Chapter 15- Nuclear Medicine Images

What is Image Quality?

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

As technologists we aim to have the highest image quality, so we can give the radiologists the most accurate picture possible.

True

What are we focusing on this chapter?

focusing primarily on planar imaging with the gamma camera

The same applications we are talking about for the planar images are also applicable to the Tomographic (SPECT) images too

True

How many methods for characterizing or evaluating image quality?

Two

What is the first method for characterizing or evaluating image quality?

quantitatively measured or calculated for the image or imaging system

What are the three characteristics for the first method for characterizing and evaluating image quality?

1. Spatial Resolution

2. Contrast

3. Noise

What is spatial resolution?

correlates to spatial resolution; detail or sharpness

What is contrast?

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

What is Noise?

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

What is the second method for evaluating image quality?

is through the use of human observer performance studies

How of the use of human observer performance studies?

is done by using images obtained with different imaging systems or under different imaging conditions.

What objectively do human observer performance studies depend on?

experience and human visuals

Is human observer performance related to the physical measures of image quality?

True

What does the computer observer performance uses to predict human observer studies and what it can do?

Uses mathematical model and speed up the time to quantify image quality

What spatial resolution refers?

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

What are the factors affecting spatial resolution?

1. Collimator Resolution

2. Intrinsic Resolution of the imaging detector

3. Source to detector distance

What is collimator resolution?

potentially the principal limiting factor when absorptive collimators are used for spatial localization.

collimator hole diameters must be relatively large to obtain reasonable collimator efficiency

True

Another factor affecting collimator resolution also depends on the source to detector distance

True

The second factor affecting spatial resolution is the intrinsic resolution of the imaging detector

True

Why limitation arises when using the gamma camera relating to intrinsic resolution?

because of the statistical variations in the distribution of light photons among the photomultiplier tubes.

Intrinsic resolution is a function of gamma-ray energy with the gamma camera, becoming poorer with decreasing gamma ray energy.

True

What determines the intrinsic resolution of the device?

the size of the individual detector elements

Can patient motion affect the image sharpness?

Yes

Can respiratory and cardiac motion can be difficult?

Yes, because its lengthy imaging times and relatively great excursions in distance (like the chest rising and roping 2-3 cm per respiration cycle)

What will help combat patient motion to help minimize motion blurring, especially in cardiac studies?

Gated-Imaging techniques

In Nuclear Medicine, what creates images?

pixels or discrete matrix location. This can cause pixelation effects.

Why is it desirable to have at least two pixels per full width at half maximum (FWHM) of system resolution?

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

How the phantom test evaluates spatial resolution?

utilizing phantoms that mimic organs or desired camera parameters can be used.

How the bar phantoms constructed?

lead or tungsten strips, which are generally encased in a plastic holder.

Does the bar phantoms have strips have widths equal to the spaces between them?

True

5 mm bar phantom separated edge to edge by 5 mm spaces.

True

How many quadrants does the bar phantom?

four quadrants

Does each quadrant of the bar phantom have four different strip widths and spacings?

True

The phantoms are placed directly on the collimated detector.

True

Spatial resolution can be defined up to the smallest bar pattern visible on the image.

True

The smaller the thin lines of the bar phantoms that the gamma camera will see in that sharpness quality

True

How to properly evaluate spatial resolution with bar phantoms?

one must ensure that the thickness of lead strips is sufficient so that they are virtually opaque to the gamma rays being imaged

What happens if the thickness of the bar phantom is not sufficient?

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

Tenth value thickness for gamma cameras (Tc99m 140 Kev) that use lead are approximately?

1 mm or less

Tenth value thickness for gamma cameras that are using I-131 (364 Kev), or annihilation photos (511 Kev) & higher are closer to

1 cm in thickness

What does full width at half max represent?

a narrow line on the Photopeak of the isotope on the emission spectrum.

We know that characteristic values for each isotope.

True

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.

True

Does the tuning of the half max improves the image quality greatly?

True

What are the Point Spread Function (PSF) or Line Spread Function (LSF) methods used for?

Used for SPECT and/or PET imaging cameras

Is Point Spread Function (PSF) or Line Spread Function (LSF) are tied together with the full width at half max (FWHM) value of the desired radionuclide

True

What does MTF stand for?

Modulation Transfer Function

The MTF is the most detailed specification of spatial resolution.

True

How the MTF done?

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

What does MTF create?

a sine-wave distribution of activity.

What is spatial frequency?

Instead of varying time (if it was sound) we use the activity distribution and its variation with distance.

What is contrast?

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

What is the major factor affecting contrast?

is the added background counting rates that are superimposes more of less uniformly over the activity distribution of interest.

In the absence of background counts there is an intrinsic contract of the object

True

What is image contrast?

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