Exam Review:

ABR- board exams

ABR certification, state licensure (FL included), eligibility: NEED a campemp residency,

EXAM- part 1: written (during grad school), part 2: specialty written (post residency), part 3:

specialty oral. Maintenance of certification: every 10 years another ABR exam, continuing

education.

Radiation Sources

-atomic: bremsstrahlung, charged particle into high z material, as it decelerates ejects photons

(Xray)

-continuous/linear spectra (Kramer) Emax/3=Eavg

-nuclear: nuclei (p and n) in excited state- relaxes into a lower energy state. Loss of EM radiation, a

gamma ray.

-discrete peaks

particles: a+2: discrete 5MeV, +/-: continuous 2MeV (produces a beta + and an antineutrino/beta -β

and a neutrino), no: 0-1 MeV, p: 200 MeV accelerated for proton therapy

Photon Interactions with Matter

Radioactivity: Bq = decays/ sec, but 1 decay doesnt necessarily mean 1 particle

Absorded Dose: Gray (Gy) = (Energy Deposited)/Mass = J/Kg

Effective Dose or “Whole body” dose:

-Takes into account tissue weighting factors

- Unit is the Sievert (Sv); Sv = Wr * Gy

Exposure: Coulomb per mass = C/Kg

-Measure of charge created in mass air

-unit is Roentgen ( R ) = 2.5*10^-8 C/Kg

- photoelectric, - compton, K- pairτ σ

production

This graph illustrates energy dependence of

photon interactions. at low energies PE

prevails, middle energies compton and PP at higher energies (remember PP requires a 1.022

MeV threshold to occur which explains its domination in the higher energy realm)

Clinical- Diagnostic & Therapeutic

diagnostic- apply low levels of radiation to image and diagnose internal problems, mainly worried

about stochastic side effects. contrast lower energies, ~20-140 keV

therapeutic- apply higher levels of radiation to treat an existing condition, mainly worried about

deterministic effects. High Lethal doses, minimize tissue attenuation, lower contrast, higher

energies.

Machine Produced Xrays

Bremsstrahlung= radiative component of stopping power.

totalradiative = TZ

800 T= kinetic energy, z= atomic number

remainder of energy gets lost in the form of HEAT. (thermal energy)

radiative= primary concern with the production of xrays, collisional= primary concern for tissue

and dose

characteristics of Xrays: Emax is equal to the potential

difference or KE of electrons, F=qv (force on an

electron when in a field)

characteristic xrays- when electron de excite from

higher excited energy levels and drop back down to a

lower energy shell, they emit characteristic xrays.

These xrays are a function of the material. appear in

discrete peaks superimposed on the continuous

bremmstrahlung spectrum. (see image to L)

intensity: # of xrays produced, a value of quantity

Linear Accelerators

accelerated electrons along the length of a linear tube

electron gun shoots the electrons into tube and conducting material

the first plate is set with positive charges, this

forces the negatively charged electron to

accelerate towards that positive conducting

material interface. Once it is close to the

interface the polarity switches and it becomes

negative and the next segment becomes positive… again forcing the electron to speed away from

that positive charge (like OPPOSES like) towards the negative. illustrated in pic above. ** we now

use microwaves to force electrons through the tube. in region of wave peaks the e- experiences a

positive field and it pushed forward, towards the peaks of the waves on the ‘outside’ the electron

experiences a negative field and pushes away into absorbing medium.

microwave source: Klystron, Magnetron

bending magnets: bend the beam and direct the beam within the gandry

270 degree rotation (using bending magnets) of electrons to transmission target (thicker so it can

avoid allowing e- through) , slower e-: smaller radius, faster e-: bigger rad

Radiation Biology (HIntenlang Lecture 4) - Dan

- The body has essentially one of two types of responses to radiation : stochastic or deterministic.

Deterministic effects are quantitative, guaranteed radiation effects based on certain

threshold limits. For example, we know that , say, 50 Gy applied to a tumor x amount of

times will kill the tumor. OR a dose applied of 1-2 Gy will result in skin burns.

Stochastic radiation effects deal with those possible effects for which no certain outcome

is known, or can be applied to a population of people. For example, small amounts of

radiation administered to patients in imaging type procedures may or may not cause cancer.

The cancer rate will vary from person to person.

Chemical changes: DSB, SSB, ect. Biological Changes: skin reddening/burns

DSB- when 2 SSB occur witihin 10 base pairs of each other, require 1-2 hr to repair (THIS is

what causes radiation damage to be seen)

SSB- DNA backbone breaks, takes ~10 min to repair

Surviving Fraction (for high LET- )= , ,  α p n o  e −D/Do Do~1-3 Gy

4-5 Gy is a LETHAL dose to HUMANS

~0.3-0.4 Gy background radiation/year

Fractionated doses for RT (low LET): Break RT into smaller doses and deliver over

period of about 30 days. this works because healthy cells repair better than tumor cells.

Let healthy cells repair over a day.

Dose Response: depends on dose, type of radiation, response, dose rate, cell type,

hypoxic state. Expect biological response for Low LET to be linear-quadratic function

(linear at first, quadratic later on). For High LET: expect LNT (Linear No Threshold)

response function. Some evidence points to a Linear Threshold (LT) model, as with

cataracts. Radiation Hormesis model indicates benefits at low levels of radiation.

Radiation protection uses most conservative model (LNT) for regulatory purposes.

Email Listserve Major Threads and Themes -Dan

Sadije: I remember there being a lot of emails about a company called Landauer medical starting

to do consulting, but I erased them

Since Dr. Hintenlang told us to keep an eye on major threads or themes in the email listserves, I

thought I would try to pull out some notable threads for us.

GE Essential FFDM QC manual 8. "Test for flexible paddle deflection in compression (thread started

09/25/13, 18 total messages in this thread)

Quick summary: Doug Simpson cites what he thinks is a new QA test for mammography in

the FFDM QC manual 8 which was released in March 2013. The purpose of the test in

question is to ensure that the flexible compression paddle is capable of applying effective

compression to the breast. The test procedure states that we use 2 foam phantoms of

specific shapes and material, and measure the compressed flex-paddle offset from the

breast support tray using machinist's gauge blocks.

Doug comically wonders, “Where the %$#^ is the average physicist supposed to get the toys

to perform this test?”

Michael Yester responded that he used relatively stiff foam and it worked fine for that test.

Another respondent suggested using an iPhone app for a level instead of having to buy a

telescoping gauge.

Dental Cone Beam CT (thread started 09/24/13, 7 total messages in the thread)

Ira Miller writes, “One of my clients recently replaced their dental panoramic units with

what they told me were to be new modern pano units. They are very modern pano

units, they are actually cone beam dental cts. Does anyone have a protocol for testing

one of these. Are they classified as pano units or ct’s?”

The consensus from the discussion was that they are in general classified as CTs, and

most places are just using CT protocols for testing. However, there are variations in

regulations based on individual states and countries. For example, Wisconsin requires

shielding plans submitted for dental CTs. In Ohio, on the other hand, Dental cone Beam

units are considered CTs. The UK has some specific guidelines on these devices.

Revisit: Dose Tracking Software (thread started 09/18/13, 11 total messages in the thread)

Quick summary:

Scott Dube revives a discussion from last year on different dose tracking software

programs utilized for tracking dose to patients in diagnostic imaging, and whether they

are really worth the cost and hassle.

Discussion highlights:

One notable software program from Radimetrics has been purchased and used by many

but a lot of people end up turning off some of the main features of the program, such as

effective dose calculations. The reason for turning this feature off is calibrated based

on a short, thin (150 lb) patient, which is not realistic for many of the patients that are

actually imaged.

Responses are mixed on dose tracking software. Some people seem to really like it and

think it is valuable. Some people think it is mostly just a waste of money (ie. Has a lot of

fancy buttons but doesn’t really add much value). The reason for the latter attitude may

be that some governing body has stated that diagnostic imaging does not contribute any

real risk to patients!

Others use inexpensive, free, or in-house software for dose tracking simply to help them

comply with established laws for dose.

Landauer and Premier Healthcare (lots of threads over lots of time)

Summary:

Landauer is a large consulting company which has just entered the medical physics market.

Diagnostic Machines: X RAY Tube

Radioactive sources (non-machine) can be sealed or unsealed and are regulated by the federalNRC

X Ray sources are machine produced (through Bremsstahlung – need charged particles that aredeaccelerating)

Diagnostic X Ray Tubes (20 to 150 kv)

Therapy (MV’s)

Accelerated electrons don’t have a Bragg Peak, so are only good for skin level therapy

Target material is usually Tungsten, Mammography is lower energy and uses something likeMolybdenum

Characteristics of X-Rays:

-On the picture, the ideal (thick target/Kramer) spectrum would be linear, with low energy Xrays

having highest intensities

- Thin target spectrum would be if you only looked at small sections of the of the thick target

spectrum

-The max E that the lines end on is the Emax of the electrons

-Inherent filtration – Low X-rays don’t escape the target material

-Force on electron in electric field and magnetic field (F=qE+qvXB)

ignore the 6.3 AC volt note

How X-Ray tube works

-A current is put through the filament that heats it up and releases electrons through thermionic

emission. Filament is negatively charged cathode. (Some Xray tubes have short and long filamants

can decide which you want to use. The large gives better penetration depth of X-Rays, the small

gives better spatial resolution)

-The electrons accelerate through vacuum (a cup around filament helps with keeping away electron

back scatter)

- The Tungsten target is the anode, is beveled, it also spins to dissipate heat (so heat doesn’t build

up in one spot). The bevel impacts focal spot size. Its beveled to direct X-Rays toward patient.

-X-rays exit through ground glass window, which helps to keep the field more uniform

- The entire apparatus is encased in lead

Design considerations

- High Z material with high melting point

- Appropriate energy for what you want to do

- Focal spot size effects heat dissipation and resolution

- Beam window- where the X-rays escape

- More heat dissipation (cooling oil bath and fan)

- Isolation of voltage (Need to keep insulted because lethal amount of voltage involved)

- Added filtration (can add Al-about 2mm slab between patient and X-Rays to further get rid of

low E X-rays that would only contribute to dose). With added filtration you get beam hardening

because you take off even more low energy x-rays that you do with just inherent filtration

- A hardened beam is a better quality beam

Beam quality beam is quantified through Half Value Layer

- HVL = “The thickness of material required to reduce intensity to one half its initial value”

Collimators – adjustable lead plates around the beam that let you square it out and control size

based on what you want to look at

Heel effect- has to do with the asymmetry of the target (Tungsten) some X-rays are more

attenuated by the target (those that are released closer to the target on the patient plane are less

intense)

Off focus radiation – Caused by incorrect voltage or damaged electron cup = electrons going off in

wrong direction

Because we use AC voltage need to use generators and look at waveforms

- Use transformer to boost voltage

- Single phase – sine wave, so get forward and backward motion

This is fixed by adding a diode that makes all of the function positive, but still have voltage ripple

- Three phase – use an RC circuit to your Kvp

- Newer high frequency units keep kvp relatively constant

Exposure (X) = has to with amount of x-rays emitted from tube

- Measure in R (C/kg)

- X is proportional to ((kvp)^n)(mAs)/d^2

o   Kvp = max electron voltage, n is ideal two, in real word about 3

o   mA = miliamps

o   s = seconds

o   Can change intensity by adjusting those values

SPECT (Edmond Olguin)

-Tc-99m is most common radioisotope used. Emits 140 keV gamma ray. Half life = 6hr

-Nuclear medicine uses TRACE amounts of radioisotopes (Th-201 is rat poison)

-Detector:

a)Photomultiplier tube-Converts light energy (created in scintillator detector) into an electrical

signal. Spatial resolution is improved by Hal Anger: 1 large slab of NaI with a bank of PMT behind

it. Light hits more than one tube and a gamma ray interacts with the scintillator, emits light

photons towards the PMT and hit more than one tube. “Its the pattern of pulses that come out of

these more than one tubes that allows us to pinpoint the location” (ie uses multiple peaks in

different PMT from one event to localize event).

SPECT attenuation effect: More attenuation in the middle of an object as opposed to the edges. (In

image, the lines should really be a straight line at 1, worse attenuation with larger objects). This

happens with myocardial perfusion (illustrates the function of the heart) and requires an

attenuation correction by using CT combined with SPECT

b)Collimator-Lead channels that allow us to determine photon trajectory and creates spatial

resolution. This collimator is necessary for SPECT. 1/10,000 photons actually pass through the

collimator and hit the detector. In SPECT, the collimator only gives us spatial resolution, NOT used

to prevent scatter. The collimator spatial resolution is proportional to: Rc proportional to d/l, The

collimator efficiency (Ec) is proportional to (d/l)^2 You want to minimize distance from source to

collimator (ie you want the detector as close to the patient as possible) See below

c)Scintillator: NaI is used most frequently. Critical issues: 1)Detector efficiency: must have high

enough stopping power to detect photons (high Z material). Detector (intrinsic) efficiency (Ei) =

1-exp(-mu*x) at 150 keV, Ei for 0.25inch NaI is about 0.85, but for the same detector, at 511 keV Ei

is about 0.1 2)Energy conversion efficiency (light output, NaI is about 50%, thus a detected xray of

140 keV will convert to about 70 keV of light). 3)Decay Time: determines how frequently you can

detect photons. System efficiency Es = Ec*Ei. Ei is defined above and Ec is the collimator efficiency

(about 1/10,000)

PET (Edmond Olguin)

Radioactive nuclide emits a beta+ particle and annihilates with a beta- (positron and electron

annihilate) and emit two photons at 180 degrees at 511keV each.

F-18 is used most frequently (110 minute half-life) usually tagged to a glucose analog: F-18

Fluoro-deoxy-glucose (FDG). Excellent because the molecule appears to be glucose and is brought

into the cell but isn’t metabolized or excreted by the cell. Thus the radionuclide is placed in the cell

until it decays.

No collimator is necessary for PET. Achieves spatial resolution without collimator, because the

photons are emitted at 180 degrees. If photons hit detector within 10 ns of each other, we consider

that a coincidence event and draw a line between the two detectors to form the trajectory.

Random Coincidence Event is when two separate decays register one coincidence event and create

a wrong trajectory.

Time-of-Flight PET: Uses differences in time of arrival for a coincidence event to help localize the

decay.

PET geometric efficiency (analogous to collimator efficiency for SPECT): (Area of detector)/(Area of

sphere)

PET scintillator materials: NaI NOT used because the attenuation coefficient is low (~.3) but high

conversion efficiency (100%). BGO has excellent stopping power (~.9) but low conversion efficiency

(~12-14%). GSO/LSO is good with both: good stopping power (~.7-.8) with good conversion

efficiency (~40-75%)

PET detector efficiency: (1-exp(-mu*x))^2

2D vs 3D PET - 2D PET uses axial collimators to avoid noise from events outside the region of

interest but 3D PET is currently used because it allows you to use events that emit photons axially.

PET attenuation: exp(-mu*d), where dis the diameter or total distance of the patient/object

(Chao Guo) lecture 9 ,11

Magnification effect:

  L objectL image =   a

a+b =   SIDSOD

Spatial Resolution:

b -> smaller,

get smaller focus spot, better spatial resolution

magnification is different than Spatial resolution

f: point spread function(PSF) width

f= but PFS affected by magnificationF OIDSOD

Better spatial resolution:

1. smaller F,

2. smaller OID

3. max number of points resolved per distance

(units in ) R =  d1 =   1

F ∙OID  mm−1

Two points blur together

Quantify spatial resolution:

minimum distance between two point but still resolved( not overlap)

d quantify spatial resolution

ID  d = F OIDOID+SOD = 1 = F ∙O    

after de-magnification≡ f

better measure of spatial resolution means not affected by

magnification

d =   f mag = FOIDSOD = F OID

OID+SOD

Spatial resolution measurement

1. Point Spread Function

2. d (minimum distance between two point but still resolvable)

3.  R =  d1

Measuring spatial resolution

1. pinhole (PSF)

2. Phantom (line pair test/ section of a star pattern)

3. Edge phantom (ESF edge spread function) SF (x)  dxdESF(x) = P

like PSF, ESF could be magnified

Contrast:

Without Scatter C = (P - P’)/P where P is the Intensity max and P’ is the minimum intensity

with Scatter (  )  Cscatter = Cno−scater1

1+s/p

where is called the contrast reduction factor   11+s/p

S/P is the scatter primary ratio

Mike

Detectors:

Screen Film

● 15% conversion efficient

● Convenient and cheap

● Higher spatial resolution

● Dose awareness

● Small dynamic range

● No post processing

● Degridate over time

● Take up space for storage

Computed Radiography (CR)

● Photostimulable Phospher Tube

○ Made of BaFBr, BaFI

● Wider dynamic range

● Reusable cartridges

Digital Radiography (DR)

● Indirect

○ Cesium Scintillator

○ Produces light which cause a charge

● High Detection efficiency

○ Lower dose needed

Digital Images and Display:

● Pixel size �X ≦ FWHM/2

Image Reconstruction:

● Intensity on detector

○ (t)  I0 e  I =   −∑u �X *

○ n( I0/I(t) )  ∑u X  l =   *�

● Matrix and Iterative Method

○ A 1 x m matrix of unknown Attenuation coefficients X

○ A m x n matrix of pixel values A

○ A*X equal an m x 1 matrix G, So A*X=G

○ Therefore X = Inverse( A )*G

○ For Matlab use the “pinv” function to do the inverse of matrici that dont have

one

● Filtered-Back Projection

○ Traditional Method

○ Fourier based

○ the back projection is the convolution of the image and function f

■ The deconvolution function of f is K( s ) = s

● it “De-blurs” the real image

● s is frequency in “frequency space”

● its basically the line y = x but in frequency space

■ Example:

● the real function is a square in the xy plane and a rod extending

in the z axis

● then the back projection is function that fall off as

1/sqrt(x^2+y^2) extending from the z axis out towards the xy

plane

■ In frequency space filters can be applied

● Low-pass: removes high frequency, allows low

● High-pass: removes low frequencies, allows high

● Combination is the Ramp Low pass

○ Kills very low and high frequencies