X-ray Attenuation Mechanisms
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Questions and Answers

What happens to the energy of X-rays as they pass through matter?

They lose some or all of their energy to the atoms or molecules in their way.

What is the term used to describe the decrease in the energy or intensity of the X-ray beam as it passes through matter?

Attenuation

Why can tissues be contrasted by measuring the intensity of the attenuated X-ray beam?

Different materials attenuate X-rays to a different extent.

What happens to the X-rays that collide with bone in an X-ray beam?

<p>They are more likely to be absorbed.</p> Signup and view all the answers

Why do digital detectors show a clearer image of the patient's skeleton than photographic film?

<p>The images are easier to process, store, and transfer.</p> Signup and view all the answers

How does the intensity of a collimated beam of X-rays change as it passes through bone?

<p>It decreases exponentially.</p> Signup and view all the answers

What happens to the intensity of an X-ray beam if it passes through 2cm of bone, given that it halves in 1cm?

<p>It will quarter.</p> Signup and view all the answers

Why is it important to understand the attenuation of X-rays in medical imaging?

<p>It allows for the creation of contrasted images of the body, such as bones and soft tissues.</p> Signup and view all the answers

What is the purpose of the dynodes in a photomultiplier?

<p>To generate a cascade of electrons, amplifying the signal.</p> Signup and view all the answers

How does a PET scanner generate a 3D image?

<p>By placing a ring of gamma cameras around the patient and detecting the emission sites of the gamma photons.</p> Signup and view all the answers

What is the result of the electron-positron annihilation in a PET scan?

<p>A pair of gamma photons are produced, emitted in opposite directions.</p> Signup and view all the answers

How is the exact location of the annihilation event calculated in a PET scan?

<p>By recording the arrival times of the gamma photons and using the known speed of the photons.</p> Signup and view all the answers

What is the purpose of the gamma camera in a PET scanner?

<p>To detect the emission site of the gamma photons.</p> Signup and view all the answers

Why is a PET scanner able to produce a higher resolution image than a gamma camera?

<p>Because it is a ring of gamma cameras, allowing for the detection of gamma photons from multiple angles.</p> Signup and view all the answers

What is the relationship between the annihilation event and the beta emission in a PET scan?

<p>The annihilation event occurs soon after the beta emission.</p> Signup and view all the answers

What is the final output of a PET scan?

<p>A 3D model of the tracer locations.</p> Signup and view all the answers

What is the main disadvantage of CAT scans compared to other medical imaging techniques?

<p>They take a significantly longer time and expose the patient to a far greater dose of ionising radiation.</p> Signup and view all the answers

What is the purpose of the ring of electronic detectors in a CAT scanner?

<p>To detect the X-ray beam intensity.</p> Signup and view all the answers

How is a full 3D image of the patient's body created in a CAT scan?

<p>The X-ray tube and detectors rotate about the patient and move up and down their length, and images of each slice are stitched together.</p> Signup and view all the answers

What is the purpose of medical tracers in medicine?

<p>To collect in particular locations in the body and aid in diagnosis and therapy.</p> Signup and view all the answers

Why are gamma-emitters most useful in non-invasive diagnosis?

<p>Because they are least ionising and most penetrative.</p> Signup and view all the answers

What is the drawback of using beta and alpha emitters in medical imaging?

<p>They are more ionising and would cause significant damage.</p> Signup and view all the answers

What is the process of converting X-ray beam intensity into electrical signals in a CAT scanner?

<p>The information is processed to reconstruct the tissues that the beam has passed through.</p> Signup and view all the answers

Where are medical tracers placed in the body for non-invasive diagnosis?

<p>Inside the patient's body.</p> Signup and view all the answers

What is the purpose of the tracer fluorodeoxyglucose in PET scanners?

<p>To locate areas in the body with high rates of respiration, such as cancerous tumors or active parts of the brain.</p> Signup and view all the answers

What is a major advantage of ultrasound as a diagnostic technique?

<p>It is non-ionising and non-invasive.</p> Signup and view all the answers

What is the minimum frequency of sound waves used in medical ultrasound?

<p>20 kHz</p> Signup and view all the answers

How are the properties of a medium determined in ultrasound?

<p>From measurements of the waves after they have been in contact with the medium.</p> Signup and view all the answers

What is the function of a transducer in an ultrasound device?

<p>To produce an electrical signal from the sound waves.</p> Signup and view all the answers

Why is PET scanning considered expensive?

<p>It requires tracers to be synthesised on-site.</p> Signup and view all the answers

What is the advantage of PET scanning in terms of observing the effects of medications?

<p>It can accurately demonstrate organ function and observe the effects of various medications.</p> Signup and view all the answers

What is the typical frequency used in medical ultrasound diagnosis?

<p>5 MHz</p> Signup and view all the answers

What happens when the impedance of the two media is very different?

<p>Most of the wave energy is reflected.</p> Signup and view all the answers

What is the purpose of the impedance matching gel in ultrasound transducers?

<p>To maximize the transmission of the ultrasound into the patient and minimize reflection.</p> Signup and view all the answers

What is the Doppler Effect?

<p>The change in frequency of a wave when it is reflected or produced by a moving source.</p> Signup and view all the answers

How does Doppler imaging measure the speed of blood flow?

<p>By using ultrasound waves to detect the reflected waves from moving blood cells.</p> Signup and view all the answers

Why is there a large amount of reflection when an ultrasound transducer is operated in air next to the patient's skin?

<p>Due to the differences in the impedances of the two media.</p> Signup and view all the answers

What is the role of iron in Doppler imaging?

<p>It reflects the ultrasound wave back to the transducer.</p> Signup and view all the answers

What is the benefit of using Doppler imaging in medical imaging?

<p>It provides a non-invasive technique to measure the speed of blood flow.</p> Signup and view all the answers

What happens when the impedance of the two media is similar?

<p>Most of the wave energy is transmitted.</p> Signup and view all the answers

Study Notes

X-ray Attenuation Mechanisms

  • X-rays lose energy as they ionize matter, resulting in a gradual decrease in intensity (power per unit cross-sectional area) known as attenuation.
  • Different materials attenuate X-rays to varying extents, allowing for contrast between tissues by measuring the intensity of the attenuated beam.
  • Bone attenuates X-rays more than flesh or soft tissues, resulting in a white outline of the patient's skeleton on a photographic film.

X-ray Intensity Decrease

  • The intensity of a collimated X-ray beam decreases exponentially with distance.
  • If the beam intensity halves in 1cm of bone, it will quarter in 2cm and be an eighth of its original value in 3cm.

Ultrasound and Impedance

  • When Z1 is similar to Z2, the fraction of intensity reflected is small, and most of the wave energy is transmitted.
  • When Z1 is very different from Z2, the fraction of intensity transmitted is small, and most of the wave energy is reflected.
  • Impedance matching gel is used to maximize transmission of ultrasound into the patient and minimize reflection.

Doppler Effect

  • The Doppler Effect is the change in frequency of a wave when it is reflected or produced by a moving source.
  • Doppler imaging is used to measure the speed of blood flow non-invasively.

Ultrasound

  • Ultrasound is a longitudinal sound wave with a frequency greater than human hearing range (>20 kHz).
  • Ultrasound waves can be refracted, reflected, Doppler shifted, and diffracted, allowing for the determination of medium properties.
  • Ultrasound is a non-ionizing, non-invasive, quick, and affordable technique useful for finding the boundary between two media.

Positron Emission Tomography (PET)

  • PET scanners produce accurate 3D images by detecting gamma photons emitted in opposite directions from positron-emitters.
  • The exact location of the annihilation event can be calculated using the arrival times of the photons and the speed of the photons.
  • Repeated detections produce a 3D model of the tracer locations.

CAT Scanners

  • CAT scanners contain an X-ray tube that generates a fan-shaped beam directed onto the patient.
  • A ring of electronic detectors detects the X-ray beam intensity, which is converted into electrical signals and processed to reconstruct the tissues.
  • The X-ray tube and detectors can rotate and move to create a full 3D image of the patient's body.

Medical Tracers

  • Radioactive isotopes are combined with specific elements to form compounds that collect in particular locations in the body, known as medical tracers.
  • These tracers are used in both diagnosis and therapy, and are most useful for non-invasive diagnosis when gamma-emitters are used due to their low ionizing and high penetrative properties.

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Description

Learn about how X-rays interact with matter, causing energy loss and attenuation. Understand how different materials affect X-ray beams and how this is used for tissue contrast. Test your knowledge on X-ray attenuation mechanisms!

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