biomedical Imaging Texted and notes

5/19/2018 biomedical Imaging Texted and notes

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MEDICAL IMAGING

BME 229F2014

(with slides from Drs. H. Keller, A. Clerk, P. Sprawls)


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Imaging Chain

Properties of medical images

Visibility

PSF and MTF

Computer Thomography

Outline


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Generic Imaging Model


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Imaging chain


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blurring and noise sources of error causes


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Converting Tissue Characteristics into a Visual Image


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Contrast sensitivity

Increasing Contrast Sensitivity Increases Image

Contrast and the Visibility of Objects in the Body

contrast-->---


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Effect of Contrast Sensitivity on Object Visibility

too much constant--- you loose details


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Effect of Blur on Visibility of Image Detail


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Effect of Noise on Object Visibility


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Viewing Condition Factors That Affect Object Visibility


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Effect of Viewing Conditions on Object Visibility


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Sensitivity and Specificity

--for damages

we have a damage normal tissue

not

100%

sure


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True and False Decisions

Relationship of True and False Diagnostic Decisionsto Sensitivity and Specificity


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Sensitivity and Specificity


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ROC curve I

Comparison of ROC Curves for an Ideal Diagnostic Procedurewith One that Produces No Useful Information

------>means method

does not work

100%speci and sensi


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ROC curve II

An ROC Curve for a Specific Imaging Procedure. The Actual

Operating Point is Determined by Characteristics of the Observer

-75 % and higher just acceptable-less than that not acceptable as

less than 50% not at all accpetable-

with computer tomography

accuracy ha improved


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Setting a threshold

is low when

specificity is much higher

?

is high whensensitivity much higher

than specif?

the tresh hold

---faulse positive specifity


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X-ray Imaging Chain


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Each step adds sources of

imperfections that will

subsequently be propagated

through the chain. The main

sources are:

! Blur

!

Noise

X-ray imaging chain


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Imaging Process


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Parameters that describe image quality:-Object and image contrast

-Spatial resolution: determined by blur

-Contrast resolution: determined by noise

-Field-of-View, Voxel Size: determined byrecording process (detector) and reconstruction.

-Temporal resolution (for dynamic or 4D

imaging)

Descriptors of Medical Images


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Contrast

In x-ray images, contrast refers to the

difference in visible gray scales (imagecontrast) seen as a result of differences in

attenuation (object contrast).

Image contrast is determined by: density,

material, thickness (projection imaging only),

x-ray tube settings (voltage, mAs), scatter.


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Object contrast and image contrast

ligh

worse contrast

Bl d i


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Every imaging system produces a blurred(unsharp) image.

Sources of blur:

-Geometry: finite source size, distance

between source and object, distance between

object and detector

-Detector blur-Object/patient motion

Blurred images


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(benz)

Bl i


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Blur size


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Bl Sh


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Blur Shape


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Bl Di t ib ti P tt


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Blur Distribution Patterns

Bl d i


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Blurred images

Bl d i d


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Blurred images: edges

Blurred images: Point Spread Function


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Blurred images: Point Spread Function

Vi ibilit f d t il


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Visibility of details


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Effect of Blur on Resolution


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Effect of Blur on Resolution


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cycle per mmsometimes


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MODULATION TRANSFER FUNCTION (MTF)


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MODULATION TRANSFER FUNCTION (MTF)

The Relationship

of an Image

Spatial Frequency

Spectrum to theObject Spectrum

and the MTF of the

Imaging System


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1:2:

3:

Blur defines spatial resolution


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Spatial resolution is the ability to sharply andclearly define the extent or shape of features

within an image.

Spatial resolution defines how close two

features (usually high contrast objects) can be

and still be resolved.

Blur defines spatial resolution

Spatial Resolution


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Spatial Resolution

how well canwe distinguish

these two

objects?

Spatial Resolution


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Spatial Resolution


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Comparison of Blur and Resolution Values for


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p

Different Imaging Methods

Image Noise


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Image Noise

Quantum Noise


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Quantum Noise

Random Image Noise


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Random Image Noise

Low-Contrast Resolution


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Low Contrast Resolution

Contrast Resolution


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Contrast Resolution

High- and low-contrast resolution


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High and low contrast resolutiontil here

High- and low-contrast resolution


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High and low contrast resolution

Field of view (FOV)


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Field of view (FOV)

Voxel size


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Voxel size

Voxel size


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Voxel size

Voxel size


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Voxel size

Voxel size


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o e s e


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Digital Image Processing


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g g g

Display: Level and Window


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p y

Temporal resolution


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Images take time to acquire

~1 min for CT

~ 3 min for MR

~ 8 min for PET

Motion can occur as an artifact withdifferent characteristics in each

modality

p

Computed Tomography


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p g p y

Beers Law


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Tomography image reconstruction


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g p y g

The concept of back projection


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p p j

Image reconstruction in 2 views


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g

First Generation CT


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First Generation CT


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Contemporary CT


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y

Contemporary CT


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Conventional X-ray versus CT


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Conventional X ray versus CT

Conventional X-ray CT

3D CT


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3D CT

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