week 11 lecture 2

difference between MRI and CT

• lack of radiation→ difference between MRI and CT, MRI has none

components of MRI

• super conducting magnet

• gantry (the giant tube shaped thing that houses everything)

• RF coils around the patient (radiofrequency)

• gantry contains the magnentic gradient coils

What do we do in MRI for different body parts

• use different specialised coils

• there are differing MRI scanner designs (e.g. open scanners for claustrophobia)

how are hydrogen atoms used in MRI

• body is made up of water (Hydrogen and oxygen)

• the behavior of these hydrogen atoms in a magnetic field is what is recorded

• MRI uses hydrogen because it’s nucleus has a single proton and is abundant in the body→ when magnet. field is applied, the proton axis all line up, creating a magnetic vector along the MRI scanner

explain the MRI physics

• the atoms line up→ radiofreq. pulses from the machine excites then relaxes the hydrogen atoms→ the resultant radio signals are computer processed to form an image

• the different types of tissue can be distinguished due to relaxation times→ denser tissue has slower relaxation times

MRI sequences

• by changing exam parameters, tissues can take different appearances

• e.g. in T1,it is anatomically detatiled, with bright fat and black calcium/fluid

• T2: fluid is bright and the muscles are dark

• STIR: suitable for brightening fluid

• contrast media is used in MRI but not iodine based, it is gadolinium based

MRI advantges

MRI disadvantages

MRI indications (why u would need one)

MRI contraindications (why u might not be able to get a MRI)

MRI safety considerations

• magnetic strength:

  • require safety zones around the magnet with safety protocols (make sure we dont transfer metallic things into the area)

  • things like O2 tanks, mop buckets and crash trolleys have all been pulled into the magnet

• patient protection:

  • need to complete a screenign questionaire

  • do a metal implant assessment

  • use ear protection

what is ultrasound

• uses non-ionising radiation

• the frequencies are above the range of human hearing

• we use different frequencies for different body parts, with

  • - higher FREQ. having better spatial resolution but less tissue depth (they get absorbed quickly) thus they are used for superficial body parts

  • lower freq. petter penetration but less resolution→ used for deep organs

How does the ultrasound work

• when the soundwave meets a interface, it can suffer reflecting or refractions

• the reflect/refract varies based on tissue type/density

• the images are created by the interpretation of sound reflections, where the image is made up on dots (each dot=echo of structure in patient)

component of ultrasounds

• transducer

• monitor for display

• recording device

• housing with controls

what does the transducer do in ultrasound

• produces sound waves that interact with body tissue

• reflected soundwaves also get sent back to transducer

• houses piezoelectric crystal which is the transmitter of pulses and reciever of echos

• also converts elect. into mechanical energy to transmit the pulses

• converts mechanical energy into electrical. recieved echos

what is echogenicity?

• how bright a tissue/structure is

• thus how intense the echos are

• three types:

  • anechoic→ no echos (black)

  • hypoechoic→ low echo, grey

  • hyperechoic (white, intense echo)

what is echotexture

• the description/pattern of echo

  • can be homogeneous (even pattern)

  • heterogenous (mixed pattern)

scan planes of sonography

doppler studies in ultrasound

• shows directional info about vascular flow

• blue→ direction of transducer

• red→ away from transducer

advantages and disadvantages of ultrasound (list 5 for each)

clinical indications for ultrasound

what is nuclear medicine

• attaches radioisotopes to compounds. the compounds are then taken up by specifc organs (heart, kidneys) and areas of high metabolic activity (bones, tumours etc)

• it is functional imaging, imaging the radiopharmaceutical distribution in a organ. it relied on microcirculation

• the resolution and anatomical detail is poor

• very sensitive imaging modality because it shows HOW the tissues are working rather than anatomical detail

• often provides complementary info to other scans

what is molecular imaging

• imaging modality that provides metabolic and functional info

• when disease occurs the biochemical activity changes, and molecular imaging detects the celluar changes

list the two scans in nuclear medicine

• PET scans

• SPECT scans

what is SPECT scan

• single photon emission computerized tomography

• tracer (radiographic injection) put in, and SPECT machine used to scan specific area of body

• tracer highlights blood flow

• used to diagnose brain disorder, heart problems, bone disorders, aldo progression of cancer in bones

• uses gamma emitting radioisotope for tracer

what is the image receptor used in nuclear medicine

• called gamma cameras

• it records the image of activity

• detects the gamma coming from the body, doesn’t inject/emit anything

PET scans

• a molecular imaging technique

• uses positron emitting radioisotope (tracer)

• uses tracer that emit positrons during decay→ position hits electron in body, they destroy each other and emit 2 photons which are picked up by detector

• it demonstrates the metabolism and function of organs/identifies changes

• use specific radioactive compounds for specific tissues

name 5 advantages of PET and SPECT scans

name disadvantages of nuclear medicine

● Low resolution images

● Although highly sensitive, NM is not specific

● NM departments are highly specialised with limited locations outside major population areas.

● Due to the long uptake time the whole procedure takes about 4 hours to complete (SPECT

clinical indications for NM

contraindications for NM

list the NM team

• nuclear medicine technologist (handling and administers radionuclides)

• nuclear medicine physician (interprets the NM procedures)

• radiation sfety officer (reviews imaging protocols and dosimetry records

• health physicist (calibrates and maintains the equipment)

list two hybrid techniques in moelcular imaging

• PET/CT

• mMR (PET/MRI)

describe PET/CT hybrid technique

• provides the detail of disorder and function information (PET)

• and detailed image of anatomy (CT)

PET and CT are two completely different imaging systems built into one scanner, but they collect different types of information:

• CT → anatomy (structure)

• PET → physiology (function/metabolism)

Because they are acquired in the same session, the computer can align (co‑register/fusion) the PET and CT images so they fuse into one image

clinical applications of PET/CT