bioimaging

biomedical imaging has allowed us to

visualize biological structure and function at macroscopic and microscopic levels

what was the first biomedical imaging technology for seeing inside the body

-William Roentgen, German physicist

-x-ray

-1895

x-rays

-Wavelengths of about 0.1 nm

-A form of ionizing radiation

-Useful for imaging

how do x-rays work on human

Human body is translucent to x-rays of

correct wavelength : can partially penetrate

or be absorbed depending on density of

tissue

x-ray computed tomography

• X-ray images taken as previously

described but from multiple projections; Rotating gantry with x-ray source and detector array around axial plane of patient

• Computer can be used to reconstruct

data into 3-D image; Usually displayed in axial plane, Weighted numbers assigned to

pixel values to represent different materials (bone, water, air)

X-ray Imaging (Plain Film Radiography)

-Creates a projection image: shadow of a 3D image

• Can show fractures and breaks in bones, cavities in teeth, fluid in lungs, cancer in breasts.

• Good contrast when there is a difference in

density of tissue: not good at visualizing

soft tissue (do not vary much in density)

• Contrast agents can be added to enhance

differences in density

advantaged of tomography vs planar radiation

• No superimposition of images outside area of interest

• Higher contrast

• Data from single CT imaging can be viewed in multiple planes

contrast agents

Radiocontrast agents are used to enhance contrast in certain features

iodinated agents

intravascular imaging

barium-based agents

gastro-intestinal imaging

ultrasound imaging

uses sound instead of light to create imaging

ultrasound image formation

-Frequencies between 2 MHz and 18 MHz are often used

• Sound waves are produced by the transducer made of a piezoelectric material: Resonate in response to a voltage, and when stimulated with a wave, creates a voltage

• Sound waves bounce off tissue interfaces and return to the transducer, generating a voltage

timing of arrival of pulses indicates

depth of echogenic material

speed of sound assumed to be constant in tissue

c = 1540m/s

time taken related to distance from transducer

c = 2d/t

doppler imaging

uses ultrasound to visualize and measure blood flow

doppler imaging process

-Transducer produces a signal of a fixed frequency, but the wavelength of the echo can change depending on the direction and angle of the movement of the objects producing the echo; Toward transducer= shorter wavelength, Away from transducer=longer wavelength

• Faster motion will generate larger change in frequency

• Angle that motion makes with ultrasound pulse will also change frequency (greatest effect when transducer is parallel to motion)

other applications of ultrasound imaging

• Determining elastic properties of tissues

• Microscopic bubbles as contrast agent, or targeting agent

• 3D imaging with additional rotation of transducers or 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 the detection of radioactive molecules, molecules that are unstable and spontaneously decay to release radiation energy, such as gamma rays

alpha decay

• Emits an alpha particle (2 protons + 2 neutrons)

• Can be blocked by a 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

ionizing radiation

Radioactive molecules are ingested, inhaled, or injected and radioactivity

results in ionizing radiation

nuclear imaging methods

• Planar Imaging

• Single Photon Emission Computed Tomography (SPECT)

• Positron Emission Tomography (PET)

Planar and SPECT imaging

Uses molecules that are, or are chemically linked to radioactive elements that emit gamma rays upon decay (γ decay)

PET imaging

• Uses very short-live radioisotopes that emit positrons (β decay) E.g., 11C, 13N, 18F

• Emitted positron encounters an electron, and when the two particles annihilate each other, two gamma rays are generated in exactly opposite directions

-positron-emitting tracer element is conjugated to a biologically active molecule

gamma camera

detects gamma radiation generated in planar, SPECT, and PET

what is a gamma camera assembled from

Collimator, Scintillation detectors, Photomultipliers

how is a cross-sectional image created for SPECT

imaging at many angles (e.g., with multiple cameras), and reconstructing cross-sections from projections

coincidence detection

needed for PET to identify the position of the positron from the simultaneous detection of gamma rays on opposite sides of the body

nuclear medicine imaging applications

• Location of damage caused by heart attack or stroke

• Decreased activity to indicate damage or restriction of blood flow (stroke, infraction, etc.)

• Bone growth, fractures, tumors and infections using bone scans

• Increased activity to indicate tumor or fracture

• Size, shape, position, irregularities in liver and spleen

• Blood flow, metabolism, neurotransmitter binding, in brain scans

what are functional images from positron electron tomography often combines and superimposed with

images from Computed Tomography or Magnetic Resonance Imaging

magnetic resonance imaging

-Uses a 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

• No ionizing radiation

• 2-D or 3-D images of the body

• Excellent for soft tissues

generation of an MRI image

• Use a spatially varying magnetic field

• This leads to different precession frequency of the protons at different regions of the tissue

• Resulting MR signal from different regions of a tissue would have a characteristic frequency

• Frequency → location mapping

T1 Image

• Grey matter = gray

• White matter = whit(er)

• CSF = black

T2 Image

• Grey matter = white

• White matter = dark

• CSF = white

what do T1 vs T2 images reflect

differences in relaxation rates between tissues

MR Contrast Agents

• Intravenously injected to enhance visibility of blood vessels, inflammation, tumors

• Typically composed of gadolinium compounds; Alter the relaxation times of tissues when present

MRI applications

excellent for soft tissues (brain, cartilage, muscle, etc)

fMRI (functional magnetic resonance imaging)

• Imaging of the brain by detecting changes to blood flow

• Magnetization of iron (Fe) in hemoglobin is used to detect oxygenation of blood

MRI strengths

-imaging of entire body at any depth

-excellent soft tissue visualization

-no ionizing radiation

MRI weaknesses

-extremely expensive machines

-magnetic precautions must be taken

-long time to perform scans

optical imaging

Allows human vision to see inside the body and at small scales

types of optical imaging

• Microscopy (optical, fluorescence, confocal)

• Endoscopy (fiber optics)

• Optical Coherence Tomography (near Infrared)

what does refraction of light through lenses allow us to see

magnified images of objects through microscopy

speed of light in a vacuum

c=299,792,458 m/s

refractive index

𝑛 = 𝑐/𝑣, where 𝑣= speed of light in given medium

snell’s law

𝑛1 sin 𝜃1 = 𝑛2 sin 𝜃2

compound microscope

• Objective Magnification = M1

• Eyepiece lens Magnification = M2 ;Lens of eye forms real image on retina

• Total system magnification = M1 * M2

histology

used to examine cells and tissues

common stains for histology

Hematoxylin (nucleus) and Eosin (cytoplasm)

fluorescence microscopy

Uses fluorophores (fluorochrome) that selectively stain to obtain functional information in images

fluorescence

1. Absorption of a photon excites the fluorophore, creating an excited electronic singlet state

2. Excited state lasts for a few nanoseconds, and fluorophores go to a relaxed single excited state

3. A photon is emitted, returning the fluorophore to its ground state.

-Due to energy dissipation, energy of emitted photon is of lower energy (longer wavelength)

fluorophores

• fluorescein (FITC), Alexafluors, rhodamine (TRITC), cyanine, eosine

• Often conjugated to antibodies for immunofluorescence

biological fluorescent fluorophores

• Nucleic acid stains are used to stain the cell nucleus (e.g., DAPI, Hoechst)

• Phalloidin stains actin fibers

fluorescent proteins

• E.g., Green fluorescent proteins (GFP), RFP, etc.

• 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 a three- dimension image

• Light emerging from points above and below a selected focal plane are filtered out using a pinhole.

endoscopy

uses fiber optics to bring light into and out of the body through passageways, allowing views of internal structures

endoscopes

long snakelike devices with internal fiber optics, and channels for other instruments

examples of endoscopes

Bronchoscopy, colonoscopy, arthroscopy, etc

optics fiber

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

fiber optics

• Based on the principle of total internal reflection

• Total internal reflection occurs at boundaries between materials

• Can occur when light comes from a high-refractive index material to a low-refractive index material, and if incident angle is shallow enough, below a critical angle

collimator

filters gamma rays

scintillation

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

photomultipliers

convert light energy to electrical energy

mason trichome purpose

differentiate collagen (and other connective tissues) from muscle fibers, cytoplasm, and nuclei. helpful for identifying fibrosis or tissue scarring

mason trichome principle

3-dye staining technique; stains nuclei dark blue/black, muscle and cytoplasm red, and collagen or mucus blue or green (depending on dye variant)

mason trichrome applications

commonly used in liver, kidney, and heart biopsies to evaluate fibrosis, smooth muscle changes, and structural organization of tissues