Fadahunsi Ct Seminar

8/6/2019 Fadahunsi Ct Seminar

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.

(a) briefly describe the difference

between a 4th

generation and a Helical CTscanner. What are the advantages of

Helical scanner over conventional

scanner.(b) Concerning CT, explain the following

including factors affecting them.

(i) CT contrast(ii) spatial contrast

Why is molybdenum usually used as

DF1


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Filter. Illustrate your answer with an appropriate x-ray spectra diagram.

Describe the properties of screen/film combinationused in mammography.

ANSWER

In the 4th generation CT scan the x-ray tube has to

be provided with a high voltage supply and thedetectors have to pass their signals to thecomputer, the gantry is also wired to an externalequipment, because of these, at the completion ofevery slice after accelerating the x-ray tube to a

rotational scanning speed, the system mustdecelerate to a gradual stop and then repeat theprocess by rotating in the opposite direction inorder to unwind the high voltage, control, powerand signal cables take up assembly.A


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In the helical scanner, a slip ring technology has

been incorporated which allows gantry control

and data signal to pass from and into acontinuously rotating gantry. The high voltage

supply used by the x-ray generator uses a

compact high frequency transformer that ismounted within the rotating gantry. All power

required by the gantry is passed through the slip

rings thereby allowing it to rotate continuously

without the need to stop after every slices.


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ADVANTAGES

The basic advantage of continuous rotation is to permitfaster rotation times.

The continuously rotating gantry allows continuousacquisition of data, reducing the inter scan delays. It is

possible to do dynamic scans at up to 12 scans per minute.A patient volume is scanned continuously without gaps and

with fewer problems with patient motion particularlypatient breathing and this is helpful in reformatted sagittaland coronal images.

Any slice can be selected within the volume to display insteadof the fixed slice position of the conventional 4th generationscan.

There is reduced use of contrast medium.


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CT contrast or contrast resolution or densityresolution is the ability of an imaging system to

display an image of a relatively large object thatis only slightly different in density from itssurroundings. For the image to be visible, theobject must produce enough change in the no of

transmitted photons to overcome statisticalfluctuation in transmitted photons caused bynoise i.e. in the computation of any one pixelvalue there is error in the form of statisticalvariation and it is this variation which limits theultimate contrast resolution and this variation isreferred to as image noise which manifest itselfas a grainy background or mottle.


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Since noise is the ultimate limitation in the accuracy ofcontrast resolution, factors that affect or induce noise affectcontrast and these factors are

(1) photon flux--- in any ray measurement the statisticalvariation associated with that measurement is directlyproportional to the number of photons detected, the relativeimage noise is affected inversely as the square root of thedetected photon flux. large photon fluxes therefore reduces

relative noise although increases patient dose. The photon intensity is affected by the efficiency of the

detector, photon flux, size of patient and the presence of highattenuation material and in turn the uncertainty in themeasurement. A large patient thus will have a more mottled

image than a small one for the same tube output. Presence ofhigh contrast objects such as bone will cause an increase innoise by reducing the transmitted photon flux.


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X-ray scatter---- Because any particular detector cannot discriminatebetween primary photon directly from the source and a scattered photon

arising from an area not in line of the ray projection, this alters the recordedvalues in adjacent detectors and produces a decrease in difference betweenadjacent measurements with a concomitant decrease in contrast resolution.

Voxel length---- if the slice thickness is reduced by 50% the detectorcollimator size must be reduced accordingly which in turn reduces theintensity by the same amount. To retain the same accuracy requires

restoration of the detected photon intensity which means doubling thedose.

Similarly decreasing pixel size increases the relative inaccuracy unless thephoton flux is raised. If the pixel side length is reduced by half the voxelvolume will be reduced by a factor of four, the SD of the CT value woulddouble and the image will become more mottled in appearance and the

contrast will deteriorate. Filters ------- These are used to remove image blurring created by back

projection process by accentuating high frequency component found in thedata and image noise increases with this thereby decreasing contrast.


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Filters---- the various filters control the amount

of image blurring created by accentuating high

frequency component found in the data. For a

crisp image the high spatial frequencies are

accentuated, and this has the effect of

sharpening the edges and improving spatialresolution.

Opening size of detector aperture----- This is the

effective size of each detector in the imageplane. A small detector aperture produces a

narrow ray, less blur and better image detail.

When a portion of the detector is covered,

however, the geometric efficiency is reduced.


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An increase in radiation exposure to the patient

is then required to produce the same image

quality with respect to noise.

Ray sample interval----- If the spacing between

rays significantly exceeds the dimension of small

objects or anatomical detail, the detail will notappear on the image. The rays must be

sufficiently close during the scanning procedure

to measure any anatomical detail that is toappear in the image.

Voxel and pixel size-------since a specific pixel can

have only one CT number value, there can be no

detail within a ixel.


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In other words all detail within the tissue voxel

represented by a pixel is blurred together andassigned a single value. Anatomical detail within

the voxel cannot be imaged therefore small

voxels are needed when image detail is required.

Three factors determine voxel size: field of view,

matrix size and slice thickness.

Since mammography is a technique for the

radiographic examination of the breast whichconsist of connective tissue, glandular tissue,

skin and fat, and all must be visualized but all

have very similar attenuation coefficient.


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In order to maximize difference in attenuation

coefficient of these various structures a low

energy x-rays spectrum needs to be used. For

this imaging situation, the use of a broad

spectrum bremsstrahlung spectrum from

tungsten is far from ideal and it is now standardpractice to make use of the characteristic

radiation fro a lower atomic number material.

Molybdenum has an intermediate atomicnumber of 42, has a k edge at 20keV and

produces two rather intense characteristic x-ray

energies: k-alpha radiation, at 17.9keV, and k-

beta, at 19.5keV.


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The x-ray tube potential is about 30kVp.

When the tube is operated at 30 to 40 kVp, a

molybdenum tube will also produce

considerable bremsstrahlung with high energies

greater than 20keV. This higher energy radiation

will reduce contrast in breast soft tissuestructures. To reduce the amount of higher

energy radiation in the molybdenum tube

spectrum, a molybdenum filter of 0.030mmthickness is commonly used.

This filter will attenuate x-rays just above the

20keV K edge of Mo very strongly, but will

transmit 57% of the 17.5K-alpha and 67%of the


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19.6keV K- beta radiation of molybdenum.

The screens used in mammography are rare

earth screens in which the principal interaction is

the photoelectric effect with the L shell electrons

(the K edge for rare earth materials is in the

region of 50keV). Because of the low x-rayenergy, the screen are thinner than for routine

radiography, thus reducing unsharpness.

A single screen is used with a single sidedemulsion. The screen (and emulsion) is on the

distal side of the film, furthest from the x-ray

tube and patient. This arrangement is used

because the highest proportion of the photon


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interactions and thus light production, will be

on the side of the screen closest to the film and

this serves to minimize further the screen

unsharpness.

They are able to produce a limiting resolution

of 151p/mm or better.

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Fadahunsi Ct Seminar