Medical Imaging- Comp Exam
Medical Imaging 1
Introduction
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Medical imaging can be used to make a diagnosis, follow the progression, and
assist in medical interventions
Some techniques either measure the anatomical structures or the function
(physiology) of the system, and some methods do both
Methods to obtain anatomical info: US, X-ray, Computerized Tomography
(CAT/ CT scan), and Magnetic Resonance (MRI)
Methods to obtain physiological info: positron emission tomography (PET),
single photon emission computerized tomography (SPECT), and functional
MRI.
Image Basics
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Pixel= picture element
Grayscale image- 0=black and the larger values indicate lighter shades
o The more Bits the detail in the image is improved, and this means the
larger the file; need a compromise between image resolution and file
size
Spatial Resolution
o Another way to improve the resolution is to increase the # of pixels
per space; again compromise b/n # of pixels vs. file size
Color= light traveling in given wavelengths that can be detected by the
human eye (400-700 nm)
o Hue= actual color of the object
o Saturation= purity of light (i.e. light that is all one wavelength is very
pure and fully saturated)
o Intensity= relative lightness or darkness of an image
Medical Imaging 2: Image Processing
Noise Reduction
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Noise= differences in pixel values that are not related to the anatomy or
physiology of the subject; noise is BAD
Filters= applied to an image to reduce noise
Registration
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Def.= process of bringing two or more images into spatial correlation
(“matching”)
Intermodal Registration= comparing 2 images taken with 2 different
techniques (i.e. CT scan and fMRI done on the same pt.)
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Intramodal Registration= longitudinal monitoring of a disease; images taken
of the same person with the same techniques but at different times.
Doppler Imaging
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Based on the principles of ultrasound
The detector measures the frequency shift caused by a moving reflector
Vascular Doppler the moving reflector is the blood cells in the vessels
They can measure blood velocity (blood flow) and produce maps of
vasculature
Preferred Doppler angle ranges from 30*-60*; <30*= refraction and critical
angle interactions and >60*= small errors in angle can results in large errors
in velocity
Safety- at high intensities biological effects are caused by thermal and
mechanical mechanisms
o Can causes heat deposition at the bone-soft tissue interface
o Mechanical- cavitation can occur along with creating free radicals
Magnetic Resonance Imaging
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Uses magnetic fields to create images
When protons are placed in the field, they can receive and then transmit
electromagnetic energy
Protons in the magnetic field absorb energy and they release or reradiate this
energy and return to equilibrium (ground state)
Relaxation= return of the nuclei to their resting state= May be T1 or T2
Bone is DARK in an MRI but white in an X-ray or CT scan
Different tissues have different relaxation times
MRI is a sensitive test for detection of diseases because it is sensitive to
water: more water in tissue= more intense MRI signal
o The exact pathology is more difficult to determine
Instrumentation
o Tube= bore of the magnet
o Strength of the magnet is measured in Tesla; another measure
commonly used is the Gauss (1 tesla= 10000 gauss)
o Currently used magnet strengths for MRI’s are 0.5 tesla to 2.0 tesla;
Earth’s magnetic field= 0.5 gauss
Magnets
o Resistive magnets: many coils wrapped around the bore through
which an electric current is passed, electricity turned off= magnetic
field dies
o Permanent magnet: magnetic field is ALWAYS there and at full
strength
o Superconducting magnets: most COMMONLY used, coils made with
electric wire (which is bathed in liquid helium) through which a
current passes
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Risks:
o Really NONE, doesn’t use radiation or contrast so NO tissue damage
o Contraindications: pregnant, anything metal, pacemakers or metal
valves, joint replacement or internal clips
GOOD anatomic resolution and can be used for making 3D images from
various angles
BAD: machines are slow and expensive
TECNIQUE of CHOICE for imaging SOFT TISSUE
Functional Magnetic Resonance Imaging
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By looking at blood flow, functional changes in the tissue can be detected
o Based on a major assumption that tissue that is more active will have
more blood flow; not been proven to be true in all situations
Most frequently used in the brain; more blood flow to an area is assumed to
mean positive neural action
1st a traditional MRI image is produced, then the area of higher blood flow is
highlighted and overlaid on traditional MRI image
Limitations w/fMRI:
o Blood flow= more active; not proven
o Time limitation of the scan; blood flow may be increasing to one part
of the brain during the beginning of the scan, but flow may be
increased to another part of the brain by the end of the scan
o Certain parts of the brain are more difficult to image; i.e. cerebellum
CONTRAST RADIOLOGY
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Uses X-rays and a contrast dye to enhance images of certain tissues
allows structures to be seen that aren’t normally visible with X-rays
Oral Contrast medias are to highlight the GI tract
o Patient asked to fast for several hours before procedure, patient
drinks chalky thick drink (~48 ounces)
o Barium sulfate is the most common oral agent used, tends to
precipitate when in solution and in body- acid/base balance must be
exactly correct and mixed with no ions (ions increase aggregation)
IV contrast media and procedures
Intravenous pyelography (great job illustrating detail within renal system)
o Examines the function of the kidneys by measuring the rate of
elimination of the media from the kidneys
Angiography
o Contrast media injected into the artery
o The vessels and the organ (or tumor) it supplies is illustrated
o GOLD STANDARD for vessel examination
Histology
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Def.= the analysis of the anatomy of cells and tissues using microscopic
techniques
General methods
o Obtain the tissue by needle biopsy or more commonly incision biopsy
o Needle biopsy especially difficult b/c the cells have lost their normal
arrangement within the tissue
o Smears- sample that are physically smeared across the slide; typically
done with blood
o Tissue preservation and preparation
 Must be a fixative—a chemical that stops the degradation of
the tissue that would normally occur
 Fixatives cross link the molecules in the tissue so they don’t
come apart
 Microtome- special instrument used to cut tissue
 Stains—bring out different features of a sample
 H & E stain- most common; stains the nuclei blue-purple
and the cytoplasm of the cell pink
 Masson’s Trichrome- collagen and other fibrous
molecules deep blue and stains the cell red
 Mallory’s Trichrome- collagen and extracellular matrix
proteins deep blue and other cells red and orange; used
for staining keratin
 Weigert’s elastic- specifically stains the elastic
extracellular matrix proteins like elastin
 Silver Stains- great for showing neurons; also used to
search for fungi
 Wright’s stain- used for blood cells stained red or pink,
nuclei is blue or purple and cytoplasm of cells is blue or
gray
 PAS (periodic acid-Schiff stain)- important for
identifying mucopolysaccarides (glycogen); blue nuclei
of cells and pink collagen
 Fontana-Masson or Schmorl’s method- to stain melanin
 Metachromasia- cellular cytoplasm red and the nuclei
and extracellular matrix blue
X-rays
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most common type of medical imaging
X-ray production
X-ray imaging
o Absorbed, reflected, refracted, penetrated
o X-rays rely on photons that are transmitted
o Most soft tissue looks the same b/c of the large amounts of water
o Bones- high absorption, white
o Air- very low absorption, black
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o Soft tissues- similar shades of grey
o Best used for examining tissues with extremely different atomic
characteristics and density
o Method of choice for identifying bone fractures
Risks of X-rays
o Total risk is small but higher than many of the other imaging
techniques
o Avoided in pregnant women
Computed Tomography (CT)
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Based on the same principles as x-rays; series of x-rays from different angles
WORSE spatial resolution than x-rays
BETTER contrast resolution so that certain types of tissue can be
distinguished
3-D image
Current CT scanners work at very high speeds; less than 30 seconds for a
complete scan of the abdomen-> important because this can allow the pt. to
hold breath and decrease the noise of the image
The thickness of the slice can be altered on a CT scan
Multiplanar- stack of CT images, displayed in sagital and coronal planes
Precautions:
o Very low risk to patient
o Don’t use on pregnant women
Ischemic stroke appears dark vs. hemorrhagic stroke appears white
Nuclear Diagnostics
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A.K.A. scintography or radionuclide imaging
Gamma Counters- imaging machines; DETECTS radiation, does not emit
radiation
Radiation is produced by trace amounts of radioactive material that are
usually injected into the patients
For Nuclear medicine it is important to use elements with short lifetimes;
otherwise the pt. would have lingering radioactivity in their bodies
Stable element- number of protons and the number of neutrons are equal
Isotope- # of neutrons does not equal number of protons
o Some isotopes have a stable nucleus (the imbalance of protons and
neutrons is not enough to cause the nucleus to change)
Decay- when a nucleus is in an excited state an is returning to its more stable
state
Radionucleotide Production
o Cyclotron- machine where stable elements are bombarded with
charged beams
o Inside the machine the charged particles are accelerated to high
energies
o They hit target materials and produce nuclear reactions
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*****LOOK UP NUCLEAR DIAGNOSTICS TO UNDERSTAND MORE CLEARLY**
Ultrasound Imaging
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uses high frequency sound waves to form images
sound waves are emitted by a handheld transducer, or probe, and the
returning wave, reflected from the interfaces b/n different tissues in the
body, is detected and used to form an image
Sound Waves
o A vibration passing through material
o Wavelength- distance b/n peaks
o Frequency- how fast the wave moves over time
o Amplitude- height of the wave
o Resolution and attenuation of the US beam depend on the wavelength
and frequency
o Wavelength determines the resolution along the direction of the beam
o High frequency wavelengths provide superior resolution and more
image detail than low frequency beams
o Low frequency beams have less attenuation with depth; thus they
have greater penetration than high frequency beams
o Thick body parts- 3.5 to 5 MHz is used
o Small body parts that are close to surface- 7.5 to 10 MHz use
o Attenuation can be measured to determine wave absorption
 2 MHZ beam has ~twice the attenuation of 1 MHz beam
Image processing
o May result in the LEAST clear images
o Signal-to-noise ratio: ability to detect small changes in the tissue
o Formation of shadows- NOISE
o Extremely difficult to get good US images on obese/overweight people
Most common use- viewing the fetus of a pregnant woman
US is thought to be the safest technique for pregnancy
Echocardiographs- specific type of US used to diagnose problems with the
heart
o Size, shape, and movement of the heart wall during contraction
o Can identify abnormal heart valves
o 1-D M-Mode- uses 1 beam of US and is the most common type to view
the left ventricle
o 2-D- produces a broader picture of the heart