gamma cameras (I131 364 keV), or annihilation photons (511 keV) and higher that use lead are approx ____ in thickness

full width at half max

this represents a narrow line on the photopeak f the isotope on the emission spectrum

true

true or false: we know FWHM for each isotope

frequency

The gamma cameras are able to tune into the specific variation of half max of the intensity and the counts it records at that specific ________

image quality

The gamma cameras are able to tune into the specific variation of half max of the intensity and the counts it records at that specific frequency, this improves

point spread function or line-spread function

quantitative approach for spatial resolution can be found using the:

SPECT and/or PET imaging

PSF and LSF methods can be used for

point spread function

PSF

Line-spread function

LSF

full width half max

PSF and LSF are ties together with what other value?

modulation transfer function

MTF

modulation transfer function

the most detailed specification of spatial resolution

by imaging the analog of the frequency response curve used for evaluating audio equipment

how does MTF specify spatial resolution

modulation transfer function

what used the analog of the frequency response curve used for evaluating audio equipment

spatial resolution

what does imaging the analog of the frequency help us measure?

a sine-wave

how is activity distributed during MTF

distance

with MTF, instead of time, like with sound, we use the activity distribution and its variation with ____

spatial frequency

the activity distribution and its variation with distance is known as the:

contrast

the ratio of signal change of an object of interest such as a legion, relative to the signal level in surrounding parts of the image

the ratio of signal change of an object of interest such as a legion, relative to the signal level in surrounding parts of the image

explain contrast

background counting rates

what is the major factor that affects contrast

they superimpose more of less uniformity over the activity distribution of interest

how do background counts affect contrast

intrinsic contrast

without background counts, there is an _____ of the object

image contrast

the differences in intensity in parts of the image corresponding to different levels of radioactive uptake in the patient

the differences in intensity in parts of the image corresponding to different levels of radioactive uptake in the patient

explain image contrast

a radiopharmaceutical with a high lesion-to-background uptake

the major component of image contrast for NM

lesion to background uptake

concentration ratio

reduce

background counts can ____ image contrast

noise

background counts add to the ____ levels in the image

septal penetration, scattered radiation, and inadequate shielding of radiation sources

background counts can arise from

using the proper collimator for energy of isotope

how can septal penetration be avoided?

using pulse height analysis

how can scattered radiation be avoided?

15%

what energy window should be used to achieve the best efficiency?

larger

the ____ the window the more photons will be accepted for increase counts per second

quality is reduced

even though a larger window can accept more counts what can happen?

radioactivity above and below the object of interest

An important contributor to background radiation in conventional planar imaging is

emission computed tomography (SPECT &PET)

Image contrast is improved in _____ due to the fact that it permits imaging of an isolated slice without the super imposed activities in overlying and underlying structures

Tomographic

_____ imaging allows for significant improvements for the detection of low-contrast lesions

low-contrast

Tomographic imaging allows for significant improvements for the detection of ______ lesions

random and structured

types of imaging noise

random noise

refers to the mottled appearance of nuclear medicine images caused by random statistical variations in counting rate

structured noise

refers to nonrandom variations in counting rate that are superimposed on and interfere with perception of the object structures of interest

the radionuclide distribution

Types of structure noise can arise from

structured noise

the organ of interest is superimposed by another organ with natural bio distribution of the radiopharmaceutical is an example of

In a cardiac stress test scan, bowel uptake can block the heart from being accurately imaged

an example of structured noise

structured noise

imaging artifacts is a type of what kind of noise?

non uniformities

rings or streaks during tomography are

statistical noise or quantum mottle

random noise is also known as

everywhere

where is random noise located in nuclear medicine