Medical Imaging

X-rays: very good for imaging bone, but not soft tissue
Still useful on kidneys - would see kidney stones

Before Roentgen's X-ray discovery, no way to look at body besides cutting it open

Some medical imaging methods (like X-ray) provide a picture, some (like ultrasound) provide a real-time picture

Where in the electromagnetic spectrum do we look?

Two parameters: resolution and attenuation (attenuation: how the radiation changes as goes through the body during the imaging process). Often, it is a tradeoff between the two.
Both of these parameters are a function of wavelength

We can't medical-image with visible light, since the human body is opaque to it

-How X-rays work-
Tungsten cathode: bombards tungsten target with electrons, the target is hooked up to a copper anode
X-rays produced by the electron/target collision

X-rays are 2D representation of a 3D object; thus aren't good for analyzing abnormal structures where you aren't sure what structure to expect

-Computer tomography-
Uses X-rays
Takes 2D images of a 3D volume (like the human body) - "slices" - target needs to be aligned straight with the beam generator
Several of these images, aggregated into one picture

Backprojection: Looking at the same slice of the body from different angles

-Positron emission tomography (PET)-

Proton --> Neutron + Neutrino + Positron
[Part of radioactive decay: inject radioactive agent during PET to cause this to happen]

Positron + Electron: antimatter annihilation that produces a gamma-ray signature. The latter is detected

Areas of higher metabolism (like cancer tumors) give off a stronger signature

-MRI-
This has been covered before
Hydrogen protons on particular become excited in a strong magnetic field, then turn the field off, which thus changes the signal behavior.
You image based on this.

-Ultrasound-
Useful on pregnant women because it's safe/not radioactive
Has other uses as well

We know the speed of sound in the body because know the speed of sound in water, and the body is mostly water

You image based on the ultrasound's echoes

Doppler ultrasound can image blood flow

Visible Human Project: part of NIH
Gathers data about human body parts that can be use din designing and tweaking these imaging processes

modality: a term for a different medical-imaging process

Often get complementary images from using different processes on the same object
Could look at these images side-by-side or no top of each other

Images often pseudocolored to help humans read + analyze them

Simulated/"phantom" images are often useful for gathering general information

Outlines [Vision And Mind example]

This came to mind as I'm working on Topic Summary #3:

An outline is a good way to prep for doing the topic summary, but it's also a good way to think about the topic, in general.

So here goes:

-Vision And Mind-

* the physiology of the vision system

** details on structure of the eye
*** Rods & Cones

** details on the visual cortex and on V1 through V5
*** where it's located compared to the rest of the brain.

** "lateral inhibition", and how it accentuates the edges of an object

** eye movements
*** depends on the size of the object/scene to be visualized (the object/scene to be visualized is counted in degrees squared)
*** oculomotor system
*** saccades
*** various ways of tracing a moving object

** field-of-view vs. acuity conflict
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* the psychology of the vision system

** How can we visualize brain psychology?
by various medical-imaging techniques:
*** EEG
*** MRI vs. fMRI, differences

** How certain optical illusions work
*** really should be 'neural illusions'?
*** name a few big-name optical illusions & discuss them
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* relation to other senses
** brain lobe where inputs from other senses 'meet'
** use of visual system to compensate for other sight loss
** use of other sensory systems to compensate for vision loss

Halstone's electron-microscope lab

This lab uses TEMs (tunneling electron microscopes)

Can't use glass lenses to focus electron beams; have to use magnetic lenses; this *is* a tradeoff

X-ray microscopes don't have the quality we want yet, gamma-ray microscopes are still atleast a few years down the road

Mosely's law - 1914: hit something w/ electrons and produce X-rays

* Radiation issues inherent
* Depending on the energy level of the X-rays, you can tell what element emitted it


Electron gun fires a beam down the main column
Tungsten filament is excited and

200,000 volts
200,000 kilo-electron volts

Moving at ~ 1/3 the speed of light, need to use Relativity-based equations

Specimen in center of column, on the end of a handle

Whole column has to be under vacuum

One magnetic lens controls spot size of beam; there are several other lenses aswell

Objective lens nearest the bottom of the column; it's what actually produces the image.

^ How do you *see* electrons?
At bottom of column, a screen covered with zinc sulfide...Electrons put some part(s) of that into an excited state, and they flouresce green.

Green-ness caught by film

X-ray detectors chilled to liquid-nitrogen temperatures.
This can be used to analyze what elements ar ein each section of the sample

Specimens extremely small (~3mm diameter, and very thin (<200nm, even thinner for heavy elements))

What is 2,000,000x magnification? A person magnified this much would be two thousand miles tall?
What is a nanometer versus a meter? Size of a golf ball versions the size of the Earth

Electron microscopes very sensitive. Even sound waves form talking, etc. can cause the image to 'jump' a bit

Electron diffraction - electron beam scattered by hitting a crystalline structure. Depending on how they scatter, you can determine what the crystal structure is. Often a ring pattern of some sort.

Lattice planes in asbestos are 9 angstroms (.9 nm) apart

Easy to detect dead space in a crystalline structure vs. the location of the atoms themselves

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*Scanning electron microscope*

Shorter column
20k eV up to 30k or 40k eV

Secondary electrons kicked off by the sample itself


Can vary distance between sample and electron beam

General good microscope idea: start at low magnification to get an overview, then zoom in

A high frame rate leads to more noise "static" (spend less time scanning a particular pixel)

Low frame rate - less "noise", but takes longer to produce the image

Low voltage; less penetration; should mean more surface detail [and vice versa]

SEM useful for building a picture of where exactly atoms of element X are located throughout the sample

H, He, Li, and Be can't be detected via this X-ray detector equipment

astigmatism: vertical image in separate focus from the horizontal image

Spehrical and chromatic aberation you need different equipment to correct
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Halstone uses a srtaightforward same (copper+aluminum disk) to calibrate the settings

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The material you used to prepare the specimen (gold coat, etc), can screw with the view

How do they work?

Okay, how do these types of medical imaging work?

* Ultrasound

How it works: Bombardment of sound waves above 20,000 hertz (20khZ), 20khz being the upper limit of human hearing

What is actually measured: Ultrasound actually measures the echoes received by the ultrasound machine after they bounce back from hitting body parts.

An electrical pulse transducer in the machine causes it to vibrate at a certain frequency. Likewise, the echoes cause the transducer to vibrate in various ways, converted into electrical pulses which are then converted into a digital image

Advantages: Light if any side effects, very good at imaging soft tissue (vary frequency for different kinds of soft tissue), can produce a "live video" feed during the procedure

* fMRI (Functional Magnetic Resonance Imaging)

How it works: Images the brain and brain activity, though the method would technically work on other areas of the body
What is actually measured: It does this by detecting the increased blood flow typically associated with neural activity

How does it act as a proxy: Neural activity kicks up blood flow, with the areas of increased blood flow being measured

* SPECT

How it works: Radioisotopes fed into body, and the gamma rays are detected by the SPECT equipment
What is actually measured: The rays received by the machine are measured as 2-D images, but are measured form different angles, enabling a 3D image to be produced

How does it act as a proxy?: See info about 3-D reconstruction method above. Also, the radio decay particles behave differently when passing through different body structures, hence the ability for this to measure anything at all

* electromicroscopy


How it works: Magnifies samples to a much higher degree of resolution than is possible with a light microscope.

What is actually measured: Very small/thin sample bombarded with an electron beam. As electrons go through the object (in tunneling electron microscopy, or TEM), they are recorded as how they were 'behaving' as they come out

How does it act as a proxy?: Impediments inside the sample modify the outgoing electron beam.

Types of Medical Imaging

We'll be investigating some forms of medical imaging in class on Wed. (4/25) and Fri. (4/25)

Now what are the different types of medical imaging?

-Things that analyze brain waves, even though they aren't imaging something visible, are still a form of medical imaging-

* such as electroencephalography (EEG)

Many techniques exist that take images of physical body parts...

* X-rays (Classic!)
* Magnetic resonance imaging (MRI) + fMRI
* Positron emission tomography (PET)
* Computerized tomography (CAT)
* Ultrasound

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endoscopy
* SPECT
* radioisotope-based techniques: nuclear medicine + flouroscopy
* thermography
* electron microscopy
* Raman spectroscopy
* image guided biopsy
* angiography

Electron Microscopy

Electron MIcroscopes come in two forms: The Tunneling Electron Microscope (TEM) and the Scanning Electron Microscope (SEM).

What is the difference? Both use beams of electrons.
However, the difference resides in the fact that the different types of machines direct the electron beam differently.

Tunneling Electron Microscopes send an electron beam *through* the object. This means theta they are good at gathering details on the inside of the object (which is especially important with cells), and of the surface of small objects.


Scanning Electorn Microscopes send their electron beams along the outside of objects. They are most effective for analyzing the surface of large/thick objects

The STEM (Scanning transmission electron microscope) combines some of the advantageous features of each, logically enough.

There are online simulators, based on Flash, Java, or the like, that give you an impression of how an electron microscope works. By adjusting settings common to an electorn microscope, and chosing from a premade set of 'samples', you see the resultant image.

http://micro.magnet.fsu.edu/primer/java/electronmicroscopy/magnify1/index.html is an example

Questions answered by Andy & Jeff

TMS = transcranial magnetic stimulation

Blinded or blindfolded people - this frees up processing capacity in the visual cortex, some of which is used for other types of sensory processing

In deaf people, the auditory cortex probably does something

Same parts of brain for thinking about something, and perceiving it. So thinking about something during a dream may helpyou perceive it when you're awake

During sleep, the memory-forming hippocampus is more-active
Sleep is mental regeneration (see above), as well as physical regeneration

Cramming - does not work for long-term retention

Eye -->LGN --> V1 --> V2 --> V3 --> V4 --> V5
Not always a linear pathway, there are some connections that skip/go backwards in this chain.

The different V's process somewhat different types of signals (have different capacities), but these roles are interconencted. (Removing V4 doesn't

LGN - laterogenetic nucleus of the thalamus

Allan Cowe - vision experiments on monkeys

The brain is to some extent a dynamic self-repairing system

At a certain point, the things [experiments] we want to do are illegal or unethical - Andy Herbert
Don't overdo the computer analogies here in brain psychology

SOme optical illusions are the result of the brain taking necessary processing shortcuts

"Is what we see really there?" - That gets into philosophy

Integrating the senses is important - Sense A compensates for a weakness of Sense B
Normally, the senses paint a complementary picture...very disorienting when they don't

Contrast sensitivity - vision of shadows vs. vision of discrete objects
Different among different species, because different species need visions systems adapted to their different environment

Can see what capacity is missing when a certain section of the brain is gone.
First started relazing this en masse with Civil War musketball-to-the-head injuries

Cortex has higher-order roles than things like the thalamus. Things liek the thalamus handle basic body functions for the most part.

Drugs,and not just the funky illegal ones - have effect on neurotrasmitters.

Range of visual sensitivity amongst normal humans
This may explain how a lot of drugs produce haalucinations and the like