• Medical imaging has been used since the discovery
of x-rays (1895)
• Other methods than x-rays (NM, US, CT, MR) have
become available during last 50 years
• Digital images on a monitor have replaced films
• New methods allow images (digital) to be
manipulated in variety of ways
• Measurements that use radiopharmaceuticals (radioactive form
of drugs) or another radioactive tracer
• Radiopharmaceutical has two components, a carrier and a
radionuclide. The carrier component (e.g. glucose)
concentrates in parts of the body with increased metabolic
activity. Therefore, over time the radioactivity accumulates in
the area with increased metabolic activity.
• Imaging doesn’t only show the anatomy (structure) of an organ
or body part, but the function of the organ as well.
• This "functional information" allows to diagnose certain
diseases and various medical conditions much sooner than
other medical imaging examinations.
• The radionuclides are absorbed by or taken up at varying rates
(or in different concentrations) by different tissue types. E.g.
the thyroid gland (kilpirauhanen) takes up more radioactive
iodine than other parts of the body.
• The amount of radiation that is taken up and then emitted by a
specific body part is linked to the metabolic activity of the
organ or tissue. For example, cells which are dividing rapidly
may be seen as "hot spots" of metabolic activity on a nuclear
medicine image, since they absorb more of the radionuclide.
• Radionuclide emits gamma rays (energetic photons of
electromagnetic radiation emitted in nuclear-energy-state
• Spatial distribution of radionulides is determined by γ-detector
• Current radionuclide imaging system uses a gamma camera
• A nucleus with measurable half-life (T½) of
• Nuclei with unfavorable neutron/proton ratio will
decay into stable nuclei by spontaneous emission of
• 99mTc (mostly used), 131I, 67Ga (soft-tissue tumors),
133Xe (lung ventilation), 101Tl (ischemia)
= λ/693.0T½ =
• Require the oral or intravenous introduction of very low-level
radioactive chemicals (radionuclides) into the body.
• The radionuclides are taken up by the organs in the body
• Radionuclides emit faint gamma ray signals which are
measured by a gamma camera.
• The gamma camera has a large crystal detector, which detects
the emitted radiation signal and converts that signal into faint
light. The light is then converted to an electric signal, which is
digitized and reconstructed into an image by a computer.
• The resulting image is viewed on the system monitor and can
be manipulated (post-processed) and filmed.
• Consist of collimator, scintillation detector, PM-tubes and
position logistic circuits
– is a pattern of holes through gamma ray absorbing material, that allows
the projection of the gamma ray image onto the detector crystal.
– allows only gamma rays traveling along certain directions to reach the
• Scintillation detector
– A gamma ray photon interacts with the iodide ions of the crystal by
means of the Photoelectric Effect or Compton Scattering. This
interaction causes the release of electrons which in turn interact with the
crystal lattice (kidehila) to produce light. (scintillation)
• Photomultiplier tube (PMT)
– instrument that detects and amplifies the electrons that are produced by
– At the face of a photomultiplier tube (PMT) is a photocathode which,
when stimulated by light photons, ejects electrons. This electron is
focused on a dynode which absorbs this electron and re-emits many
more electrons (usually 6 to 10). These new electrons are focused on the
next dynode and the process is repeated over and over in an array of
dynodes. At the base of the photomultiplier tube is an anode which
attracts the final large cluster of electrons and converts them into an
– Each gamma camera has several photomultiplier tubes arranged in a
• During nuclear medicine imaging body emits gamma
rays, which are similar to x-rays but have a shorter
• In x-ray imaging the radiation comes from an external
radiation source and then passes through the patient's
body before being detected.
• Nuclear medicine uses the opposite approach: a
radioactive material is introduced into the patient, and
is then detected by a machine called a gamma camera.
• Visualization of organs and regions within organs that
can not be seen on conventional x-ray images
• Space occupying lesions are seen as areas of reduced
radioactivity ("coldspot"); however, in some
instances, like bone scanning, areas of increased
activity ("hotspot") represent disease or injury.
• Therapeutic uses are: treatment of hyperthyroidism
(kilpirauhasen liikatoiminta), thyroid cancer, blood
imbalances and pain relief from certain types of bone
• Bone scanning:
– It can reveal if the cancer has spread beyond its primary site
and developed secondary cancer growths in the bones
– Metabolic changes caused by fine fractures, small tumors,
or degenerative diseases such as arthritis (niveltulehdus) can
• Heart Disease:
– cardiac angiography yields images of the beating
heart and the blood vessels (coronary arteries) that
supply the heart muscle (myocardium) with blood.
– A stress thallium nuclear medicine study shows the
function of the myocardium.
Single-photon emission tomography (SPET)
• To produce 3D-images data are collected by rotating
the gamma camera around the patient
• Profile is taken from each image of the sequence
• Image of the radioactivity distribution through the
patient is done with filtered back-projection
• SPET is used for cardiac imaging and imaging of
• Uses positron-emitting isotopes (11C, 15O, 13N)
• A positron emitted by a nucleus annihilates with an electron to form two, in
opposite direction moving γ-rays.
• Involves cross-sectional data acquisition and reconstruction much like CT
• Positron Emission Tomography, (PET) allows physicians to view biological
functions. Areas with increased metabolic activity show up as "hot areas" in a
• Physicians use metabolic information from PET in conjunction with other
diagnostic tests to assess certain cancers, cardiac diseases and brain disorders.
Abnormal increased metabolic activity may indicate a malignant cancerous
tumor. Abnormal metabolic activity may also indicate heart disease or brain
• Lung cancer imaging is a new, emerging application of PET and involves
inhalation of the radionuclide.
• Study of epilepsy (nervous system disorders that cause
• Evaluation of stroke (blood clot or bleeding in the brain)
• Study of dementia (for example in patients with Alzheimer's or
• Imaging and evaluation of brain tumors
• Evaluation of coronary artery disease and detection of transient
ischemia (poor blood flow)
• Differentiation between recurrent, active tumor growth and
necrotic (dead) soft tissue masses in cancer treatment patients
• Ultrasound is sound at frequency above 20kHz
• Diagnostic imaging uses frequencies 1-15MHz
• Based on reflection of sound waves at an interface
between two media of different acoustic impedance
• Relatively inexpensive, fast and radiation-free
• Pulse-echo technique produces images of soft tissues
and fluid filled spaces (e.g. heart, pelvis, kidneys)
• Doppler imaging provides a means of measuring
blood flow in arteries and veins
• No ionising radiation
• Soft tissues can be imaged directly without the
injection of any sort of radio-opaque substances or
• Entire abdomen and pelvis can be rapidly scanned
while the patient is lying on the table and images can
be made of the area in question
• Real-time images
• Organs filled with air (e.g. lungs, stomach and intestines) and
hard tissues (bone) are opaque to sound
• Sound is not able to travel through certain organs → the
interiors of these organs and those lying directly beneath them
cannot be imaged
• Formerly it was almost impossible to view the cervix
(kohdunkaula) and lower uterus (kohtu) because they lay under
the air-filled intestines. If patient drinks fluid one hour before
an ultrasound exam, the distended bladder will push the air-
filled intestines out of the way and permit sound to reach the
• The air layer between the patient’s skin and the transducer is a
barrier for the sound. To overcome the reflections, some
lubricant with similar acoustic impedance to tissue is applied
on the patient’s skin.
• An ultrasonic pulse emitted from a transmitter moves through
the medium and some of its energy reflects back from the
objects within the body
• Interfaces between different structures in the body produce
echoes at different times
• If the velocity of the pulse is known, the time taken for the
echo to return can be translated into distance from transmitter.
• When the position and the orientation of the transducer and
receiver are known, image of the stuctures within the body can
• The skin is covered with mineral oil
• Transducer is connected to a console with a television screen and
placed against the skin near the region of interest.
• Transducer emits sound and receives sound.
• Transducer produces a stream of inaudible, high frequency sound
waves which penetrate into the body and bounce off the organs
• Transducer detects sound waves as they
bounce off or echo back from the internal
structures and contours of the organs.
• Different tissues reflect these sound waves
• Received waves converted to electrical signals
and turned into live pictures with computers.
• Piezoelectric crystal is used to excite and detect ultrasound
• Applying AC voltage across the crystal faces causes a changing
mechanical strain → pressure wave at certain frequency
• Conversely, ultrasound waves received at the transducer causes
the crystal to vibrate → alternating electric voltage
• Detailed images of the fetus and uterus.
• Ultrasound is very operator-dependent.
• Transabdominal prenatal ultrasound is used to check on the
development of the fetus and look for abnormalities e.g..
– Determining the age of the pregnancy
– Examining the baby's physical development and functions
– Imaging the limbs and spinal column to check for proper formation and
– Imaging the development of the brain and other major organs
– Determining whether the pregnancy is ectopic or whether there is a
– Guiding other prenatal tests (e.g.. Amniocentesis)
– Guiding prenatal surgery and the safe delivery of medications to the
fetus the uterus)
• Used to determine velocities.
• Based on the Doppler frequency shift (fd), which is
related to the velocity of the object (v)
• Can be used to measure blood flow
• An application of nuclear magnetic resonance
• Can be used to image nuclei with magnetic moment
• Produces images with high contrast between
• Nuclear density and recovery times vary from a
tissue to another
• Safe procedure (non-radioactive)
• Can measure properties which reflect the chemical
environment of the nucleus
• Can also visualize the flow in the blood vessels
• Measures the density of protons (1H) in the body
as a function of the position
• In the human body there is a lot of hydrogen
(mainly in water and fat)
• MRI is used to image soft tissues with high
proton concentration (e.g. brains)
• A fundamental property of nature
• Comes in multiples of ½ and can be + or -
• Almost every nucleus has an isotope with nuclear spin
• Nuclear spins having the opposite sign can pair up to
eliminate the observable manifestation
• Outside magnetic field nucleus are randomly oriented
• In magnetic field (B0) nuclei align themselves parallel or
antiparallel to the magnetic field
– There are little more nuclei in lower energy state
– Net magnetic moment (sum of individual magnetic moments) parallel
• In magnetic field protons’ magnetic moment vector
precess around the magnetic field at Larmor frequency
• At equilibrium the net magnetization is parallel to B0
• When placed in a magnetic field, a particle with a net
spin can absorb RF-pulse’s photon at Larmor
frequency → net magnetization turns to tranverse
• After RF-pulse the longitudial
magnetization recovers and net
magnetization returns to the
• Precession of transverse magnetization induces a
electrical signal in coil
• Depends on nucleus’ gyromagnetic ratio γ and the
strength of the magnetic field B0
• Similar nuclei can be differentiated from each
other by changing the magnetic field strength
γω = B0
• Net magnetic vector M consists of two
components Mz and Mxy
• Two relaxation processes: longitudial T1 and
• Describes the recovery of the longitudinal
magnetization (Mz) to its equilibrium state
• Nuclei returns by dissipating their excess
• Depends on the surrounding having at Larmor
frequency fluctuating magnetic field
z eMM −
• Net field B0 around the protons differ from proton to proton
→ protons don’t precess at the same angular velocity
• Transverse magnetization (Mxy) decays because its magnetic
moments get out of phase
• Loss of coherence is irreversible and results in Mxy falling to
• T2 is always less than or equal to T1
• Large molecules, which move slowly, promote T2 relaxation
AMP LIT UDI-
SYNT ETISAAT T ORI
Structure of MR-scanner
• Main parts are: magnet,
gradients and coils
• A magnetic field that increases in strength along a particular
• There are x, y and z gradients
• Gradients are resistive electromagnets consisting of metallic
coils driven by power amplifier
• The strength of the gradient refers to the rate at which its
magnetic field changes with distance
• If a sample is in uniform magnetic field, the Larmor
frequency will be the same of all parts of the sample.
• The size of the returning signal reflect the total number
of protons in the sample
• To localize the signal to a particular point in space, the
applied field is made non-uniform.
• The localization is done with slice selection, phase and
• Z-gradient generates an extra field Gz in z-direction
• Larmor-frequency is now a function of the position in z-
( ) ωωγω ∆+=+= 00 GzB
9.7 MHz 9.9 MHz9.8 MHz
• Phase encoding in slice
– Protons in higher field has a higher angular velocity of precession
than protons in lower field
– Initially all components are in phase, but gradient field Gy causes
the components become out of phase
– After a while gradient field is removed and all the components
precess again at the same frequency but in different phase
• With frequency encoding the protons are localized in x-direction
9.7 MHz 9.9 MHz9.8 MHz
• Slice selection with a gradient Gz
• Phase encoding with Gy and frequency encoding with Gx
• Echo signal from the nuclides received bu a receiving
• Received signal is amplified and digitized for the image
• After time TR the process is repeated with different Gy
• This is continued until all the data are collected
• An image is done from the data with help of Fourier
• Computed Tomography (CT) imaging combines the
use of a digital computer together with a rotating x-
• Creates detailed cross sectional images or "slices" of
the different organs and body parts such as the lungs,
liver, kidneys, pancreas, pelvis, extremities, brain,
spine, and blood vessels.
• CT has the ability to image a combination of soft
tissue, bone, and blood vessels.
• CT imaging provides both good soft tissue resolution
(contrast) as well as high spatial resolution.
• A rotating frame has an x-ray tube mounted on one side and
a serie of detectors on the opposite side.
• A fan beam of x-ray is created as the rotating frame spins the
x-ray tube and detector around the patient.
• As x-rays pass through the body they are absorbed or
attenuated (weakened) at differing levels creating a profile of
x-ray beams of different strength.
• The amount of attenuation of the beam depends on the
density of the structures the beam passes through at different
angles. The densest structures in the human body (like bones
and teeth), which attenuate x-rays most, appear most brightly
in CT images.
• A rotating frame has an x-ray tube mounted on one
side and a detector mounted on the opposite side.
• A fan beam of x-ray is created as the rotating frame
spins the x-ray tube and detector around the patient.
• As x-rays pass through the
body they are absorbed or
attenuated (weakened) at
differing levels creating a
profile of x-ray beams of
• As the x-ray tube and the detector make a 360°
rotation, the detector takes numerous profiles of the
attenuated x-ray beam.
• ”Slice" is collimated (focused) to a thickness between
1 mm and 10 mm using lead shutters in front of the x-
ray tube and x-ray detector.
• Each profile is subdivided spatially by the detectors
and fed into channels.
• Each profile is then backwards reconstructed by a
dedicated computer into a 2D-image of the “slice”.
• Enables direct imaging and differentiation of
soft tissue structures (liver, lung tissue, and
• Useful in searching for large space occupying
lesions, tumors and metastasis (etäpesäke) and
examining of their size, spatial location and
extent of tumors.
Virtual reality 3-D image of the
lungs. The bronchial trees are
colored in green and the heart,
aorta and vertebrae are colored in
• Scanner collects data for a single slice, so the position
of the appropriate slice needs to be known.
• The x-ray power was transferred to the x-ray tube
using high voltage cables.
• The rotating frame would spin 360° in one direction
and make an image (or a slice), and then spin 360°
back in the other direction to make a second slice.
• In between each slice, the gantry would come to a
complete stop and then reverse directions while the
patient table would be moved forward by an
increment equal to the slice thickness.
• In the mid 1980's, an innovation called the power slip
ring allows electric power to be transferred from a
stationary power source onto the continuously rotating
• CT scanners with slip rings can rotate continuously.
• Patient is moved continuosly through the detector ring
while the source traces a spiral around the patient.
• It acquires a volume of data with the patient anatomy
all in one position.
• This volume data set can be computer-reconstructed
to provide 3D-pictures of complex blood vessels like
the renal arteries or aorta.
• The primary digital technique for imaging the chest,
lungs, abdomen and bones due to its ability to
combine fast data acquisition and high resolution in
the same study.
• It can provide detailed information of nearly every
organ in the upper abdomen and pelvis in one quick
• New "multi-slice" spiral CT scanners can collect up to
four slices of data during spiral CT mode and some
rotate at speeds up to 120 rpm (previously 60rpm).
• Measures the distribution of impedance in a cross-
section of the body.
• Does not use ionising radiation
• Safe and often pleasant method for the patient.
• Electrical resistivities of different body tissues
varies widely from 0.65 ohm m for cerebrospinal
fluid to 150 ohm m for bone.
• Measure of how electricity travels though a given
• Every tissue has different electrical impedance
determined by its molecular composition.
• Eg. cancerous breast tissue has a much lower
electrical impedance than normal tissue and non-
• Cancerous tissue causes alterations in intracellular and
extracellular fluid compartments, cell membrane surface area,
macromolecules, ionic permeability, and membrane associated
• These changes cause measurable changes in tissue electrical
• When a small alternating current is placed across the area of
interest, the focal increase in electrical conductance and
capacitance of the cancer tissue distorts the electric field.
• The impedance map shows the cancer as a focal brightness on
the gray scale image of conductivity and capacitance measured
by an array of signal sensors on the skin surface.
• A series of electrodes are attached to a subject in a
• Electrodes are linked to a data acquisition unit which
outputs data to a PC.
• By applying a series of small currents to the body
potential difference measurements can be made from
non-current carrying pairs of electrodes.
• Since electric currents take the paths of least impedance,
the currents flow depends on the subject's conductivity
•AC voltage is connected to the wand held by the patient
•Electrical current flows through the body from the wand
to the scanning probe.
•The scanning probe is moved over the breast and its
many sensors measures the current signal at the skin
• The computer reconstructs the information and shows
images immediately on the monitor.
•Impedance objects are defined as spots or regions that
are brighter (or darker) than their surrounding.
Tomography (EIT) of the
human thorax. The images on
the left are of six normal
subjects. On the right are six
images of patients with lung
water associated with cardiac
failure. The fluid in the lungs is
• Medical Physics and Biomedical Engineering (Chapter 12)
• Imaginis (http://www.imaginis.com/)
• Philips Medical Systems (http://www.medical.philips.com)