OBI Imager QA

Nov 2006 Medical Physics has a quality assurance program for the on-board imager.

Travelling vs Standing waves

· Accelerate electrons using a microwave using either a traveling wave, which absorbs the residual power with a dummy to prevent “backward reflected waves”, or a stationary wave accelerator which reflects the waves from each end towards each other to produce “standing waves”.

Mnemonic, when one is travelling they don't look backward (i.e. prevent backward reflecting waves)

When one is stationary or standing they can reflect back on their life (reflect waves to from each end)

Photon Interactions as a function of energy

Why use dref for electrons in TG-51

TG-51 gives a reference for the reference depth for electron beam, which is Ref.#17.Quote: For electron beam reference dosimetry in radiotherapy, it is shown that by choosingthe reference depth as dref=0.6R50 -0.1 cm, ..., the Spencer-Attix water-to-airstopping-power ratioat dref is given by (L/p)(water to air) = 1.2534-0.1487(R50)exp(0.2144).This is the magic.

REF YAHOO STUDY GROUP 4/23/08

Photon Interactions Part III

As far as the photon fate after the interaction with an
atom is concerned there are two possible outcomes:
• Photon disappears (i.e., is absorbed completely) and a portion
of its energy is transferred to light charged particles (electrons
and positrons in the absorbing medium).
• Photon is scattered and two outcomes are possible:
• The resulting photon has the same energy as the incident photon and no
light charged particles are released in the interaction.
• The resulting scattered photon has a lower energy than the incident photon
and the energy excess is transferred to a light charged particle (electron).

Photon Interaction (Tightly Bound)

1.4.1 Slide 5 (125/194)
1.4 PHOTON INTERACTIONS
1.4.1 Types of indirectly ionizing photon irradiations
􀀁 A tightly bound electron is an electron whose binding
energy is comparable to, larger than, or slightly smaller
than the photon energy .
• For a photon interaction to occur with a tightly bound electron, the
binding energy of the electron must be of the order of, but
slightly smaller, than the photon energy
• An interaction between a photon and a tightly bound electron is
considered an interaction between photon and the atom as a
whole.

Photoelectric effect (tightly bound)

Loosely bound electrons (Compton)

1.4 PHOTON INTERACTIONS
1.4.1 Types of indirectly ionizing photon irradiations
􀀁 A loosely bound electron is an electron whose binding
energy to the nucleus is small compared to the
photon energy
􀀁 An interaction between a photon and a loosely bound
electron is considered to be an interaction between a
photon and a free (unbound) electron.

Photon Interactions part 2

1.4 PHOTON INTERACTIONS
1.4.1 Types of indirectly ionizing photon irradiations
􀀁 Interactions of photons with nuclei may be:
• Direct photon-nucleus interactions (photodisintegration)
or
• Interactions between the photon and the electrostatic field of the
nucleus (pair production).
􀀁 Photon-orbital electron interactions are characterized as
interactions between the photon and either
• A loosely bound electron (Compton effect, triplet production)
or
• A tightly bound electron (photoelectric effect).

Photon Interactions

1.4 PHOTON INTERACTIONS
1.4.1 Types of indirectly ionizing photon irradiations
􀀁 In penetrating an absorbing medium, photons may
experience various interactions with the atoms of the
medium, involving:
• Absorbing atom as a whole
• Nuclei of the absorbing medium
• Orbital electrons of the absorbing medium.

Electron Interactions and Stopping Power

􀀁 Electrons traversing an absorber lose their kinetic energy
through ionization collisions and radiation collisions.
􀀁 The rate of energy loss per gram and per cm2 is called the
mass stopping power and it is a sum of two components:
• Mass collision stopping power
• Mass radiation stopping power
􀀁 The rate of energy loss for a therapy electron beam in
water and water-like tissues, averaged over the electron’s
range, is about 2 MeV/cm.

Activity (Basics)

Activity represents the total number of disintegrations
(decays) of parent nuclei per unit time.
􀀁 The SI unit of activity is the becquerel (1 Bq = 1 s-1).
Both the becquerel and the hertz correspond to s-1, however, hertz
expresses frequency of periodic motion, while becquerel expresses
activity.
􀀁 The older unit of activity is the curie ,
originally defined as the activity of 1 g of radium-226.
Currently, the activity of 1 g of radium-226 is 0.988 Ci.
(1 Ci = 3.7 􀀂 1010 s􀀁1)

Review of the basics Ch 1 Atomic Physics

􀀁 The constituent particles forming an atom are:
• Proton
• Neutron
• Electron
Protons and neutrons are known as nucleons and they form the
nucleus.
􀀁 Atomic number Z
Number of protons and number of electrons in an atom.


􀀁 Atomic mass number A
Number of nucleons in an atom,
where
• Z is the number of protons (atomic number) in an atom.
• N is the number of neutrons in an atom.

Radiation fundamentals

Exposure (X), 1 R= 2.58 x10-4 C/kg air

Dose, 1Gy=100 rad

Equivalent Dose, 1 Sv=100 Rem (note weighting factors are applied)

Activity 1 Bq= 1Ci/3.7x10 to the power 10

Categories of ionizing radiation

Ionizing photon radiation is classified into four categories:
􀀁 Characteristic x ray
Results from electronic transitions between atomic shells.
􀀁 Bremsstrahlung
Results mainly from electron-nucleus Coulomb interactions.
􀀁 Gamma ray
Results from nuclear transitions.
􀀁 Annihilation quantum (annihilation radiation)
Results from positron-electron annihilation.

Courtesy Pgorsak Ch1

Collisional Losses (Ionization and Excitation)

The rate of energy loss depends on the electron density of the medium.

The rate of energy loss per gram per centimeter squared, which is called the mass stopping power is greater for low atomic number (Z) material than for high Z materials (Khan pg 298). Compare the water to lead curve in Khan Fig 14.1

There are two reasons for this:

First, high Z materials have fewer electrons than low Z materials
Second, high Z have more tightly bound electrons, which are not as available for this type of interaction

The figure in Khan Ch 14 shows the the energy loss rate first decreases and then increases with increase in electron energy with a miniumum occurring at about 1 MeV. Above 1 MeV, the variation with energy is very gradual.

The energy loss rate of electrons of energy 1 MeV and above in water is roughly 2 MeV/cm.

Electron nucleus interactions

The energy loss by radiation and the radiative yield g increase directly
with the absorber atomic number Z and the kinetic energy of electrons.
The radiation yield for X ray targets in the diagnostic radiology energy
range (~100 keV) is of the order of 1%, while in the megavoltage energy
range it amounts to 10–20%.

Indirectly Ionizing Radiation

Indirectly ionizing radiation (photons or neutrons) deposits energy in the
medium through a two step process:
● In the first step a charged particle is released in the medium (photons
release electrons or positrons, neutrons release protons or heavier ions);
● In the second step the released charged particles deposit energy to the
medium through direct Coulomb interactions with orbital electrons of the
atoms in the medium.

PGORSAK

Question 3542 Yahoo site, ADCL calibration

Can anyone tell me how ADCL calibrate user's ion chambers?ADCL website explains the configuration in the following.

Please see the diagram in the website.http://rpc.mdanderson.org/adcl/absorbed.htm"

Absorbed Dose to WaterAbsorbed dose to water calibrations are performed in a 30 x 30 x 30cm3 phantom at 5 cm depth and 85 cm from the source. Water proofchambers are calibrated bare in water, other chambers are calibrated in1 mm PMMA water-proofing provided by the ADCL. The field size atthe location of the chamber is 10 x 10 cm2. The dose rate at thelocation of the chamber will be between 25 and 50 cGy/min."

I have a question here.TG-51 says that "The gradienteffects are included implicitly in the beam quality conversionfactor kQ for photons and explicitly by the term PgrQ for electrons."How the gradient effect is included for x-ray?I think it is something to do with how ADCL calibrate user's ionchamber.Can anyone explain?

Thanks,Jongmin

Neutron shielding

Ch 5 Mcginley pg 69

Most medical accelerators operating above 10 MeV use a maze with a door shielded for neutrons and photons at the outer maze entrance. A typical door consists of a steel case 0.635 cm thick containing 10.2 cm of borated polyethylene.

The polyethylene is used to moderate the fast and intermediate energy neutrons, which then react with the boron and produce a 0.473 MeV photon.

The lead is placed after the polyethylene where it will attenuate the photons where it will attenuate the photons produced in the boron and any capture gamma rays generated in the maze by neutron capture in the concrete in the concrete wall, ceiling and floor.

Recently McCall (1997) has indicated a more efficient door shield is produced by placing the lead before the polyethylene. With this arrangement, the high energy neutron component is reduced in energy by interactions in the lead before entering the polyethylene layer.

The overall result is an increased attenuation of the neutrons with approximately the same penetration for the capture gamma rays that originate in the maze. The low energy photons aristing from the (n, alpha) reaction with the boron in the polyethylene are attenuated sufficiently by the steel case of the door.

Gadolinium for MRI

Breast MRI Contrast Agent: Gadolinium
Malignant breast tumors begin to grow their own blood supply network once they reach a certain size; this is the only way the cancer can continue to grow. In a breast MRI scan, a contrast agent injected into the bloodstream can provide information about blood supply to the breast tissues; the agent "lights up" a tumor by highlighting its blood vessel network. Usually, several scans are taken: one before the contrast agent is injected and at least one after. The pre-contrast and post-contrast images are compared and areas of difference are highlighted. It is important to note that if the patient moves even slightly between the two scans, the shape or size of the image may be distorted--a big loss of information.
What is Gadolinium?
This is an FDA approved contrast agent for MRI. Gadolinium, or gadodiamide, provides greater contrast between normal tissue and abnormal tissue in the brain and body. Gadolinium looks clear like water and is non-radioactive. After it is injected into a vein, Gadolinium accumulates in the abnormal tissue that may be affecting the body or head. Gadolinium causes these abnormal areas to become very bright (enhanced) on the MRI. This makes it very easy to see. Gadolinium is then rapidly cleared from the body by the kidneys.
Does Gadolinium go by other names?
Gadolinium, gadolinium-DPTA, gadodiamide. It also goes by various brand names, depending on the pharmaceutical company that makes it:
Magnevist (Berlex Laboratories, Inc.)
Omniscan (Nycomed Amersham plc)
ProHance (Bracco Diagnostics, Inc.)
What does Gadolinium do?
Gadolinium allows the MRI to define abnormal tissue with greater clarity than ever before. Tumors enhance after Gadolinium is given. The exact size of the tumor and location are very important in treatment planning and follow up. Gadolinium is also helpful in finding small tumors by making them bright and easy to see.
Is Gadolinium safe?
Gadolinium has been used for years in adults and children in the United States, Europe and Japan, without any serious complications in thousands of patients. The FDA declared Gadolinium safe for use in MRI in 1988. A few side effects, such as mild headache, nausea and local burning can occur. Very rarely (less than one in a thousand), patients are allergic to Gadolinium. If you have kidney problems, it must be used with caution. Gadolinium should be used in pregnant patients or nursing mothers only when the benefits outweigh the risk. Gadolinium used in MRI is many times safer than the iodine type contrast used in CT scans. There is more information at the International MR Safety Central Web Site.
What are the side effects?
Side effects of the contrast agent injection include mild headache, nausea and local pain. Rarely (less than 1% of the time) low blood pressure and lightheadedness occurs. This can be treated immediately with intravenous fluids. Very rarely (less than one in one thousand), patients are allergic to the contrast agent. These effects are most commonly hives and itchy eyes, but more severe reactions have been seen which result in shortness of breath.
What about breast feeding?
According to Dr. Emanual Kanal of the International MR Safety Central Web Site: "More data are available for Magnevist than for the other agents. Magnevist is excreted in very low concentrations (i.e., 0.011% of the total dose) in human breast milk over approximately 33 hours. The concentration of this contrast agent in breast milk peaks at approximately 4.75 hours and decreases to less than a fifth of this level (to less than 1 micromol/L) 22 hours after injection. For this reason, and as an extra precaution, it is recommended that nursing mothers express their breasts and not breastfeed for 36 to 48 hours after administration of an MR imaging contrast agent, to ensure that the nursing child does not receive the drug in any appreciable quantity."
What about new contrast agents?
New contrast agents to improve breast MRI are currently being developed and studied in safety and efficacy clinical trials.