image quality bushong chapter 7. image quality image quality can be described by five...

Image QualityImage Quality

BushongBushong

Chapter 7Chapter 7


Image quality can be described by Image quality can be described by five characteristicsfive characteristics– Contrast resolutionContrast resolution– Spatial resolutionSpatial resolution– Image noiseImage noise– Linearity/DistortionLinearity/Distortion– Uniformity/ArtifactsUniformity/Artifacts

What influences image What influences image quality?quality?

X-ray beam characteristicsX-ray beam characteristics DoseDose Transmissivity of the subject/Attenuation Transmissivity of the subject/Attenuation

characteristicscharacteristics Slice thicknessSlice thickness ScatterScatter Efficiency of analog-to-digital conversionEfficiency of analog-to-digital conversion Pixel sizePixel size Reconstruction algorithmReconstruction algorithm Display resolutionDisplay resolution


Specification of image quality is Specification of image quality is usually very subjective and is usually very subjective and is described by such generic terms as described by such generic terms as detail, sharpness, or blurring.detail, sharpness, or blurring.– E.g. “the images of the patient appear E.g. “the images of the patient appear

blurry because she wouldn’t hold her blurry because she wouldn’t hold her breath”breath”


Image quality cannot be represented Image quality cannot be represented by a single numberby a single number– There isn’t one all encompassing There isn’t one all encompassing

method that can accurately measure method that can accurately measure image qualityimage quality

Image quality can be represented by Image quality can be represented by several numbersseveral numbers

Image quality can be described Image quality can be described numerically by noise, linearity, and numerically by noise, linearity, and uniformityuniformity

Contrast ResolutionContrast Resolution

Contrast resolution describes the Contrast resolution describes the property of distinguishing between property of distinguishing between similar tissues, for example gray-similar tissues, for example gray-white matter in the brain, or white matter in the brain, or differentiating between the liver and differentiating between the liver and spleen.spleen.

Plain film radiography tends to lack Plain film radiography tends to lack contrast resolution (it has great contrast resolution (it has great spatial resolution however)spatial resolution however)

Plain Film vs. CTPlain Film vs. CTContrast ResolutionContrast Resolution

Examine the following imagesExamine the following images The liver is very hard to distinguish by The liver is very hard to distinguish by

looking at the radiographylooking at the radiography The CT does a much better job at The CT does a much better job at

demonstrating anatomical structures demonstrating anatomical structures associated with the liverassociated with the liver

This is because liver tissue is similar in This is because liver tissue is similar in density and CT is superior at detecting density and CT is superior at detecting objects with similar contrast valuesobjects with similar contrast values

Radiograph vs. CTRadiograph vs. CT


Determined by tissue atomic number Determined by tissue atomic number (Z), mass density, and electron density(Z), mass density, and electron density

Tissues with large difference in atomic Tissues with large difference in atomic number will have high contrastnumber will have high contrast– Lead (atomic number 82)Lead (atomic number 82)– Oxygen (atomic number 8)Oxygen (atomic number 8)

These items have high contrast (basically These items have high contrast (basically appearing white and black on a radiograph)appearing white and black on a radiograph)


Tissues with large differences in mass Tissues with large differences in mass density will also have high contrastdensity will also have high contrast– Density is a measure of mass per unit of Density is a measure of mass per unit of

volumevolume– The SI unit is kg/mThe SI unit is kg/m3 3 but most people use but most people use

g/cmg/cm33 because it is more convenient because it is more convenient Sodium = 968 kg per cubic meterSodium = 968 kg per cubic meter Lead = 11340 kg per cubic meterLead = 11340 kg per cubic meter

– There would be a large difference in contrast There would be a large difference in contrast between these two elementsbetween these two elements


During radiographic imaging, contrast During radiographic imaging, contrast resolution is improved with reduced resolution is improved with reduced scatter radiation and lower kVpscatter radiation and lower kVp– Scatter only contributes to noise and Scatter only contributes to noise and

decreases overall image qualitydecreases overall image quality– When kVp is increased the resultant When kVp is increased the resultant

image has a long scale of contrast image has a long scale of contrast (appears very grey with little (appears very grey with little differentiation between tissue types)differentiation between tissue types)


CT employs a high kVp because this CT employs a high kVp because this helps to minimize patient dose by helps to minimize patient dose by reducing the amount of radiation that reducing the amount of radiation that is absorbed the patientis absorbed the patient

CT is able to maintain contrast CT is able to maintain contrast resolution because of the narrow x-resolution because of the narrow x-ray beam collimation which provides ray beam collimation which provides excellent scatter radiation rejectionexcellent scatter radiation rejection


The larger the dynamic range, for The larger the dynamic range, for example 1024 versus 512, the less example 1024 versus 512, the less contrast in the imagecontrast in the image

At low spatial frequencies the modulation At low spatial frequencies the modulation transfer function (MTF) is a measure of transfer function (MTF) is a measure of contrast resolutioncontrast resolution

Contrast resolution is the ability to image Contrast resolution is the ability to image adjacent tissues having a similar mass adjacent tissues having a similar mass density and effective atomic numberdensity and effective atomic number


Contrast resolution is improved by Contrast resolution is improved by using higher mAusing higher mA

Contrast resolution is improved by Contrast resolution is improved by imaging thicker slices imaging thicker slices

Contrast resolution is improved when Contrast resolution is improved when imaging smaller patientsimaging smaller patients

Contrast resolution is better with low Contrast resolution is better with low noise imagersnoise imagers


Contrast resolution is improved with Contrast resolution is improved with larger field of view (FOV) and smaller larger field of view (FOV) and smaller matrix size, hence, larger pixelsmatrix size, hence, larger pixels

Contrast resolution can be improved Contrast resolution can be improved with a smoothing reconstruction filterwith a smoothing reconstruction filter

Spatial ResolutionSpatial Resolution

Spatial resolution refers to the ability Spatial resolution refers to the ability to faithfully reproduce small objects to faithfully reproduce small objects having high subject contrasthaving high subject contrast

The bone – soft tissue interface The bone – soft tissue interface represents very high subject contrastrepresents very high subject contrast

The liver – spleen interface represents The liver – spleen interface represents very low subject contrastvery low subject contrast

Spatial resolution is often described by Spatial resolution is often described by the degree of blurring in an imagethe degree of blurring in an image


A bone – soft tissue interface will be A bone – soft tissue interface will be very sharp and is described as a high very sharp and is described as a high spatial frequency interfacespatial frequency interface

High spatial frequency objects are High spatial frequency objects are more difficult to image than low more difficult to image than low spatial frequency objectsspatial frequency objects

Small, high contrast objects are more Small, high contrast objects are more difficult to image than large, low difficult to image than large, low contrast objectscontrast objects


Larger pixel size results in poorer Larger pixel size results in poorer spatial resolutionspatial resolution

Lower subject contrast results in Lower subject contrast results in poorer spatial resolutionpoorer spatial resolution

Larger detector size results in poorer Larger detector size results in poorer spatial resolutionspatial resolution

Larger prepatient and postpatient Larger prepatient and postpatient collimation results in more scatter collimation results in more scatter radiationradiation

Scatter RadiationScatter Radiation

Scatter radiation reduces contrast and Scatter radiation reduces contrast and results in less contrast resolutionresults in less contrast resolution

A smaller x-ray tube focal spot A smaller x-ray tube focal spot improves spatial resolution because of improves spatial resolution because of the sharper image projection, not the the sharper image projection, not the geometry of a shadowgeometry of a shadow

Point response function (PRF) is one Point response function (PRF) is one method of evaluating spatial method of evaluating spatial resolutionresolution


Edge response function (ERF) is one Edge response function (ERF) is one method of evaluating spatial resolutionmethod of evaluating spatial resolution

Line spread function (LSF) is one Line spread function (LSF) is one method of evaluating spatial resolutionmethod of evaluating spatial resolution

The Fourier transform (FT) is a The Fourier transform (FT) is a mathematical manipulation to convert mathematical manipulation to convert an intensity versus distance relationship an intensity versus distance relationship into an intensity versus 1/distance into an intensity versus 1/distance (spatial frequency) relationship(spatial frequency) relationship

Spatial ResolutionSpatial Resolution The FT of a PRF results in the MTFThe FT of a PRF results in the MTF The FT of an ERF results in the MTFThe FT of an ERF results in the MTF The FT of a LSF results in the MTFThe FT of a LSF results in the MTF Spatial resolution is best described by the Spatial resolution is best described by the

limiting spatial frequency (lp/cm)limiting spatial frequency (lp/cm) The MTF is obtained from the FT of the The MTF is obtained from the FT of the

PSF, LSR, or the ERFPSF, LSR, or the ERF The MTF is useful when evaluating The MTF is useful when evaluating

components of a system or comparing components of a system or comparing similar systemssimilar systems


Spatial resolution is improved with a Spatial resolution is improved with a smaller x-ray focal spot sizesmaller x-ray focal spot size

Spatial resolution is improved with Spatial resolution is improved with thinner section imagingthinner section imaging

Spatial resolution is improved with Spatial resolution is improved with narrow predetector collimationnarrow predetector collimation

Spatial resolution can be improved with Spatial resolution can be improved with the use of an edge enhancement the use of an edge enhancement (convolution) reconstruction algorithm(convolution) reconstruction algorithm


Spatial resolution is improved by Spatial resolution is improved by increasing the number of projection increasing the number of projection profiles acquired per scanprofiles acquired per scan

Spatial resolution is improved when small Spatial resolution is improved when small FOV or larger matrix size is employedFOV or larger matrix size is employed

At high spatial frequencies the MTF is a At high spatial frequencies the MTF is a measure of spatial resolutionmeasure of spatial resolution

The MTF is the principal means of The MTF is the principal means of expressing CT spatial resolutionexpressing CT spatial resolution


To understand MTF, one must first To understand MTF, one must first understand spatial frequency, which understand spatial frequency, which has units of line pairs/mm (lp/mm)has units of line pairs/mm (lp/mm)

One line and a line-sized interspace is One line and a line-sized interspace is a one line pair (lp)a one line pair (lp)

High spatial frequencies represent High spatial frequencies represent small objectssmall objects

Low spatial frequencies represent Low spatial frequencies represent large objectslarge objects


An MTF value of 1.0 represents an An MTF value of 1.0 represents an absolutely perfect imageabsolutely perfect image

As MTF value is reduced, image blur As MTF value is reduced, image blur increases and therefore image quality is increases and therefore image quality is reducedreduced

Usually, the spatial frequency at the .1 Usually, the spatial frequency at the .1 (10%) MTF is identified as the limiting (10%) MTF is identified as the limiting resolutionresolution

The total MTF of an imager is the product The total MTF of an imager is the product of component MTFsof component MTFs


Computed Tomography imagers are Computed Tomography imagers are capable of approximately 10 lp/cm (1 capable of approximately 10 lp/cm (1 lp/mm) in normal mode and up to lp/mm) in normal mode and up to approximately 20 lp/cm (2 lp/mm) in approximately 20 lp/cm (2 lp/mm) in the high resolution modethe high resolution mode

Z-axis resolution is better with spiral Z-axis resolution is better with spiral CT compared to conventional CTCT compared to conventional CT

Image NoiseImage Noise

Contrast resolution is limited by image Contrast resolution is limited by image noisenoise

Scatter radiation results in image noiseScatter radiation results in image noise An increase in slice thickness results in An increase in slice thickness results in

less noiseless noise An increase in slice thickness usually An increase in slice thickness usually

results in lower patient doseresults in lower patient dose An increase in pixel size results in less An increase in pixel size results in less

noisenoise


An increase in patient dose results in less An increase in patient dose results in less noisenoise

In statistics, noise is called standard In statistics, noise is called standard deviation deviation

High noise images appear blotchy, grainy, or High noise images appear blotchy, grainy, or spottyspotty

Low noise images appear very smoothLow noise images appear very smooth Noise in a CT image comes from the scanner Noise in a CT image comes from the scanner

electronics and the random nature of x-ray electronics and the random nature of x-ray interaction with a detectorinteraction with a detector


Anything that reduces CT noise will Anything that reduces CT noise will improve contrast resolutionimprove contrast resolution

Increased image noise at low mAs Increased image noise at low mAs can usually be accommodated by can usually be accommodated by using a low-pass convolution filterusing a low-pass convolution filter

Image noise can be reduced by using Image noise can be reduced by using 360 degree interpolation360 degree interpolation

LinearityLinearity

When water = 0, bone = 1000, and air = When water = 0, bone = 1000, and air = -1000, a CT imager exhibits perfect linearity-1000, a CT imager exhibits perfect linearity

The CT number for a given tissue is The CT number for a given tissue is determined by the x-ray linear attenuation determined by the x-ray linear attenuation coefficient (LAC)coefficient (LAC)

A plot of CT number versus LAC should be a A plot of CT number versus LAC should be a straight line passing through water straight line passing through water = 0= 0

LinearityLinearity

Good linearity is essential for quantitative Good linearity is essential for quantitative computed tomography (QCT)computed tomography (QCT)

Linearity is the ability of the CT image to Linearity is the ability of the CT image to assign the correct Hounsfield unit (HU) to assign the correct Hounsfield unit (HU) to a given tissuea given tissue

Linearity is monitored by imaging the 5-Linearity is monitored by imaging the 5-pin insert of the American Association of pin insert of the American Association of Physicists in Medicine (AAPM) and plotting Physicists in Medicine (AAPM) and plotting HU versus linear attenuation coefficientHU versus linear attenuation coefficient

UniformityUniformity

When a test object made of one When a test object made of one substance is imaged, the value of each substance is imaged, the value of each pixel should be the samepixel should be the same

Cupping is the reduction in CT number Cupping is the reduction in CT number toward the middle of a uniform test objecttoward the middle of a uniform test object

Peaking is the increase in CT number Peaking is the increase in CT number toward the middle of a uniform test objecttoward the middle of a uniform test object

Cupping and peaking are signs of poor Cupping and peaking are signs of poor image uniformityimage uniformity

UniformityUniformity

Image uniformity is essential for QCTImage uniformity is essential for QCT Uniformity is the ability of the CT Uniformity is the ability of the CT

imager to assign the same HU to a imager to assign the same HU to a uniform phantom (water) over the uniform phantom (water) over the entire FOVentire FOV

Contrast DetailContrast Detail

Contrast detail plots are instructive when Contrast detail plots are instructive when evaluating contrast resolution and spatial evaluating contrast resolution and spatial resolutionresolution

Contrast detail plots are useful when Contrast detail plots are useful when comparing different scanning protocolscomparing different scanning protocols

The low contrast region of the contrast The low contrast region of the contrast detail curve is noise limiteddetail curve is noise limited

The high contrast region of the contrast The high contrast region of the contrast detail curve is determined by the MTR of the detail curve is determined by the MTR of the imager componentsimager components

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