Intro to medical imaging

Question 1

how is an image produced using X-rays

Answer 1

• within an X-ray tube, electrons are accelerated towards a metal target
• interaction of electrons with the target produces photons (X-rays)
• some of the X-rays pass through the patient then hit a detector behind the patient
• some are attenuated by the patient (absorbed, scattered, lose energy)
• amount of attenuation depends on density and atomic number of tissue/material and energy of the X-ray beam
• detected X-rays are digitised and processed, creating an image

Question 2

imaging techniques with X-rays

Answer 2

• x-ray single pulse of X-rays passes through patient to detector creating one image
• fluoroscopy continuous/pulsed X-rays, creating real-time moving images
• CT X-ray tube and detectors move around patient creating cross-sectional images

Question 3

how does angiography work to generate an image of CVS

Answer 3

• intial X-ray image taken of anatomical region of interest
• second image taken of same region after iodinated contrast agent injected into the blood vessels
• the two images are digitally subtracted leaving only the iodinated contrast outlining the blood vessels
• radiologist compares the image to normal anatomical images

Question 4

clinical use of X-ray

Answer 4

chest
- indications: dyspnoea, cough, haemoptysis, chest pain, post-procedure
- diagnoses: infection, pulmonary oedema, pleural effusion, pneumothorax, pericardial effusion, cancer

abdomen/pelvis
- indications: neonatal, urinary tract calculi, location of foreign bodies
- diagnoses: obstruction, volvulus, perforation, colitis, AAA

MSK
- indications: trauma, pain, deformity, swelling, post joint relocation, post reduction
- diagnoses: fracture, dislocation, effusion, soft tissue injury, tumour, infection

Question 5

advantages of X-ray

Answer 5

• quick
• portable
• cheap
• simple

Question 6

advantages of X-ray

Answer 6

• quick
• portable
• cheap
• simple

Question 7

disadvantages of X-ray

Answer 7

• radiation (relatively low)
• one plane, two dimensional
• cannot see all pathology
• poor soft tissue imaging

Question 8

how does fluoroscopy work

Answer 8

• pulsed or continuous X-rays are used which creates moving images
• can examine anatomy, pathology, motion and function
• images enhanced using contrast (barium/iodine)
• high atomic number = good absorber of X-rays so dense on image

Question 9

clinical use of fluoroscopy

Answer 9

• diagnostic and interventional
• vascular (angiography) eg. cerebral, cori=onary, embolisation, angioplasty
• GI eg. barium swallow, barium meal, barium enema
• GU eg. urogram, hysterosalpinogram, nephrostomy, insertion
• MSK eg. arthrogram, therapeutic joint injections, orthopaedic surgery

Question 10

advantages of fluoroscopy

Answer 10

• dynamic studies, can assess in real time and carry out intervention
• quick

Question 11

disadvantages of fluoroscopy

Answer 11

• higher radiation dose than single X-ray
• radiation exposure to interventional clinician
• one plane, two dimensional
• cannot see all pathology
• poor soft tissue imaging

Question 12

how does CT (computed tomography) work

Answer 12

• X-rays produced
• X-ray tube on one side of a rotating gantry and detectors on opposite side
• patient table moves through gantry
• cross-sectional slices of patient imaged
• detected signal processed by computer

Question 13

clinical uses of CT

Answer 13

• diagnosis/further investigations/management trauma, bleeding, clots, ischaemia, pain, cancer, obstruction, calculi, weight loss, fever
• monitor conditions cancer, ILD
• interventional radiotherapy, CT guided biopsies, drains

Question 14

advantages of CT

Answer 14

• quick
• good spatial resolution
• can scan most parts of body well

Question 15

disadvantages of CT

Answer 15

• radiation
• does not delineate soft tissues well
• affected by artefact
• requires breath-holding
• overuse
• incidental findings
• contrast reactions

Question 16

how does PET (positron emission tomography) work

Answer 16

• radionuclides used emit positrons during decay
• emitted positrons collide with nearby electrons in patient producing two annihilation gamma photons which are detected by gamma camera
• radiopharmaceutical used commonly is 18F-FDG
• often combined with CT or MRI

Question 17

how does nuclear medicine work

Answer 17

• administration of radiopharmaceutical (pharmaceutical part takes compunt to tissue of interest and radionuclide part sends signal from tissue of interest)
• nuclear decay of radionuclide occurs within tissues of interest, emitting gamma radiation which is detected by a gamma camera
• gamma camera contains a scintillator which converts to signal to light
• light signal is amplified and processed by computers to produce an image giving functional and anatomical info
• different type: planar, SPECT, PET

Question 18

clinical uses of PET

Answer 18

• oncology detection, staging, response to treatment
• neurological early diagnosis of Alzheimer, localisation of seizure focus
• cardiac identification of poorly perfused myocardium
• infection/inflammation pyrexia of unknown origin, vasculitis

Question 19

advantages of PET

Answer 19

• good contrast and spatial resolution
• can analyse anatomy and function

Question 20

disadvantages of PET

Answer 20

• physiological uptake of radiopharmaceutical
• radiation dose to patient
• risk of radiation to others
• radioactive waste produced
• expensive and time consuming
• radionuclide shortages

Question 21

what is the natural background radiation in the UK

Answer 21

~2.3mSv/year

Question 22

how does MRI (magnetic resonance imaging) work

Answer 22

• MRI scanner creates strong magnetic field, aligning hydrogen atoms within the patient
• radiofrequency pulse is applied, tipping the aligned hydrogen atoms which creates a detectable magnetic field
• this field induces an electric current in nearby coils in the MRI machine
• varying signal intensities are produced by different tissues
• signals are processed to create images
• after the radiofrequency pulse ends the hydrogen atoms relax back into alignment with the magnetic field of the MRI machine
• can adjust the settings to exploit differences between varying tissues through weighting

Question 23

weighting in MRI

Answer 23

• T1 weighting fat is bright, water is dark
• T2 weighting water is very bright, fat is quite bright

Question 24

images produced by MRI

Answer 24

• bright ‘hyperintense’ = high signal
• dark ‘hypointense’ = low signal

Question 25

clinical uses of MRI

Answer 25

• central nervous system brain and spinal cord
• head and neck imaging
• MSK imaging bone, joints, soft tissue
• GI eg. MRCP, MRI liver
• cardiac MRI, MR angiogrpahy
• gynaecological imaging, prostate imaging
• in paediatrics/pregnancy to avoid radiation

Question 26

advantages of MRI

Answer 26

• no radiation
• good contrast resolution especially on soft tissues

Question 27

disadvantages of MRI

Answer 27

• expensive
• time consuming
• fewer machines, fewer radiographers
• contraindications (pacemakers etc), claustrophobia, lack of mental capacity
• contrast reactions
• other risks eg. overheating
• image quality relies on magnetic field strength
• risk of magnetic objects becoming missiles in room

Question 28

how does ultrasound work

Answer 28

• utilises soundwaves
• crystal in the transducer probe oscillates, creating high frequency sound waves
• sound waves travel through tissues and are reflected back from boundaries between tissues of different density (acoustic impedence mismatch)
• probe detects reflected sound waves (echoes) and converts them into electrical signals
• time taken for echo to return is used to calculate where it was reflected from
• proportion of reflected waves is used to calculate acoustic impedence mismatch in that place

Question 29

features of images produced by ultrasound

Answer 29

• white on image = more reflection (hyperechoic)
• dark on image = less reflection (hypoechoic)
• acoustic shadowing = large acoustic impedence mismatch so sound waves completely reflected back (dark area behind bone, air, stones)

Question 30

doppler ultrasound

Answer 30

• if something is moving towards or away from the soundwave (eg. blood flow in a vessel) the frequency of the reflected wave is affected
• moving towards the soundwave increases frequency of echo wave
• moving away from soundwave decreases frequency of soundwave

Question 31

what is a duplex ultrasound

Answer 31

normal 2D imaging + doppler

Question 32

clinical uses of ultrasound

Answer 32

• solid organs appearance of tissue/organ, masses, bleeds
• hollow structures function, stones, flow, obstruction, masses
• breast assessing lumps, abnormalities on mammogram
• obstetrics pregnancy dating, fetal anomaly, placental location, fetal growth
• musculoskeletal assessing muscles, tendons. ligaments, joints, nerves, soft tissue masses
• interventional guided injections, biopsies, drains, aspirations

transvaginal, transrectal, transoesophageal ultrasounds

Question 33

advantages of ultrasound

Answer 33

• lack of radiation
• low cost
• portable
• dynamic so can see movement and assess blood flow

Question 34

disadvantages of ultrasound

Answer 34

• operator dependent
• no bone or gas penetration
• difficult with obese, frail, unwell patients
• theoretical risk of overheating if misused