Bioimaging

biomedical imaging

allows us to visualize biological structure and function at macroscopic and microscopic levels

first biomedical imaging technology

x-ray (William roentgen, 1895)

x-ray

wavelength of 0.1 nm, form of ionizing radiation

how x-rays work

body is translucent to x-rays of correct wavelength, can partially penetrate or be absorbed depending on density of tissue

x ray (plain film radiography)

projection image (shadow of 3D image), can show fractures/breaks in bones, cavities, fluid in lungs, cancer in breasts; good contrast if there is difference in density of tissue (not good at visualizing soft tissue), contrast agents can be added to enhance

x ray computed tomography (CT)

x-ray images taken from multiple projections: rotating gantry with x-ray source and detector array around axial plane of patient; computer can reconstruct data into 3D image: displayed in axial plane, weighted numbers assigned to represent different materials

x-ray computed tomography usefulness

contrast still limited in soft tissues, but versatile and valuable for imaging head, lungs, abdomen, pelvis, extremities; speed enables dynamic imaging of moving structures (heart); microCT allows imaging of very small structures

CT advantages over planar radiography

no superimposition of images outside area of interest, higher contrast, data from single CT can be viewed in multiple planes

contrast agents

radiocontrast agents, iodinated agents, barium-based agents

radiocontrast agents

used to enhance contrast in certain features

iodinated agents

for intravascular imaging

barium-based agents

for gastro-intestinal imaging

ultrasound imaging

uses sound, pressure waves that transmit through a medium, uses high frequency that can penetrate tissues

ultrasound image formation

frequencies between 2 MHz and 18 MHz, sound waves produced by transducer made of piezoelectric material, waves bounce off tissue interfaces and return to transducer generating voltage

piezoelectric material

resonate in response to voltage, when stimulated with a wave it creates voltage

ultrasound image formation (small details)

timing of arrival of pulses indicates depth of echogenic material, speed of sound assumed constant in tissue, measures strength of echo

speed of sound in tissue

c = 1540 m/s

time taken related to distance from transducer (ultrasound)

c = 2d/t

doppler imaging

ultrasound used to measure velocity of a tissue (blood flow)

doppler effect

apparent change in frequency of a wave (like sound or light) as the source or observer of the wave is moving relative to each other

doppler imaging (more details)

transducer produces signal of fixed frequency but the wavelength of echo can change depending on direction and angle of movement of object producing echo; toward transducer = shorter wavelength, away from transducer = longer wavelength, faster motion = larger change in freq

ultrasound other applications

determining elastic properties of tissues, microscopic bubbles as contrast agent/targeting agent, 3D imaging with additional rotation of transducers/arrays of transducers

ultrasound strengths

no ionizing radiation, inexpensive, centimeter-range depth penetration, fast scan times, can be made portable

ultrasound weaknesses

limited resolution, limited depth penetration, poor visualization of bony structures

nuclear medicine

first imaging modality designed to measure function within the body rather than structure; based on detection of radioactive molecules

radioactive molecules

unstable and spontaneously decay to release radiation energy such as gamma rays

radioactive decay

alpha, beta, gamma

alpha decay

emits an alpha particle (2 protons 2 neutrons), can be blocked by thin sheet of paper

beta decay

emits an electron or positron, can be blocked by aluminum sheet

gamma decay

emits gamma rays, very thick dense layer needed to shield

nuclear medicine process

radioactive molecules ingested, inhaled, or injected and radioactivity results in ionizing radiation; functional targeting possible

nuclear medicine imaging methods

planar imaging, single photon emission computed tomography (SPECT), positron emission tomography (PET)

planar and SPECT imaging

uses molecules that are chemically linked to radioactive elements that emit gamma rays upon decay

PET imaging

uses very short-live radioisotopes that emit positrons (beta decay), emitted positron encounters an electron and when 2 particles annihilate each other, 2 gamma rays are generated in exactly opposite directions

PET imaging (more details)

positron-emitting tracer element is conjugated to a biologically active molecule (ex: fludeoxyglucose)

gamma camera

detects gamma radiation generated in planar, SPECT and PET; assembled from collimator, scintillation detectors, photomultipliers

collimator

filters gamma rays

scintillation detectors

detect presence of gamma rays, crystalline materials exhibit luminescence when excited by ionizing radiation

photomultipliers

cover light energy to electrical energy

gamma camera for SPECT

cross-sectional images are produced by imaging at many angles and reconstructing cross-sections from projections

gamma camera for PET

coincidence detection is needed to identify the position of the positron from the simultaneous section of gamma rays on opposite sides of body

nuclear medicine imaging applications

functional imaging possible with tracers (blood circulation)

nuclear medicine imaging examples

location of damage caused by heart attack or stroke; bone growth, fractures, tumors, infections using bone scans; size, shape, position, irregularities in liver and spleen; blood flow, metabolism, neurotransmitter binding in brain scans

PET-CT and PET-MRI

functional images from positron electron tomography are often combined with and superimposed with images from computed tomography or magnetic resonance imaging

magnetic resonance imaging

uses magnet and alternating radio frequency field to alter the magnetic spins of nuclei in the body; as these nuclei rotate, a rotating magnetic field can be detected by the scanner

MRI pros

no ionizing radiation, 2D or 3D images of body, excellent for soft tissues

generation of MRI image

use a spatially varying magnetic field, leads to different precession frequency of protons at different regions of tissue, resulting MR signal from different regions of tissue would have characteristic frequency, frequency → location mapping

MRI more details

• tissues composed largely of water, protons in hydrogen nuclei have inherent ‘spin’ directions, randomly oriented in tissue

• in strong magnetic field, these nuclei align with field lines, protons experience motion similar to spinning top hit but small force → precession

• sending an RF pulse to magnetized tissues causes spins to shift 90 degrees

• most rapid relaxation is dephasing of the spins, FR energy is released and detected

• slower relaxation - restoration or original orientation

T1 vs T2 images

images reflect differences in relaxation rates between tissues

T1

grey matter = gray, white matter = whiter, CSF = black

T2

grey matter = white, white matter = dark, CSF = white

MR contrast agents

intravenously injected to enhance visibility of blood vessels, inflammation, tumors; typically composed of gadolinium compounds

MRI applications

soft tissue (brain, cartilage, muscle)

fMRI

functional MRI; imaging of brain by detecting changes to blood flow, magnetization of iron in hemoglobin is used to detect oxygenation of blood

MRI strengths

imaging of entire body at any depth, soft tissue visualization, no ionizing radiation

MRI weaknesses

very expensive, magnetic precautions must be taken, long time to perform scans

optical imaging

allows human vision to see inside body at small scales

types of optical imaging

microscopy, endoscopy (fiber optics), optical coherence tomography

microscopy types

optical microscopy, fluorescence microscopy, confocal microscopy

refraction

refraction of light through lenses allow us to see magnified images of objects through microscopy; path of light bends or refracts when traveling between material with different refractive indices

snell’s law

n1 * sin(theta)1 = n2 * sin(theta)2

compound microscope

objective magnification = M1, eyepiece lens magnification = M2, total system magnification = M1 * M2

histology

used to examine cells and tissues, medical diagnosis possible, samples need to be fixed and processed (chemical fixation, sectioning, staining), common stain: hematoxylin (nucleus) and Eosin (cytoplasm)

fluorescence microscopy

uses fluorophores that selectively stain to obtain functional information in images

fluorescence

1. absorption of photon excites the fluorophore, creating an excited electronic singlet state (S1’)

2. excited state lasts for few nanoseconds, flourophores go to relaxed single excited state (S1): energy dissipation

3. photon is emitted, returning the fluorophore to ground state S0. due to energy dissipation, energy of emitted photon of lower energy

fluorophores

emitted light will always be of lower energy, longer wavelength than the exciting light. bc the colors vary from use of different dyes, the exciting and emitting light are different and can be separated from one another incorporating use of optical filters

fluorophores details

fluorescein, alexafluors, rhodamine, cyanine, cosine; often conjugated to antibodies for immunofluorescence

biological fluorescent fluorophores

nucleic acid stains are used to stain cell nucleus, phalloidin stains actin fibers

fluorescent proteins

ex: green fluorescent proteins; cells or organisms can be transfected to genetically express GFP as a marker

confocal microscopy

thin sections of images are taken and can be stacked to produce 3D image, light emerging from points above and below selected focal plane are filtered out using a pinhole

endoscopy and fiber optics

endoscopy uses fiber optics to bring light into and out of body through passageways, allowing views of internal structures; endoscopes are long snakelike devices with internal fiber optics and channels for other instruments

optics fiber

fiber made of quality silica or plastic that can transmit light between 2 ends of the fiber

fiber optics

based on principle of total internal reflection (occurs at boundaries bw materials), can occur when light comes from high-refractive index material to low-refractive index material and if incident angle is shallow enough below a critical angle