Medical Imaging Lecture 1 (7/14/11)

- The only history you need to is November 8, 1895 Professor Wilhelm Konrad von

Roentgen (University of Wurzburg, Bavaria) discovered X-ray.

- Expected to know this - Very high percentage of this will be on your boards - Ma (Milliamps) number of photons flowing per second. It’s a number (a quantity).

o Milliamps seconds samething, 1Ma 5 seconds is the same as shooting 5Ma at 1 second, the same number of photons.

o The number of photons determines the density

o So Ma affects the background and the object being X-rayed equally. If I double the time, the background is twice as black and the object is twice as black.

, the whiteness and blackness of the film. So the more photos that hit the film, the blacker the film will get and the blacker the object will get.

o Has NO effect on contrast (Exam question). o Contrast for radiology means the difference btwn white and black. o Ma if is too high, things will be black proportionately. If it is too low, will be light

proportionately. - Kv (kilovolts)

o Has nothing to do with number of photons, has to do with the penetrating power. The wave length.

o If I increase Kv, I shorten the wave length and increase the penetrating power. o So if I want to x-ray the heel, I am going to need more kilovolts than if I am going

to x-ray the phalanx. o So Kv and contrast are INVERSELY proportional. Kv will affect the whiteness and

blackness of the OBJECT, but will NOT affect the whiteness and blackness of the BACKGROUND. So it is only affecting contrast.

o If I increase Kv more photons will pass through the object, because they have a shorter wave length. That will make the object look blacker.

o Increasing Kv will decrease contrast. o To increase contrast, decrease Kv.

o The best example is a mammogram. It is a very high contrast x-ray. They are shot about 25-27Kv, which is really little and that makes them high contrast.

o Why haven’t any young women had mammogram? They are not good for you. High contrast x-rays, very large waves or low Kv. Those waves are going to stay in the body and cause free radicals. So over the age of 40 women have a higher risk of dying of breast cancer, than the amount of radiation she’s exposed to.

o So high contrast x-ray, means low Kv. KV AND CONTRAST ARE INVERSELY PROPORTIONAL.

- Intensity radiographically is the number of photons. - In radiology brightness is blackness. So the blacker the film, the brighter it is. - Distance- the inverse square of

o So if I am shooting an x-ray at an angle and I am twice as far. I have to quadruple the Ma, so the same amount of photon will hit the plate. If I shot from ½ the distance I would have to 4 the Ma. So distance affects the number of photons hitting the plate or the density.

- Kv: o Increase penetration (more hard X-rays). o Hard x-rays means high Kv x-rays

Are good because they will pass right through the patient, will not interact with him

o Soft x-ray means low Kv x-rays. Going to get absorbed by the body and create free radicals. That’s called

the photoelectrical affect. Or it has enough energy to pass through the body and get deflected,

that’s called Compton scatter. That fogs up the x-ray. We want to minimize soft x-ray.

- What is the SAFEST x-ray for a patient? o Low Ma (low photon interaction) and high Kv (so there is not interaction in the

body) (exam question).

- We do not really use this anymore, we use digital but we will cover them quickly. - X-ray film is made of a base and emulsion. - The whole purpose of the base is to give it rigidity (to hold the chemicals). The base is

transparent. (Don’t have to memorize other stuff about the base).

- The main chemical is silver bromide - When the silver gets hit by x-ray then it’s developed it turns black. - Speed or sensitivity of a film means how sensitive it is to an x-ray. High speed means it

will develop it quickly, a low speed film means you need more x-ray to do it. - In podiatry, we need a lot of details so we use intermediate speed film.

- Gelatin in emulsion

- When you put gelatin in a solution it swells up. That means the chemical reaction can occur more quickly.

- Screen or non-screen films, not relevant to you. Everything we use now is screen. The main

purpose of the screen is to limit the exposure to a patient. Much safer.

- This is on every x-ray exam and on your boards. So basically what that means, you have

taken the x-ray but haven’t developed it yet.

- You can do it manually or with an automatic processor. - Fixing time is usually twice the developing time. The optimal temperature for developing

manually is 68° F.

- The developing solution has 4 chemicals in it. The important chemicals are reducing agents.

The reducing agents are what turned the exposed silver black (that’s important). That is the main point, you want to turn the exposed silver black and prevent the unexposed from turning silver black.

- Potassium bromide is both in the developer and fixer.

- The sodium carbonate forces the gelatin to the exposed and sodium sulfite is the protector.

- Wash off and put in the fixer - The main purpose of the developing solution is to turn the exposed black and stop the

unexposed from turning black. - The main purpose of the fixing solution is to remove the unexposed silver so you have your

clear area and to make the image permanent. So if you walked in to the developing room in the middle of developing the image would disappear. If you walked in during the fixing phase, nothing will happen, the image has become permanent.

- Cells with highest turn over rate are more sensitive to radiation. Lymphocytes probably have

the highest sensitivity to radiation. - Nerve cells the most well differentiated are the least sensitive. - On an exam what is the least sensitive cell, the answer is nerves. - A woman is most sensitive to radiation during her first trimester because her cell turn over

rate is high. During the third trimester is a very little risk. - Last menstrual period- When is it safe to radiate a young woman of bearing age who is

sexual active and not on birth control? The ten day rule, you limit it to the first ten days of the cycle from the first day of menses (to make sure she isn’t pregnant). If a woman is abstinent or on birth control I don’t care I will shoot the x-ray.

- Downstairs you are going to see a Ma dial and Kv dial. Everything is set, but what if it is a fat

patient? The settings are set for 150lbs, so for a big guy I am going to increase the voltage. For a petite woman or kid with these settings I am going to over penetrate, so you start playing with it.

- The difference btwn somatic and genetic effect. A somatic effect is you radiate me and I lose

my hair, get leukemia or get cataracts (a direct effect from radiation). A genetic effect is something that is passed on in the genetic pool, you radiate me and the baby is born without an arm or has leukemia.

- No one is going to ask you about this number, just know that what makes x-rays really useful

is that they have very small wave length. So they can pass through things

- Remember the duality of light, it behaves as a wave and behaves as a particle (on an exam

the answer is both wave and a particle). - Travels at (186,000 m/s)

- Number 5: causes free radicals, those are bad for you. - Produces secondary radiation. When the x-ray comes out of the machine, its called the

primary beam, as soon as it hit anything, hits the blouse, the blouse gives off secondary radiation.

- This is the term for braking radiation. Inside the tube head the negative electrons get

attracted to the positively charged anode. That actually slows it down, that the conversion of one type of E to another. Kinetic E is transformed into radiation called Brem’s stahlung radiation. That’s all I want you to know about that.

- Primary beam is the beam coming out of the x-ray. - The central ray is the middle of the primary beam. - Soft x-ray: Low Kv, they are bad for you. - Hard x-rays are high Kv x-rays, they are better for you.

- This is inside the tube head. The electrons start of at the cathode. - The cathode is negatively charged, that’s your source of electrons. - Electrons are negatively charged, so they are coming from the negatively charged cathode.

So the cathode shoots the electrons at the positively charged anode. - Inside the anode is the target. The target is made of tungsten (b/c it’s giving off lots of heat).

Tungsten has a high atomic number, which means it has a high melting point. - 99% of the image is going to be dissipated as heat, only 1% is going to hit the target and

come out of the tube as an x-ray. - Problems are how to dissipate the heat. The smaller the focal spot the better the detail, so

we want a smaller focal spot. There are two options o 1- Rotate the target (so no one place is being hit exactly in the same place). o 2- The Line focus principle: if you angle the target 15-20°, the heat will be dissipated

over a larger area. o Those are the two ways we dissipate heat in an x-ray machine.

- Like I said it goes around.

- The line focus principle, you can read that.

- All I want you to know is that the anode is positively charged and that where you tungsten target is.

- On an exam to confuse you instead of saying high melting point for tungsten will say high atomic number, same thing.

- We said it’s negatively charged.

- X ray tube just leads the electron from the cathode to the anode. - What percentage gets dissipated into heat? 99.2…So if I ask you what percent becomes

useful beam the answer is 1%.

- The heel effect: useful probably not, but you have to know it. The heel effect say the

intensity of the beam when is comes out of the x-ray tube is not totally equal, it’s 1/10th at the cathode side, than the anode side. So if you knew which side you cathode on and which side your anode is on, you put the thicker side of the heel closer to the cathode.

- This is saying the x-ray beam is more intense at the cathode side than the anode side. - A very popular question on exams.

- Characteristic radiation is a type of secondary radiation

- Scattered x-rays soft waves, those that have suffered a change of direction. So scattered radiation is a type of secondary radiation.

- Energy interaction: a very low E level, below diagnostic radiology, its unmodified scatter (not

important for diagnostic radiology). - In radiation therapy, if I was treating you carcinoma, I will hit you with very high levels of

radiation and the main E transfer will be electron pairing. - In diagnostic radiology, the most two common effects are the photoelectric collision and

Compton Effect. Both of them think of soft x-rays.

- The photoelectric event. - Main type in podiatric diagnostic - We shoot at low E level. Below 70Kvp the main E interaction is photoelectric. That is when

the x-rays are soft and stay inside the body. So the main E interaction in mammography is photoelectric. That causes biological effects.

- If we increase the E a bit, but not too much, the photon would have enough E to pass

through the body. But they are going to interact with the body and come out at an angle. That is called Compton scatter.

- What that does, it hits the x-ray plate at an angle and fogs the film. - Both Compton scatter and photoelectric are bad. But If I increase the Kv I can minimize this.

- Absorption of an x-ray depends on the wave length. - So if I hit him with high Kv, it going to pass through the body, but if I hit him with low Kv, its

going to stay in the body. - Component of the object in the pass of the beam, is it soft tissue or bone. An inch of soft

tissues is not going to absorb as much x-ray as an inch of bone. - Thickness of the object: two inch of bone is going to absorb more than one inch of bone.

- Speed of sensitivity means how much x-ray has to be used.

- A high speed film a little bit of x-ray, safer for the patient.

- In the screen, we use phosphors which give off light. - He skipped a bunch of slides.

- These are important. They are aluminum strips inside the x-ray machine. - The purpose of grids is to absorb soft x-rays (absorb the scatter so it doesn’t fog the film). - Bucky grid, used for chest x-rays (grid actually moves).

- Diaphragms, cones and collimator: we want to have the smallest field. So if I want to radiate

your foot, I want the field to be a little bit bigger than your foot. - Skipped a bunch of slides.

- REM is the same as Siveert absorbed dose. - REM and Siveert have different values but the same kind of definition and measuring the

same thing (biological equivalent that is). - As doctors you are allowed 5 REM of radiation per year after the age of 18. If you were 20

years old what will be your totaled allowed occupation dose. The formula is ((the doctor’s age-18) X 5). So a 20yr will be allowed to have 10REM.

- A patient is allowed ½ REM per year (1/10th a dose of a doctor).

- An x-ray view is named according to which side of the anatomy is CLOSEST to the plate - An x-ray projection is named according to which side of the anatomy is CLOSEST to the

BEAM. - There are two major exceptions to this DP view and lateral view, which are the most

common views we shoot. - Pic above: this is set up as DP view. The foot is on the plate, the beam is coming down at

15°. We shoot at 15° because the metatarsals declinate 15° average. If you shot straight down at zero, the metatarsal will appear abnormally short.

- So we are shooting at the second ray, middle of the foot. - Remember where you put your focal spot is the best penetration. So if I am interested in the

rear foot I go more proximal. But since it is in the forefoot, I go more distal.

- Dorsal Plantar view: Middle of the first met. (Asks a question is it dorsal or plantar? You

can’t tell). There are limitations on the x-ray. You can tell me its medial or later, distal or proximal but not dorsal or plantar.

- A DP is good for biomechics, good for evaluating the angles, good for isolating the metatarsals and good for looking at digital contractions. But it isn’t good for plantar or dorsal.

- This x-ray was under penetrated (look how white the foot is). So the Kv was too low. - If the Ma was low, the foot would look white like this, but the background would also look

white. - So when we talk about something white on the x-ray, we talk about “scoradic” a radio

dense, a radiopaic (means it is whiter than it should be). - When something is black its radiolucent.

- What did I do wrong? - If the background is okay. The Kv is too high. - If the took the x-ray for the mid-foot that’s okay. - There a problem, the foot is mixed dense. So if you take a perfect x-ray of the heel, you

going to over-penetrate the toes. If you take a perfect x-ray of the toes, you going to over-penetrate the heel.

- Look at that radio-density in the soft tissue by the neck of the third metatarsal. - Dorsal or plantar? You can’t tell.

- This is kinda cool, a transverse fx of the seamoids, horizontal. Shows up best on a dorsal

plantar view. - A longitudinal or oblique fx would show up on a seamoidal view.

- Remember the exception: a lateral view. - Lateral view: the medial side of the food is up against the plate. - The beam is coming in at 90°. It’s coming in straight shooting at the 5th met-cuboid

articulation. - The biggest mistake you going to make in clinic is if you don’t shoot at 90°.

- Here is a lateral. - Look at that sclerosing at the subtalar joint (red circle). - A lateral view you can look at dorsal, you can look at plantar, but you can’t tell if it is medial

or lateral. - This view is good for looking at the top of the foot, looking at the bottom. It isn’t good for

looking medial or lateral. - That was a kid, has an open apophysis.

- This patient has a cyst of the posterior tuberosity (where the arrow is).

- Hagalund’s deformity

- A fx of the styloid, 5th met.

- A fracture of the calcaneus. This is what’s called the joint depression fracture, where the

calcaneus gets pushed out on itself.

- This is important - A medial oblique view: the medial side of the foot against the plate is at 45°. - The beam is coming straight down at zero. - The medial oblique view pronates the foot; it opens up most of the joints. - The only thing that’s going to overlap on the medial oblique is the 1st and 2nd rays.

Everything else is going to be axle-d. - Impt point: The medial oblique view shows the dorsal medial aspect of the foot and plantar

lateral in terms of the mid-line. o So if I want to see a ganglionic cyst on the dorsum of the 1st met-cuneiform joint.

Medial oblique view will be good for it. o I want to see a sub-ungal exostosis on top of the hallux (medial dorsal)= medial

oblique. o I want to rule out osteomylesis under the 5th met. head that will be plantar later

(that’s a good view for it). o If I want to look at a “dura-atkin corn” that’s dorsal lateral (won’t be a good view for

it).

o A heel spur (plantar medial) not a good view. o It will be a great view for calcaneal navicular coalitation, because it pronates the

foot. o It will be a great view for the styloid process.

- This is a medial oblique, one and two overlap. You can see dorsal medial and plantar lateral.

Right: DP Left: Medial oblique.

- If you have an uncooperative patient, sit him down and turn the machine 45° from lateral to

medial, medial side away from the beam. The image is going to be a little distorted, but it’s going to be a medial oblique.

- In radiology distortion means it isn’t its true size. That isn’t necessary bad.

- Lateral oblique: lateral side of the foot against the plate at 45° - The beam is coming down at zero. - This is going to supinate the foot. - The 2nd, 3rd and 4th are going to overlap, but you can isolate the 1st and the 5th. - Good for dorsal lateral and plantar medial - So If I want to see a heel spur, comes of the plantar tubercle that’s a good view. - If I want to see an exostosis on the 5th toe dorsal laterally, that’s a good view. - If I want to see an os tibiale externa or the insertion of the PT tendon, that’s a good view.

- Here is your distorted lateral oblique. - The patient can be sitting down. - Usually when we shoot a distorted oblique we have to increase the Ma to make up for

the make up for the increased distance.