msc medical physics medical imaging exam 2

ANDREW JOHNSTONE B3459426 S809 TMA02

1 (a)

1st Generation CT is a translation / rotation setup where a pencil beam x-ray and its corresponding detector (single source / single detector with the detector travelling parallel to the source on translation) is translated across the subject then rotated and another slice scan is taken. This continues till the image is built up.

This method is very slow at approx. 5 mins per slice and is best suited for head imagery .

2nd Generation CT is also a translation / rotation setup only this time instead of using a single source single detector; the beam is fanned out so that it becomes single source / multiple detector. As a result fewer rotations are needed e.g. 3 degrees of rotation per slice as opposed to 1 degree for 1st Gen. Because of the shorter scan times it is the first generation to allow the body trunk to be scanned.

3rd Generation CT is a rotate / rotate setup. The source is a fan beam and the detectors are arranged in an arc opposite. Both the source and detectors rotate around the patient. With the advent of slip ring technology both of the source and detector can freely rotate 360 degrees indefinitely.

4th Generation CT is a rotate / stationary setup. The fan beam source rotates around the patient but within a 360 degree ring of stationary detectors surrounding them. This setup was formed to try and alleviate ring artefacts which would occur with 3rd gen.

This setup requires many more detectors.

(b) Blurring ( point spread function ) occurs when back projection without filtering occurs, this is due to the convolution of many images each with their own degree of blurring. To counter this, filtered back projection uses fourier techniques to analyse the resulting images in the frequency domain particulary the transfer function of special frequency known as the modular transfer function. A spacial frequency co-ordinate (q) can be derived i.e. the source point sepated from noise. Noise can then be filtered from each image resulting in a clearer end image.

ANDREW PAUL JOHNSTONE B3459426S809 IMAGING IN MEDICINE

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(c)

The CT number remains constant and isn’t affected by change in keV for this range of frequencies (monochromatic X – Ray)

Photon energy is indirectly proportional to wavelength. It cannot be said that the attenuation coefficient is directly proportional to wavelength across the entire spectrum. Using Table 1( Values coefficient for different tissues at different energies) of Question 1 of TMA 1 concludes this. Therefore is can be said that each tissue type will have a CT number range for a polychromatic X – Ray. QED

These ranges of CT numbers overlap for various tissues thus making it difficult to unambiguously detect a target tissue type.

(d)

The Hounsfield Unit is named after Sir Godfrey Newbold Hounsfield (1919 – 2004), Inventor and pioneer of the first CT scanner whilst working at EMI. He won the Nobel prize for physiology or medicine along with Alan Cormack in 1979. [1][2]


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2.

(a)(i) Investigations where a contrasting agents’ dynamics are tracked when moving through the body are best suited to fluoroscopic systems as conventional film radiography can only take pictures of static events.

(ii) Angiography is a technique that can be used to image (but not restricted to) arterial blood flow in the body. Iodine is generally the contrasting media.

A gastrointestinal series uses barium as the contract agent to image the intestinal tract.

Both imaging techniques make use of Digital Subtraction, whereas an xray image is taken before the contrasting media is introduced then as the media is introduced. Subtraction of the initial image leaves only the contrasting agents flow open to analysis.

(b)

Figure 1 Image Intensifier

Figure 1 shows a simplified Image intensifier with the various parts explained below:

A. Input window made of thin metal (approx. 1mm thick) possibly aluminium or titanium for strength and absorption properties, the input window is convex also for structural strength but also to minimise the distance from the patient.

B. Input Phosphor. Made of cesium iodide doped with sodium (CsI:Na). The luminescent phosphor is a scintillator turning x-ray photons into optical photons .


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C. Photocathode. A cesium – antimony compound (CsSb3) which converts the optical photons from the input phosphor and converts them to photoelectrons for acceleration to the output phosphor.

D. Focusing Electrodes. Used to focus the photoelectrons onto the output phosphor.

E. The housing is made of Mu – metal, a nickel iron alloy permeable to magnetic fields to reduce magnetic interference. Basically a magnetic shield.

F. Output phosphor Which has a thin aluminium covering on the inside face to prevent noise caused by stray light photons. The output phosphor is very thin typically 4 – 8 micrometres and is made of silver activated zinc cadmium sulphide (ZnCdS:Ag). The output phosphor converts the accelerated electrons into optical photons with gain. These optical photons are transferred to recording medium via fibre optics.

G. Is the accelerating anode typically at a potential of relative to the input photocathode of 30keV

(c) An absorption edge relates to the probability of photoelectric absorption of a photon in each energy shell of an atom. Photoelectric absorption occurs at an energy level either equal or very slightly over the ionisation energy level. The greater the energy of the incoming photon the less likely the probability of photoelectric absorption will take place and the absorption decreases. However when the incoming electron reaches the ionisation energy level of another shell in the atom the absorption dramatically increases again.

These increases are names in accordance with the shells energy levels. L-edge , K-edge etc.

An x ray taken at energy levels slightly below and slightly above a contrasting agents absorption edge will show the greatest amount of contrast between the two images.


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(d) The equation used for finding output intensity can be re-arranged to find the attenuation coefficient if the output intensity and thickness of the medium is known.

The absorption coefficients taken just below the absorption edge (333m-1) and just above (1489m-1) supports the answer given at (c). The huge difference in absorption will lead to a large amount of contrast between images allowing the radiographer to distinguish between differentiate between mediums.


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3.

I Reference ResultRadiologist result Positive Negative

Positive 98 66Negative 2 34

II Reference ResultRadiologist result Positive Negative


III Reference ResultRadiologist result Positive Negative


IV Reference ResultRadiologist result Positive Negative


Table 1. Radiologist Vs Reference Results(b)

I Reference ResultRadiologist result Positive Negative

Positive TP FPNegative FN TN

Table 2. True/False


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Threshold Sensitivity % Specificity %I 98 34II 96 44III 74 76IV 36 98

Table 3. Sensitivity Vs Specificity(c)

Threshold Sensitivity % Specificity % 100-SpecificityI 98 34 66II 96 44 56III 74 76 24IV 36 98 2

Table 4. 100%- Specificity

Figure 2. ROC Graph of Tresholds


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(d)

Figure 3. ROC Graph Showing Guess (chance) Line and Ideal Result Line

5.(a) 77pixels


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(b)

(c) Images stored in the DICOM format are of a lossless format and do not succumb to the degradation of quality as suffered by compressed images. The edges of the samples would not be burred and the image would have a greater spacial resolution. DICOM images are true to the original.

(d)

Sample Diameter (mm)1 11.822 11.823 11.364 11.825 10.916 11.827 11.368 11.369 10.91

Table 5. Sample Diameters


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(e)


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Sample C number1 0.285 0.239 0.13

13 0.03

Table 6. C – numbers of samples

(f) A user could make use of the 18 discs for contrast calibration in order for the user to distinguish between materials of a different C number. Especially useful in fluoroscopy using a contrast medium.


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References.

1. http://www.britannica.com/EBchecked/topic/272989/Sir-Godfrey-Newbold- Hounsfield (accessed 27/02/2011)

2. http://nobelprize.org/nobel_prizes/medicine/laureates/1979/hounsfield- autobio.html# (accessed 27/02/2011)


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http://nobelprize.org/nobel_prizes/medicine/laureates/1979/hounsfield-autobio.html#

http://nobelprize.org/nobel_prizes/medicine/laureates/1979/hounsfield-autobio.html#

http://www.britannica.com/EBchecked/topic/272989/Sir-Godfrey-Newbold-Hounsfield

http://www.britannica.com/EBchecked/topic/272989/Sir-Godfrey-Newbold-Hounsfield

msc medical physics medical imaging exam 2

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