Minna Tuominen12.3.2002

• Introduction• Radionuclide imaging• Ultrasonic imaging• Magnetic resonance imaging• CT imaging• Electrical Impedance Tomography (EIT)

• Medical imaging has been used since the discoveryof x-rays (1895)

• Other methods than x-rays (NM, US, CT, MR) havebecome 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 formof drugs) or another radioactive tracer

• Radiopharmaceutical has two components, a carrier and aradionuclide. The carrier component (e.g. glucose)concentrates in parts of the body with increased metabolicactivity. Therefore, over time the radioactivity accumulates inthe area with increased metabolic activity.

• Imaging doesn’t only show the anatomy (structure) of an organor body part, but the function of the organ as well.

• This "functional information" allows to diagnose certaindiseases and various medical conditions much sooner thanother 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 radioactiveiodine than other parts of the body.

• The amount of radiation that is taken up and then emitted by aspecific body part is linked to the metabolic activity of theorgan or tissue. For example, cells which are dividing rapidlymay be seen as "hot spots" of metabolic activity on a nuclearmedicine image, since they absorb more of the radionuclide.

• Radionuclide emits gamma rays (energetic photons ofelectromagnetic radiation emitted in nuclear-energy-statetransition)

• Spatial distribution of radionulides is determined by γ-detector• Current radionuclide imaging system uses a gamma camera

• A nucleus with measurable half-life (T½) ofradioactive decay

• Nuclei with unfavorable neutron/proton ratio willdecay into stable nuclei by spontaneous emission ofnuclear particles

• 99mTc (mostly used), 131I, 67Ga (soft-tissue tumors),133Xe (lung ventilation), 101Tl (ischemia)

,eNN(t) to

λ= λ/693.0T½ =

• Require the oral or intravenous introduction of very low-levelradioactive 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 faintlight. The light is then converted to an electric signal, which isdigitized and reconstructed into an image by a computer.

• The resulting image is viewed on the system monitor and canbe manipulated (post-processed) and filmed.

• Consist of collimator, scintillation detector, PM-tubes andposition logistic circuits

• Collimator– 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 thedetector.

• Scintillation detector– A gamma ray photon interacts with the iodide ions of the crystal by

means of the Photoelectric Effect or Compton Scattering. Thisinteraction causes the release of electrons which in turn interact with thecrystal lattice (kidehila) to produce light. (scintillation)

• Photomultiplier tube (PMT)– instrument that detects and amplifies the electrons that are produced by

the photocathode– At the face of a photomultiplier tube (PMT) is a photocathode which,

when stimulated by light photons, ejects electrons. This electron isfocused on a dynode which absorbs this electron and re-emits manymore electrons (usually 6 to 10). These new electrons are focused on thenext dynode and the process is repeated over and over in an array ofdynodes. At the base of the photomultiplier tube is an anode whichattracts the final large cluster of electrons and converts them into anelectrical pulse.

– Each gamma camera has several photomultiplier tubes arranged in ageometrical array.

Forte (Philips)

• During nuclear medicine imaging body emits gammarays, which are similar to x-rays but have a shorterwavelength.

• In x-ray imaging the radiation comes from an externalradiation source and then passes through the patient'sbody before being detected.

• Nuclear medicine uses the opposite approach: aradioactive material is introduced into the patient, andis then detected by a machine called a gamma camera.

• Visualization of organs and regions within organs thatcan not be seen on conventional x-ray images

• Space occupying lesions are seen as areas of reducedradioactivity ("coldspot"); however, in someinstances, like bone scanning, areas of increasedactivity ("hotspot") represent disease or injury.

• Therapeutic uses are: treatment of hyperthyroidism(kilpirauhasen liikatoiminta), thyroid cancer, bloodimbalances and pain relief from certain types of bonecancers.

• 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) canbe seen

• Heart Disease:– cardiac angiography yields images of the beating

heart and the blood vessels (coronary arteries) thatsupply the heart muscle (myocardium) with blood.

– A stress thallium nuclear medicine study shows thefunction of the myocardium.

Single-photon emission tomography (SPET)

• To produce 3D-images data are collected by rotatingthe 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-projectionreconstruction algorithm

• SPET is used for cardiac imaging and imaging ofbrain perfusion

• Uses positron-emitting isotopes (11C, 15O, 13N)

• A positron emitted by a nucleus annihilates with an electron to form two, inopposite direction moving γ-rays.

• Involves cross-sectional data acquisition and reconstruction much like CTscanning.

• Positron Emission Tomography, (PET) allows physicians to view biologicalfunctions. Areas with increased metabolic activity show up as "hot areas" in aPET image.

• Physicians use metabolic information from PET in conjunction with otherdiagnostic tests to assess certain cancers, cardiac diseases and brain disorders.Abnormal increased metabolic activity may indicate a malignant canceroustumor. Abnormal metabolic activity may also indicate heart disease or braindisorders.

• Lung cancer imaging is a new, emerging application of PET and involvesinhalation of the radionuclide.

• Study of epilepsy (nervous system disorders that causeconvulsive seizures)

• Evaluation of stroke (blood clot or bleeding in the brain)• Study of dementia (for example in patients with Alzheimer's or

Parkinson's disease)• 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

imaging modality• 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 orisotopes

• Entire abdomen and pelvis can be rapidly scannedwhile the patient is lying on the table and images canbe made of the area in question

• Real-time images

• Organs filled with air (e.g. lungs, stomach and intestines) andhard tissues (bone) are opaque to sound

• Sound is not able to travel through certain organs → theinteriors of these organs and those lying directly beneath themcannot be imaged

• Formerly it was almost impossible to view the cervix(kohdunkaula) and lower uterus (kohtu) because they lay underthe air-filled intestines. If patient drinks fluid one hour beforean ultrasound exam, the distended bladder will push the air-filled intestines out of the way and permit sound to reach thereproductive organs.

• The air layer between the patient’s skin and the transducer is abarrier for the sound. To overcome the reflections, somelubricant with similar acoustic impedance to tissue is appliedon the patient’s skin.

• An ultrasonic pulse emitted from a transmitter moves throughthe medium and some of its energy reflects back from theobjects within the body

• Interfaces between different structures in the body produceechoes at different times

• If the velocity of the pulse is known, the time taken for theecho to return can be translated into distance from transmitter.

• When the position and the orientation of the transducer andreceiver are known, image of the stuctures within the body canbe obtained

tcd 2=

• 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 organsinside.

• Transducer detects sound waves as theybounce off or echo back from the internalstructures and contours of the organs.

• Different tissues reflect these sound wavesdifferently

• Received waves converted to electrical signalsand turned into live pictures with computers.

• Piezoelectric crystal is used to excite and detect ultrasoundwaves.

• Applying AC voltage across the crystal faces causes a changingmechanical strain → pressure wave at certain frequency

• Conversely, ultrasound waves received at the transducer causesthe 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

growth– Imaging the development of the brain and other major organs– Determining whether the pregnancy is ectopic or whether there is a

multiple pregnancy– 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

cd v

vf θ=

• An application of nuclear magnetic resonancephenomenon (NMR)

• Can be used to image nuclei with magnetic moment• Produces images with high contrast between

different tissues• 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 bodyas 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 highproton 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

to B0

• In magnetic field protons’ magnetic moment vectorprecess around the magnetic field at Larmor frequency

• At equilibrium the net magnetization is parallel to B0(z-axis)

Y

Z

X

• When placed in a magnetic field, a particle with a netspin can absorb RF-pulse’s photon at Larmorfrequency → net magnetization turns to tranverseplane

Y

Z

X

• After RF-pulse the longitudialmagnetization recovers and netmagnetization returns to theoriginal equilibrium

• Precession of transverse magnetization induces aelectrical signal in coil

• Depends on nucleus’ gyromagnetic ratio γ and thestrength of the magnetic field B0

• Similar nuclei can be differentiated from eachother by changing the magnetic field strength

BL γω = B0

• Net magnetic vector M consists of twocomponents Mz and Mxy

• Two relaxation processes: longitudial T1 andtransverse T2

Y

Z

X

MXY

MZM(t)

B0

1

• Describes the recovery of the longitudinalmagnetization (Mz) to its equilibrium state

• Nuclei returns by dissipating their excessenergy

• Depends on the surrounding having at Larmorfrequency fluctuating magnetic field

1/0

Ttz eMM −−=

2

• 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 magneticmoments get out of phase

• Loss of coherence is irreversible and results in Mxy falling tozero

• T2 is always less than or equal to T1

• Large molecules, which move slowly, promote T2 relaxation

2/)( TtxyeMtM −=

PÄÄMAGNEETTI

LÄHETYSKELA

GRADIENTTIKELAT (X,Y,Z)

PÄÄMAGNEETTI

LÄHETYSKELA

GRADIENTTIKELAT (X,Y,Z)

ESIVAHVISTIN

MIXERI

ADC

AM P LIT UDI -MODULAATTORI

TE HOVAHVIST IN

SYNT ETISAAT T ORI

GRADIENT T I -VAH VISTIN

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Structure of MR-scanner

• Main parts are: magnet,gradients and coils

• A magnetic field that increases in strength along a particulardirection

• 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 Larmorfrequency will be the same of all parts of the sample.

• The size of the returning signal reflect the total numberof protons in the sample

• To localize the signal to a particular point in space, theapplied field is made non-uniform.

• The localization is done with slice selection, phase andfrequency encoding

• Z-gradient generates an extra field Gz in z-direction• Larmor-frequency is now a function of the position in z-

direction( ) ωωγω ∆+=+= 00 GzB

LeikkeenvalintaGz VaihekoodausGyTaajuuskoodausGx

9.8 MHz

Lukugradientti

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

LeikkeenvalintaGz VaihekoodausGy TaajuuskoodausGx9.8 MHz

Lukugradientti

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 receivingcoil

• Received signal is amplified and digitized for the imageformation

• 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

transform

Panorama 0.23T (Philips) Intera (Philips)

Philips

Philips

• Computed Tomography (CT) imaging combines theuse of a digital computer together with a rotating x-ray device

• Creates detailed cross sectional images or "slices" ofthe 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 softtissue, 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 anda serie of detectors on the opposite side.

• A fan beam of x-ray is created as the rotating frame spins thex-ray tube and detector around the patient.

• As x-rays pass through the body they are absorbed orattenuated (weakened) at differing levels creating a profile ofx-ray beams of different strength.

• The amount of attenuation of the beam depends on thedensity of the structures the beam passes through at differentangles. The densest structures in the human body (like bonesand teeth), which attenuate x-rays most, appear most brightlyin CT images.

• A rotating frame has an x-ray tube mounted on oneside and a detector mounted on the opposite side.

• A fan beam of x-ray is created as the rotating framespins the x-ray tube and detector around the patient.

• As x-rays pass through thebody they are absorbed orattenuated (weakened) atdiffering levels creating aprofile of x-ray beams ofdifferent strength.

• As the x-ray tube and the detector make a 360°rotation, the detector takes numerous profiles of theattenuated x-ray beam.

• ”Slice" is collimated (focused) to a thickness between1 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 detectorsand fed into channels.

• Each profile is then backwards reconstructed by adedicated computer into a 2D-image of the “slice”.

(Siemens) CT detector

X-ray tube

• Enables direct imaging and differentiation ofsoft tissue structures (liver, lung tissue, andfat).

• Useful in searching for large space occupyinglesions, tumors and metastasis (etäpesäke) andexamining of their size, spatial location andextent of tumors.

Virtual reality 3-D image of thelungs. The bronchial trees arecolored in green and the heart,aorta and vertebrae are colored inred

Siemens

• Scanner collects data for a single slice, so the positionof the appropriate slice needs to be known.

• The x-ray power was transferred to the x-ray tubeusing high voltage cables.

• The rotating frame would spin 360° in one directionand 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 acomplete stop and then reverse directions while thepatient table would be moved forward by anincrement equal to the slice thickness.

• In the mid 1980's, an innovation called the power slipring allows electric power to be transferred from astationary power source onto the continuously rotatinggantry.

• 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 likethe renal arteries or aorta.

• The primary digital technique for imaging the chest,lungs, abdomen and bones due to its ability tocombine fast data acquisition and high resolution inthe same study.

• It can provide detailed information of nearly everyorgan in the upper abdomen and pelvis in one quickexamination.

• New "multi-slice" spiral CT scanners can collect up tofour slices of data during spiral CT mode and somerotate 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 cerebrospinalfluid to 150 ohm m for bone.

• Measure of how electricity travels though a givenmaterial.

• Every tissue has different electrical impedancedetermined by its molecular composition.

• Eg. cancerous breast tissue has a much lowerelectrical impedance than normal tissue and non-cancerous tumors.

• Cancerous tissue causes alterations in intracellular andextracellular fluid compartments, cell membrane surface area,macromolecules, ionic permeability, and membrane associatedwater layers.

• These changes cause measurable changes in tissue electricalimpedance.

• When a small alternating current is placed across the area ofinterest, the focal increase in electrical conductance andcapacitance of the cancer tissue distorts the electric field.

• The impedance map shows the cancer as a focal brightness onthe gray scale image of conductivity and capacitance measuredby an array of signal sensors on the skin surface.

• A series of electrodes are attached to a subject in atransverse plane.

• Electrodes are linked to a data acquisition unit whichoutputs data to a PC.

• By applying a series of small currents to the bodypotential difference measurements can be made fromnon-current carrying pairs of electrodes.

• Since electric currents take the paths of least impedance,the currents flow depends on the subject's conductivitydistribution.

•AC voltage is connected to the wand held by the patient

•Electrical current flows through the body from the wandto the scanning probe.•The scanning probe is moved over the breast and itsmany sensors measures the current signal at the skinlevel.• The computer reconstructs the information and showsimages immediately on the monitor.•Impedance objects are defined as spots or regions thatare brighter (or darker) than their surrounding.

Electrical ImpedanceTomography (EIT) of thehuman thorax. The images onthe left are of six normalsubjects. On the right are siximages of patients with lungwater associated with cardiacfailure. The fluid in the lungs isclearly visible.

• Medical Physics and Biomedical Engineering (Chapter 12)

• Imaginis (http://www.imaginis.com/)• Philips Medical Systems (http://www.medical.philips.com)