Medical Imaging

Gary Arbique, Ph.D.

UTSW Medical Physics Division
 Medical Imaging Time Line

• 1895 - First x-ray image of human hand

• 1914 - First practical fluoroscope
• 1924 - Blumgart & de Hevesy perform first
clinical tracer studies
• 1948 – Fluoroscopic intensifier tube
• 1952 - Wild & Reed: echo ultrasound image
• 1957 - Anger invents gamma camera
• 1972 - Hounsfield: x-ray computed tomography
• 1973 - Lauterbur: magnetic resonance imaging
 Electromagnetic Spectrum
• aside from ultrasound, medical imaging relies on
electromagnetic radiations
• high and low energy portions of the spectrum used in
medical imaging
• mechanism of interaction depends on energy of the
radiation

Wavelength (nm)
1015 1012 109 106 103 1 10-3 10-6

103 106 109 1012 1015 1018 1021 1024

Frequency (Hz)

Radio, Television, Radar, MRI Ultra

violet
Infra Gamma rays
red
Radiant Heat X-Rays
Cosmic Rays

10-12 10-9 10-6 10-3 1 103 106 109

Energy (eV)
Visible light

red yellow blue

orange green violet
 Contrast

• imaging relies on contrast differences

• in diagnostic imaging, contrast must
distinguish anatomy, and/or physiological
processes
• different imaging modalities produce
contrast through differing physical
processes
• various modalities offer advantages and
disadvantages
 X-Ray Modalities

• x-ray modalities are the most common

imaging modalities in medical diagnostic
imaging
• modalities include:
– Radiography
– Fluoroscopy
– Computed Tomography
 X-Ray Contrast

• low energy x-rays produce contrast through

absorption in tissue
• relative absorption depends on tissue density
and atomic composition
• down-side: absorption and scattering results
in ionization (radiation dose) and potential
biological damage, however, benefit outweighs
risk
 X-Ray Imaging Basics Source

• source produces collimated beam

of x-rays
• x-rays absorbed, scattered or
transmitted through patient
• if imaged, scattered x-rays
reduce contrast, typically
removed by a grid
• receptor captures an image of patient
the transmitted x-rays
grid

receptor
 X-ray Production
kVp
rotating
x-ray tube anode
vacuum jacket

electron
filament x-ray
emissions

• X-ray vacuum tube: apply an DC voltage (kVp) between

a cathode (electrode filament) and anode
• high energy electrons striking the anode produce
– heat (typically > 99% of electron energy),
– bremstrahlung radiations, and
– characteristic x-ray radiations
 Anode Target X-Ray Spectrum
• polyenergetic bremstrahlung (i.e.,
braking radiation) spectrum, and
• monoenergetic characteristic Tungsten Anode X-Ray Production (100 kVp)

(fluorescent) spectral lines K shell Characteristic

X-rays

• upper energy limit set by

Kα

Relative Photon Intensity

generator kVp (typical diagnostic Kβ

energies 50 – 120 kVp)

• in practice, lower energy x-ray
spectrum preferentially
attenuated (filtered, hardened)
by inherent and added filtration
• attenuation desirable since low
energy x-rays otherwise totally 20 60 80 100
40
absorbed in patient, and Photon Energy keV
contribute disproportionately to
patient dose
 Radiography
• radiography or plain x-rays, the
most common x-ray imaging
modality
• in radiography, static anatomy
images produced, typically on
Wall Stand
film
• film not very sensitive to x-
rays, fluorescent “screen” used
to convert x-rays to visible
light and expose film
• typical radiography suite
comprises a gantry mounted
tube, a table, and a wall stand
Table
 Radiograph Example

• plain x-rays used to image most

aspects of anatomy
• chest x-ray a common
radiographic procedure
• negative image produced for
reading by radiologist
• dark image regions correspond to
high x-ray transmission
• image visualizes lung field and
silhouette of mediastinum
• used to diagnose lung and
mediastinal pathologies (e.g.,
pneumonia, and cardiomegaly) Pnuemocystis
 Contrast Enhancement
• contrast agents (dyes) can be
injected into the blood vessels
(angiograms) and cavities to improve
visibility
• for example: iodine and barium
absorbs more x-rays than tissue

Air-Contrast Barium
Enema

Cerebral Arteries
 Fluoroscopy
• fluoroscopy used to obtain real time x-ray images
• image receptor converts x-ray image into a TV signal
• video images can also be recorded (film, video-tape)

Input
flourescent
screen Vacuum Output
jacket flourescent
screen

Lens

X-ray
TV
photons Photoelectrons
Camera

Visible
light

Focusing
Photocathode electrodes
 Fluoroscopy Suites

• table and c-arm arrangements

available
• fluoroscopy typically used for
observing the digestive tract,
catheter guiding, and cardiac
angiography
 X-ray Computed Tomography (CT)
x-ray
tube
• conventional x-rays are projection
collimated images, and overlying structures can
x-ray beam
obscure anatomical details
• in CT slice projections (profiles)
through patient measured by a
detector array
detector
array
tube and detector
array rotated
around patient
• by rotating the tube and detector
array, profiles are taken at multiple
angles
• a computer then processes the
profiles using a mathematical
algorithm (convolution) to create a
cross-sectional image on a video
screen detector
array
 CT Scanner

• cowling covers rotating tube and detector electronics

• central port and table for patient
• computer console for control and image viewing
 CT Slice Images

abdominal scan
spleen/liver level

abdominal scan at
kidney level head scan showing
ventricles

• CT eliminates the shadow overlap problem of conventional X-rays

• contrast agents commonly used in CT
 Helical CT
• modern CT scanners use continuous
tube rotations and table translation
• with respect to patient, the tube
follows a helical path Simulated helical x-ray beam path for a
• results in faster scans (e.g., a scan of the of the abdomen. The
highlighted area is a man's stomach (man
single breath hold lung scan) is lying on his back with his arms over
• helical scan profiles are interpolated his head).

to form slice images

• modern computer reconstruction can
reformat data to view slices at
arbitrary angles
• three-dimensional rendered images
of complex blood vessels like the
renal arteries or aorta are also
possible

3D rendering of kidneys
3D Rendered CT Images

Heart Colon Fly Through

 Nuclear Medicine Imaging
• radio-isotopes are natural and artificially produced
unstable isotopes that decay through gamma-ray and/or
particulate emissions (e.g., positrons)
• ideal imaging isotopes feature low dose to the patient
(e.g., short physical and/or biological half lives)
• medical isotopes produced in nuclear reactors and by
particle accelerators
• nuclear medicine images visualize radioisotope
concentrations
• by “tagging” radio-isotopes to biological molecules,
physiological processes can be measured
• nuclear imaging is functional, not anatomic
 Planar and SPECT Cameras
• relies on isotopes that emit γ-rays (e.g., 99mTc)
• planar camera comprises a collimator, scintillator crystal (e.g., NaI)
and a light detector array
• by rotating a planar camera, data for tomographic images acquired
• SPECT an acronym for single photon emission computed tomography

light detector array

side view of
scintilator crystal planar
detector
assembly
collimator

source
 SPECT Camera & Images

sagittal

transaxial
rotating planar SPECT camera
coronal

Tc-99m HMPAO SPECT perfusion

images, showing decreased blood
perfusion to posterior frontal and
anterior temporal lobes
 PET Imaging
• some radio isotopes decay with the
emission of a positron (e.g., 18F)
• positrons annhilate with electrons detector
element in
shortly after emission, resulting in detector
emission of two coincident 511 keV ring

photons traveling in opposite

directions
• positron emission tomography (PET)
camera detects coincident photon source
emission
emissions to form tomographic data
sets for computer image
reconstruction
• PET has higher sensitivity and PET camera
resolution than SPECT
• 18FDG commonly used in PET to
detect increased cellular metabolism
(e.g., detecting and staging cancer)
State of the Art PET-CT Scanner

• PET-CT systems generate PET

functional and CT anatomy
images of a patient in a single
study
 3D Image Co-registration

PET Uptake CT Anatomy Co-Registered

• functional and anatomical images registered and fused to

form a single image
 Magnetic Resonance Imaging (MRI)
Basic Physics

• protons and neutrons have a quantum

property called spin
• in a classical description, the spin
property is similar to a tiny magnet
• in a strong DC magnetic field, spin
moments align with the magnetic field
to form a net magnetization

• hydrogen atoms have a net spin

moment and the high content of H2O in
tissue makes it ideal for anatomical
imaging
 Magnetic Resonance Imaging (MRI)
Basic Physics
• if disturbed (tipped) from steady-state alignment, the
magnetization vector will precess at a characteristic (Larmor)
frequency about an applied magnetic field (much like a nutating
spinning top)
• the Larmor frequency is proportional to the magnetic field
• relaxation mechanisms cause the tipped magnetization to relax
back to the steady state
• the magnetization can be tipped by transmitting electromagnetic
waves at the Larmor frequency
• signal from the precessing magnetization can be detected by a
receiver antenna

• magnetic fields used in MRI are typically > 1 Tesla (i.e., ~20000 x
earth’s magnetic field), here the hydrogen Larmor frequency is ~
42 MHz (i.e., close to TV and FM frequencies)
 MRI Image Aquisition
• imaging details very complex, however, “pulse sequences” are
used to excite and receive magnetization signals
• small magnetic field gradients are applied in addition to the
strong DC magnetic field
• the resulting Larmor frequency shifts encode signal levels as a
function of position

• pulse sequences are characterized by a repetition time (TR)

and a receiver delay time (TE)
• TR and TE are used to control signal levels from different
tissues

• receiver signals computer analyzed to form an image

• mathematical algorithms based on the Fourier Transform
• tomographic and volume images can be aquired
 MRI Image
Contrast
• contrast in MRI produced by variations
in proton density and by variations in
relaxation times in different tissues
• paramagnetic contrast agents can be
Spin-Echo Sequence
employed to enhance contrast TR = 250 ms
TE = 20 ms

• MRI offers anatomical diagnostic

imaging features similar to CT, but, with
“tuneable” contrast
• however, MRI does not image bone well
because of low water content

• magnetic and rf fields levels used in

MRI have no known biological risks
Spin-Echo Sequence
TR = 2000 ms
TE = 80 ms
 MRI Scanner

• solenoid magnet field scanner looks

much like a CT scanner
• rf transmitter/reciever coil typical in
scanner cowling, however, special coil
assemblies for head and extremities MRI Scanner
imaging used
• scanner situated in a rf shielded room
• ferromagnetic metals, pacemakers
are not compatible with MRI exams
• MRI is noisy (knocking noises) due
rapid application of imaging field
gradients

“bird cage” coil assembly

for head imaging
 BOLD Imaging (fMRI)
• MRI can measure
physiological processes (i.e.,
functional MRI)
• oxy- to deoxy-hemoglobin
results in changes to local
relaxation processes thus
affecting signal level
• technique called blood
oxygen level-dependant
(BOLD) MRI
• hemodynamic changes can
be imaged and correlated
to neuronal activity
• subjects given specific
challenges during imaging to
isolate associated activity motor strip localization
areas in the brain
(co-registered 3D image)
 Ultrasound (US) Imaging
• US uses high frequency (> 1 MHz) ultra-sound waves (i.e., not
electromagnetic) to create static and real time anatomical images
• contrast results from reflections due to sound wave impedance differences
between tissues
• at diagnostic levels, no deleterious biological effects from US pulses
• technique similar to submarine ultrasound, a sound pulse is sent out, and the
time delays of reflected "echoes" are used to create the image
• image texture results from smaller scatters (diffuse reflectors)
• boundaries result from specular reflections (large objects)
diffuse
reflections
from small
US pulse objects (~ λ)

US
Probe

specular
reflections
from large
objects (>> λ)
 US Images
• by sending pulses out along different directions in a plane, slice images of
anatomy are produced for viewing on monitor
• US does not work well through lung or bone, used mainly for imaging
abdominal and reproductive organs
• one of the most well known US procedures is the examination of the living
fetus within the mother's womb
• 3D imaging scanners now available (real time, so called 4D)
 US Scanner
• scanner features probes, data
processing computer, and image
viewing monitor
• probes specialized for exam
requirements
• modern probes feature phased
US Scanner transmit/receiver arrays to
electronically steer and focus the
US beam

US Probes
Doppler Ultrasound Measures Blood Flow
• using a special form of US called Doppler (just like police speed
RADAR) the speed and direction of flowing blood can be measured and
illustrated in color images
• Doppler US allows Radiologists to image vasculature and detect
blocked blood vessels in the neck, and elsewhere
 Transition to a Digital
Imaging Environment
• modern radiology is making a
transition to a digital imaging
environment or PACS (picture
archiving and communications
system)
• advantages include efficient film (hard-copy) reading
image distribution and reduced
storage requirements
• integral to PACS, is digital image
acquisition
• computer based modalities
inherently digital
• film based modalities now being
phased out by digital technologies

PACS soft-copy reading

 PACS Environment Example
Web Archive
Main Reading Room 1 Access Server
Data

Main Reading Room 2

Reading TCP/IP
Stations Wide Area Network Printing

CT Reading Room

Information Services Acquisition

CR Station CT Station Digitizer

Hospital Radiology
 Digital Imaging Technologies
Replacing Film in Radiology

DR: digital radiography uses an

x-ray imaging detector
electronically connected to a
readout system (e.g., flat panel
detectors).

CR: computed radiography

uses a storage phosphor x-
ray imaging detector which
is read out by a separate
digital reading device
Suggested Reference Material
• Physics of Radiology, A. B. Wolbarst,
Medical Physics Publishing (ISBN 0-
944838-95-2)
• Search the Internet
Role of the Physicist in Diagnostic Radiology
(revenge of the nerds)
• Ensure equipment is producing high quality
images
– image quality control
– periodic checks of equipment
– supervise preventive maintenance

• Reduce dose to patients and personnel

– monitor radiation dose records
– evaluate typical doses for procedures
– recommend equipment changes and/or dose
reduction strategies
 Medical Physics Skills
• Measure Radiation Output
• Calculate Radiation Dose to Tissues

• Supervise Radiation Safety Program

• Evaluate Equipment for Purchase
• Image Processing
• Computer Programming & Networking
• Teach Physics to Radiology Residents