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Medical Physics

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David Young

on 17 September 2018

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Transcript of Medical Physics

And you have completed year 1 physics!
Medical Physics

The dice game for the random nature of decay
Radiopharmiceutical Manufacturing
Gamma camera
Pet Scan
Magnatic Resonance Imaging
Kidney Transplant
Radiation Therapy
Xrays and how they work
Advanced Xray
Record the total number of Dice
record every roll
After each roll record how many dice have landed black side up .... and remove them
Purpose: Each die is an atom, each die has a black side, if the die lands black side up it is 'radioactive'
keep going until you have no dice left
How to construct your graph
A Radiopharmaceutical is a Xenobiotic that can be used either for diagnostic or therapeutic purposes.
It is composed of a radioisotope bonded to an organic molecule.
The organic molecule takes the radioisotope to specific organs, tissues or cells. The radioisotope is selected for its properties.
Radioisotopes emitting penetrating gamma rays are used for diagnostic (imaging) where the radiation has to escape the body before being detected by a specific device (SPECT/PET cameras).
Typically, the radiation emitted by isotope used for imaging vanishes completely after 1 day through radioactive decay and normal body excretion. The most common isotopes for imaging are: 99mTc, I-123, I-131, Tl201, In111 and F18.
More Advanced Understanding!
Radioisotopes emitting alpha or beta particals are used for therapy due to their power to lose all their energy over a very short distance, therefore causing a lot of local damage (such as cell destruction).
This is used for therapeutic purposes: cancer cell destruction, pain treatment in palliative care for bone cancer or arthritis. Such isotopes stay longer in the body than imaging ones; this is intentional in order to increase treatment efficiency, but this remains limited to several days. The most common therapeutic isotopes are: I131, Y90, Rh188 and Lu177.
Technetium -99m is produced by bombarding molybdenum 98Mo with neutrons. This results in 99Mo which decays with a half-life of 66 hours to the metastable state of Tc.
Since 99Mo is a fission product of 235U, it can be separated from the other fission products and used to generate 99mTc. For medical purposes, the 99mTc is used in the form of pertechnate, TcO4-.
The technetium isotope 99mTc is unusual in that it has a half-life for gamma emission of 6.03 hours. This is extremely long for an electromagnetic decay - more typical is 10-16 seconds.
With such a long half-life for the excited state leading to this decay, this state is called a metastable state, and that is the reason for the designation 99m.
While the 140.5 keV gamma transition is labeled as happening 98.6% of the time, not all of those actually emit a gamma ray photon. The process called internal conversion always competes with gamma photon emission. This involves transfer of the energy of the transition to one of the atomic electrons, usually a K, L, or M shell electron.
gamma photon 87.87%
K-internal conversion 9.13%
L-internal conversion 1.18%
M-internal conversion 0.39%
In the K-internal conversion process, a K-shell electron is ejected with the transition energy 140.5 keV minus the binding energy of the K-shell electron.
Internal conversion is another electromagnetic process which can occur in the nucleus and which competes with gamma emission. Sometimes the multipole electric fields of the nucleus interact with orbital electrons with enough energy to eject them from the atom. This process is not the same as emitting a gamma ray which knocks an electron out of the atom. It is also not the same as beta decay, since the emitted electron was previously one of the orbital electrons, whereas the electron in beta decay is produced by the decay of a neutron.
203Hg, which decays to 203Tl by beta emission, leaving the 203Tl in an electromagnetically excited state. It can proceed to the ground state by emitting a 279.190 keV gamma ray, or by internal conversion. In this case the internal conversion is more probable. Since the internal conversion process can interact with any of the orbital electrons, the result is a spectrum of internal conversion electrons which will be seen as superimposed upon the electron energy spectrum of the beta emission. The energy yield of this electromagnetic transition can be taken as 279.190 keV, so the ejected electrons will have that energy minus their binding energy in the 203Tl daughter atom.
Monoclonal antibodies are man-made versions of immune system proteins that attack only a specific molecular target on certain cancer cells. Scientists have learned how to pair these antibodies with radioactive atoms. When put into the bloodstream, the antibodies act as homing devices. They attach only to their target, bringing tiny packets of radiation directly to the cancer.

Radio-labeled antibodies are used to treat some non-Hodgkin lymphomas, especially those that don’t respond to other treatments.
Please use the following journal to complete your class project......
Phosphorus 32

This form of phosphorus (also known as P-32 or chromic phosphate P 32) is put inside brain tumors that are cystic to degrade the tumor without hurting the healthy cells in the brain.

In the past, P-32 was given intravenously as a common treatment for a blood disease called polycythemia vera. P-32 was also placed inside the abdomen as a treatment for ovarian cancer. It’s rarely used in these ways today, because there are better drugs with fewer side effects.
Treatment of thyroid cancer

The thyroid gland absorbs nearly all of the iodine in the blood. Because of this, radioactive iodine (also called radioiodine or iodine 131) can be used to destroy the thyroid gland and thyroid cancer with little effect on the rest of the body. This treatment is often used after thyroid cancer surgery to destroy any thyroid cells left behind. It’s also used to treat some types of thyroid cancer that spread to lymph nodes and other parts of the body.
Treatment of bone pain

Strontium 89 (Metastron®), samarium 153 (Quadramet®), and Radium- 223 (Xofigo®) are radiopharmaceuticals that can be used for bone metastases. These medicines are given intravenously, so that they go directly into blood circulation. They travel through the body and build up in the areas of the bone where there is a cancer. The radiation they give off then kills cancer cells and eases the pain caused by bone metastases.
For cancer that have already spread to several bones, this approach can be better than trying to aim external beam radiation at each affected bone. These drugs may be used along with external beam radiation which is aimed at the most painful bone metastases. This combined approach has helped many men with prostate cancer, but it has not been studied as much for use in other cancers.
Some people notice more bone pain for the first couple of days after treatment, but this isn’t common. These drugs can also lower blood cell counts, especially white blood cells (which can increase the risk of infection) and platelets (which can raise the risk of bruising or bleeding).
The Machine
How it Works
Types of MRI
Magnetic Resonance Imaging is a diagnostic technique that uses magnetic fields and radio waves to produce a detailed image of the body’s soft tissue and bones.
An MRI images the spine by using a magnet that goes around the body to excite hydrogen atoms. After the atoms return to their normal level of excitation, they emit energy that is detected on a scanner.
An MRI scan is typically used for pre-surgical planning! Due to the scan being sensitive to information about the health of discs as well as the presence of tumors or a herniated disc.
The magnet in an MRI system is rated using a unit of measure known as a Tesla. Another unit of measure commonly used with magnets is the gauss (1 Tesla = 10,000 gauss).
The magnets in MRI are in the 0.5-Tesla to 3.0-Tesla range, or 5,000 to 30,000 gauss. Extremely powerful magnets -- up to 60 Tesla -- are used in research. Compared with the Earth's 0.5-gauss magnetic field, you can see how incredibly powerful these magnets are.
The very powerful magnet is used to align the nuclei of atoms inside the body, and a variable magnetic field that causes the atoms to resonate, a phenomenon called nuclear magnetic resonance.
The nuclei produce their own rotating magnetic fields that a scanner detects and uses to create an image. What makes this remarkable is there is no risk of Ionizing Radiation
The human body is mostly water. Water molecules (H20) contain hydrogen nuclei (protons), which become aligned in a magnetic field. An MRI scanner applies a very strong magnetic field (about 0.2 to 3 teslas, or roughly a thousand times the strength of a typical fridge magnet), which aligns the proton "spins."
The scanner also produces a radio frequency current that creates a varying magnetic field. The protons absorb the energy from the variable field and flip their spins. When the field is turned off, the protons gradually return to their normal spin, a process called precession. The return process produces a radio signal that can be measured by a Gamma camera in the scanner and made into an image
Protons in different body tissues return to their normal spins at different rates, so the scanner can distinguish among tissues. The scanner settings can be adjusted to produce contrasts between different body tissues. Additional magnetic fields are used to localize body structures in 3D.
Diffusion MRI
Functional MRI
Health & Safety
The Future of MRI
This form of MRI measures how water molecules diffuse through body tissues. Certain disease processes — such as a stroke or tumor — can restrict this diffusion, so this method is often used to diagnose them. Diffusion MRI has only been around for about 15 to 20 years.
In neurons, molecules tend to diffuse along neural fibers, so the direction of diffusion parallels the fibers themselves. A recent method called diffusion tensor imaging (DTI) allows researchers to measure diffusion in multiple directions, and can be used to map connectivity between brain areas.
In addition to structural imaging, MRI can also be used to visualize functional activity in the brain. Functional MRI, or fMRI, measures changes in blood flow to different parts of the brain. The main form of fMRI involves blood-oxygen-level dependent (BOLD) contrast. BOLD signals are widely used as a proxy for brain activity, because neurons use more oxygen when they're active. This technique has been especially useful in neuroscience.
Increasingly, researchers are using fMRI and DTI together, Filippi said. They use fMRI to map brain activity, and use DTI to map tracts of white matter — the connective cables of the brain.
Just because it does not use the risks of Ionizing Radiation doesn't mean it is 100% safe!
Because MRI uses strong magnets, any kind of magnetic metal implant poses a hazard. The implant can move or heat up in the field. Several patients with pacemakers who underwent MRI scans have died.
The constant flipping of magnetic fields can stimulate peripheral nerves. The flipping also produces clicking or beeping noises, so ear protection is necessary in the scanner room.
The radio frequency energy transmitted by the scanner can be absorbed by the body, causing heating. To prevent this from happening, there are limits on the transmitter rates than can be used.
Using combinations of types of MRI are already being used
MRI methods are constantly improving. "There are always new [magnetic] pulse sequences and different ways to image different body parts being developed. Techniques are becoming much more quantitative, and smaller and smaller regions can be imaged — scientists can now image brain areas down to 1 millimeter.
positron emission tomography. This type of scan can show how body tissues are working, as well as what they look like.
A PET scan can help to

Show up a cancer
Find out the stage of a cancer
Show whether a lump is cancer or not
Show whether a cancer has spread to other parts of the body
Decide the best treatment for your cancer
Show how well cancer drug treatment is working
Show the difference between scar tissue and active cancer tissue
You can use a CT scan synergysticly with a PET scan
CT Scans
How it works
Before carrying out a PET scan, a radioactive medicine is produced; this is then tagged to a natural chemical.
This natural chemical could be glucose, water, or ammonia. The tagged natural chemical is known as a radiotracer. The radiotracer is then injected into the human body.
When it is inside the body, the radiotracer will go to areas that use the natural chemical. Fluorodeoxyglucose is a radioactive drug, it is tagged to glucose to make a radiotracer.
The glucose travels to those parts of the body that use glucose for energy. Cancers, for example, use glucose differently from normal tissue - so, FDG can show up cancers.
A PET scan detects the energy emitted by positively charged particles (positrons). As the radiotracer is broken down inside the patient's body, positrons are created. This energy appears as a 3-D image on a computer monitor.
The image reveals how parts of the patient's body function by the way they break down the radiotracer. A PET image will display different levels of positrons according to brightness and colour.
This uses xrays to take detailed images of your structure... You can use this in conjunction with PET to see in detail which domain is dysfunctional and by what degree
The multiple X-rays from the beams are detected after they have passed through the body and their strength is measured.
Beams that have passed through less dense tissue such as the lungs will be stronger, whereas beams that have passed through denser tissue such as bone will be weaker.
technetium-99m, a metastable nuclear isomer travels through the body and emits radiation the tracer’s progress is tracked by a crystal that scintillates in response to gamma-rays.
The crystal made of sodium Iodide is mounted in front of an array of light sensors that converts Gamma rays into a flash of light which in turn is transferred into an electrical signal. Gamma cameras differ from X-ray imaging techniques in one very important respect; rather than anatomy and structure, gamma cameras map the function and processes of the body.
Gamma rays are produced by unstable nuclei when protons and neutrons re-range to a more stable configuration. Gamma decay usually follows an alpha or beta decay and does not change element.
In the body
Ultrasound imaging uses sound waves to produce pictures of the inside of the body. Ultrasound is safe, non-invasive, and does not use ionizing radiation.
ultrasound images are captured in real-time, they can show the structure and movement of the body's internal organs, as well as blood flowing through blood vessels.
Doppler ultrasound is a special ultrasound technique that allows you to see the blood flowing
Color Doppler uses a computer to convert Doppler measurements into an array of colors to show the speed and direction of blood flow through a blood vessel.
Power Doppler is more sensitive than color Doppler and capable of providing greater detail of blood flow, especially when blood flow is minimal. Power Doppler, however, does not help the radiologist determine the direction of blood flow, which may be important in some situations.
Spectral Doppler displays blood flow measurements graphically, in terms of the distance traveled per unit of time, rather than as a color picture. It can also convert blood flow information into a distinctive sound that can be heard with every heartbeat.
Ultrasound imaging is based on the same principles involved in the sonar used by bats and ships . When a sound wave strikes an object, it bounces back. By measuring these echo waves, it is possible to determine how far away the object is as well as the object's size, shape and consistency (whether the object is solid or filled with fluid).
Radiopharmaceuticals for diagnosis are labelled with a radioisotope that decays with the emission of electromagnetic radiation
Gamma radiation is of the same nature as radio waves, television-waves and light.
As electromagnetic radiation has a high penetrating power and is absorbed only to a limited extent by tissues, the gamma emitted after the administration of a diagnostic radiopharmaceutical in the patient´s body can be detected outside the body using a gamma camera or PET cameras
With the aid of powerful computer programs, this information is converted into scintigraphic images showing the distribution of the radioactive compound in the patient´s body.
If the radiopharmaceutical is taken up and handled by a pathological tissue or organ to a different extent than by healthy tissues, the scintigraphic image shows the localisation and status of a particular disease, such as a tumour, metastasis, or infection.
The images can also allow the evaluation of, for example, the functional status of an organ, the density of receptors at a particular site, or the levels of metabolism in some tissues.
Recapping Radiopharmaceuticals
The Start of Module 2
Kidney Transplant Statistics
Intervention & How it works
Visual links to see the entire transplant
Kidney transplantation is a treatment option for end-stage renal disease (ESRD).
Designed to prolong the patients health, it can cause additional problems. These are classified as either
complications include renal artery thrombosis or stenosis, renal vein thrombosis, or urinary leak.
complications include organ/tissue rejection, drug toxicity related to anti-rejection treatments, acute tubular necrosis (ATN), infection, and transplantation-related malignancies.
The most common complication of kidney transplantation is allograft dysfunction.
Acute rejection and cyclosporine toxicity are the most common causes of early transplant failure.
Compare and Contrasting
Artificial Production
A cyclotron is a particle accelerator. A cyclotron accelerates charged particles in a spiral path, which allows for a much longer acceleration path than a straight line accelerator.
A cyclotron consists of electrodes, termed 'dees' because of their shape, in a vacuum chamber.

This vacuum chamber is flat and sits in a narrow gap between poles of a large magnet which creates a perpendicular magnetic field.

A stream of charged particles is fed into the centre of the chamber and a high frequency alternating voltage is applied across the electrodes.

This voltage alternately attracts and repels the charged particles causing them to accelerate.
El Natural
The magnetic field moves the particles in a circular path and, as they gain more energy from the accelerating voltage, they spiral outwards until they reach the outer edge of the chamber.
Modern cyclotrons accelerate negative ions created in a plasma. When these negative ions reach the outer edge of the chamber the excess electrons are stripped off the ions forming positive particles such as a proton or deuteron, which can then be extracted from the cyclotron as a beam. The size of the vacuum chamber determines the length of the spiral path and hence the amount of energy attained by the particle.
We spend less than 10% of our time outside so the sun does not expose us to the most radiation.
Most of the radiation comes from the ground (Radon).
Potassium radionuclides are found in foods, most often in shellfish.
Mercury is another element that is found in the food web, also in sea foods but is much more dangerous.
Radon is a colorless, odorless gas, a radioactive byproduct of radium.
It is part of the natural radioactive decay series starting with uranium-238.
It is radioactive with a half-life of 3.8 days, decaying by the emission of alpha particles to polonium, bismuth, and lead in successive steps.
The decay of radon-222 with emission of an alpha particle is followed within about an hour by a series of four further decays, two of them accompanied by emission of alpha particles and the other two accompanied by other types of radiation.
The short-lived atoms into which a radon atom decays are actually isotopes of polonium, lead, and bismuth, but they are referred to collectively as radon daughters
Sometimes your body will reject the transplant as it sees it as foreign.... Same principal for a baby.
There are two common types of rejection:
Acute Rejection
– Usually occurs anytime during the first year after transplant and can usually be treated successfully.
Chronic Rejection
– Usually occurs slowly over a long period of time. The causes are not well understood and treatment is often unsuccessful.
Immune system repression are the most common form of manipulation to allow the host and donor to connect.
Some of the side effects of the transplant not being 100% successful are; New on-set Diabetes, Blood Pressure anomalies and renal dysfunction...
MRI intensity?
Radiation therapy techniques
Mathematics of ultrasound
More on the basics
We can hear sound between 20 - 20,000 Hz. Ultrasound is much more than that th an average of 10MHz.
For this, a tranducer is critical. it is used to shoot the the ultrasonic waves through an object. But the medium in which it passes through must be used as a guide.
While the ultrasound signal is sent through the specific part of the body, a portion of it is sent back when it interacts with a different entity.
The speed of sound is 340.29 m/s which the computer uses to determine how far away something is. It calculates this based also on the time in which it takes for the sound to bounce back
An ultrasound is used because the frequency is high while the wavelength is low.... neither of these things determine the speed of sound, it is all about the medium!!
This is important because it means you get less diffraction, meaning sound does not bend or refract on its return
Just one of the complexities surrounding ultrasound is "The Static Forward Equation"
in this instance a piezoelectric transducer is used to measure the
Acoustic Incidence Field
Ui is the acoustic incidence field, Co is the speed of sound in the medium, Az is the beam at which we hit z,
is the waveform, t is time and x, y, z imaging planes
The Basics
Looking at the math behind intensity
The Math
Xrays are electromagnetic waves that have a diverse range of 0.01 - 10nm and a range of energy of 100eV - 100keV.
the ray comes from a pair of electrodes called a cathode and anode.
The cathode acts like a light bulb in that it has a filament. It then releases electrons
Those electrons are passed to the tungsten anode disc (this attracts the electrons).
The electrons react with the tungsten to produce photons (this is because electrons hit the tungsten which knocks a lower orbital electron out of the atom, with extra energy)
These photons are filtered through a Pb cylinder
Once through the filter the beam passes through the body, only being absorbed by the dense tissue
A radiograph film is placed behind the patient and the photons that pass through the body turn the film black
The film consists of an emulsion-gelatin containg an Ag-halide crystals with a blueish base
One of the key concepts that must taken into account is Newtons Inverse Sq Law.
This law simply means that if radiation is at 100mR/Hr 1 inch from the source, at 100 inches radiation 0.01mR/Hr
The particles that hit the bone or tissue are either absorbed or refracted, this is called attenuation
The graph that is constructed is termed an attenuation curve.
While constructing your graph you need to apply 10% probability of attenuation.
This 10% probability is known as the linear attenuation coefficient
Transmission intensity varies depending on what you are scanning
If you use a narrow beam of mono-energetic photons the change in beam intensity is shown as
dI is the change of intensity, I is initial intensity, n is No. Atoms/ cm3, sigma is proportionality constant, dx is the increments in thickness
This then aligns to
Where I is intensity of photons, Io is initial intensity, U (micro) linear attenuation coefficient and X is distance traveled
As an Xray passes through the body part the ray is attenuated, meaning its intensity has been reduced.
How much it is reduced is dependent on the medium and pass through the body directly
Your standard equation is
is the distance traveled by the Xray which has an intensity of
I(s). As s
increases so does intensity. distance is critical! u(s) is optical density.
Intensity is also a major component
Computerized Axial Tomography
We need to consider this as t gives detail to dimensions by looking at attenuation
This Xray passes through at x and y
Therefore your optical density is u(x,y)
Your equation is

The function R(P,degrees) is the Radon transform of the function u(x,y)
Gradient coils are used to produce deliberate variations within the major magnetic field
There is a set of coils for each plane (x, y, z)
This variation in the field allows for a localization of an image in addition to phase and frequenct encoding
These gradients are specific in that for the z plane the coils are HelmHoltz
And for x and y the coils are saddle coils
This is a pair of coils that create uniform magnetic fields at the center space between the coils
Known as Golay coils, hey produce linear, homogeneous, magnetic field along a central axis
When turned on one end has lesser and one end has greater strength than the static magnet
These gradients play a major role in MRI safety, especially with prosthetics.
Tissue that surrounds a metallic implant is often exposed to non-negligible heating and as a result the implant absorbs electromagnetic energy
Where J is density, R is the radii of joint, micro0 is permeability. B is the Time harmonic magnetic flux peak, f is the frequency, r distance from the spherical center. (0) is the colatitude. K is the space between source and sphere.
Treatment modalities comprise of radiation therapy, surgery, chemotherapy, immunotherapy and hormonal therapy. Radiation therapy remains an important component of cancer treatment with approximately 50% of all cancer patients receiving radiation therapy & it contributes towards 40% of curative treatment for cancer.
The aim of Radiation therapy is to stop or cause significant disruption to mitosis
Radiation is a physical agent, which is used to destroy cancer cells. The radiation used is called ionizing radiation because it forms ions (electrically charged particles) and deposits energy in the cells of the tissues it passes through. This deposited energy can kill cancer cells or cause genetic changes resulting in cancer cell death.
High-energy radiation damages genetic material of cells and thus blocking their ability to divide and proliferate further. Although radiation damages both normal cells as well as cancer cells, the goal of radiation therapy is to maximize the radiation dose to abnormal cancer cells while minimizing exposure to normal cells, which is adjacent to cancer cells or in the path of radiation. Normal cells usually can repair themselves at a faster rate and retain its normal function status than the cancerous cells.
Radiation therapy delivered in a fractionated regime is based on the differing radiobiological properties of cancer and various normal tissues. These regimes amplify the survival advantage of normal tissues over cancer cells, largely based on better sublethal damage repair of radiation damage in normal cells as compared to cancer cells. Normal cells proliferate relatively more slowly compared to the rapidly proliferating cancer cells and therefore have time to repair damage before replication. Initial observations of the effects of fractionated radiation therapy in the 1920s eventually led to the development of regimes comparing different treatment schedules based on total dose, number of fractions and overall treatment time. A typical radiation therapy regime now consists of daily fractions of 1.5 to 3Gy given over several weeks.
This technique which precisely delivers very high individual doses of radiation over only a few treatment fractions to ablate small, well-defined primary and oligometastatic tumours anywhere in the body. Due to the high radiation dose, any tissue immediately adjacent to the tumour is likely to be damaged. However as the amount of normal tissue in the high dose region is small and non-eloquent, clinically significant toxicity is low. SBRT has shown excellent results in the treatment of early stage non-small cell lung cancer in patients unfit for surgery. Other tumours include in the prostate, head and neck, hepatic, renal, oligometastases, spinal and pancreatic.
Stereotactic body radiation therapy (SBRT)
Electron beams are commonly used in everyday radiation therapy treatment and are particularly useful to treat tumours close to a body surface since they do not penetrate deeply into tissues. External beam radiation therapy is also carried out with heavier particles such as: neutrons produced by neutron generators and cyclotrons; protons produced by cyclotrons and heavy ions (helium, carbon, nitrogen, argon, neon) produced by synchrotrons. Proton beams are a newer form of particle beam radiation used to treat cancer. It can offer better dose distribution due to its unique absorption profile in tissues, known as the Bragg's peak, allowing deposition of maximum destructive energy at the tumor site while minimizing the damage to healthy tissues along their path.
Particle radiations (electron, proton and neutron beams)

Particle radiation has higher LET than photons with higher biological effectiveness. Therefore, these forms of radiations may be more effective to the radioresistant cancers such as sarcomas, renal cell carcinomas, melanomas and glioblastoma. However, equipment for production of particle radiation therapy is considerably more expensive than for photons. The decreasing costs of cyclotrons are likely to result in a wider use of proton beam therapy in the future.
These have particular clinical use in pediatric tumors and in adults tumors located near critical structures such as spinal cord and skull base tumors, where maximal normal tissue sparing is crucial. Neutron beams are generated inside neutron generators after proton beams are deflected to a target. They have high LET and can cause more DNA damage than photons. The limitations have been mainly due to difficulty in generating neutron particles as well as the construction of such treatment facilities.
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