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Basic Science > Imaging Techniques > Flashcards

Imaging Techniques Flashcards

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1
Q

What are X-rays ?

A
  • Are a form of HIGH ENERGY RADIATION belonging to the electro magnetic spectrum, 10-10 lambda(m)
  • They pass thru the body undergoing Differential ABSORPTION by tissues
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2
Q

How do X-rays in radiography work?

A
  • X-rays fall onto fluorescent film- screen combinations -> black silver crystals ( film blackening) these processed to give an image.
  • The tissues containing high atomic no- e.g. calcium in bone, absorb a higher amount of X-rays (high beam attenuation) -> resulting in less crystals formation on the film & therefore a white radiodense appearance.
  • Tissues where less absorption - e.g. fat- ( low beam attenuation- give black radiolucent appearance as more crystals are formed on the film. Muscles appear grey as intermediate attenuation
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3
Q

How are X-rays produced?

A
  • Heating a fine filament usually made of Tungsten ( negative cathode) to INCANDESCENCE in a VACUUM to around 220oC-> Emission of ELECTRONS ( thermionic emission)
  • Electrons leave cathode move to positive anode- a smooth metal fragment of tungsten.
  • Electrons hit tungsten at about half speed of light on the FOCAL SPOT where they either interact with
    • 1) outer electrons of target nucleus -> heat
    • 2) inner electrons- knocking them off orbit-> X-RAYS
    • 3) nucleus - >electrons slow down! change direction -> X-rays from BRAKING radiation ( 80% production of xray made this way)
    • As X-rays pass thru pt they can be Transmitted, absorbed, scattered -> attenuation After passing thru pt fall onto X-ray plate
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4
Q

What is the X-ray cassette made up of?

A
  • Carbon fibre/ aluminium front - filter to minimise beam attenuation , remove low energy X-rays , reduce pt exposure
  • Lead sheet back- decrease backscatter of radiation
  • Film- polyester based coated with fine photographic emulsion - silver iodiobromide- sensitive to X-rays
  • Two intensifier screens on each side of the film-polyester base coated with dense layer of phosphor crystals that absorb X-rays and converts them into visible light which exposes the film
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5
Q

What is involved with processing of the film?

A
  • Development by alkaline immersion
  • Fixative in acid immersion
  • Wash Dry
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6
Q

What is the difference with digital imaging?

A
  • Digital Xrays is a form of X-ray imaging, where digital X-ray sensors are used instead of traditional photographic film.
  • Indirect FPDs. Amorphous silicon (a-Si) is the most common material of commercial FPDs.
  • Combining a-Si detectors with a scintillator in the detector’s outer layer, which is made from caesium iodide (CsI) or gadolinium oxysulfide (Gd2O2S), converts X-rays to light.
  • Because of this conversion the a-Si detector is considered an indirect imaging device.
  • The light is channeled through the a-Si photodiode layer where it is converted to a digital output signal.
  • The digital signal is then read out by thin film transistors (TFTs) or fiber-coupled CCDs. The image data file is sent to a computer for display.
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7
Q

What are the advantages of digital radiography?

A
  • Advantages include time efficiency through bypassing chemical processing
  • ability to digitally transfer and enhance images.
  • Also less radiation can be used to produce an image of similar contrast to conventional radiography.
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8
Q

What are the advantages of X-ray radiology?

A
  • Good for assessing bone
  • Cheap
  • Easily obtained
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9
Q

What are the disadvantages of X-ray radiology ?

A
  • Not sensitive to subtle bony destruction or abnormalities
  • Difficult to interpret in areas of complex anatomy
  • Form of ionising radiation -aim to reduce dose
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10
Q

What does Pacs stand for ?

A
  • Picture archiving and comminution systems
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11
Q

What are the disadvantages of digital X-rays ?

A
  • Do not have the high spatial resolution of radiographic film- screen combinations
  • Expensive though costs reducting
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12
Q

What is ultrasound ?

A
  • An imaging modality which used HIGH frequency (3-50Mhz) SOUND Waves generated by a PIZOELECTRIC crystal transducer.
  • The waves are reflected and refracted at the tissue interface
  • Reflected waves return to the Transducer where they are converted into electrical signals in order to -> image
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13
Q

How does ultrasound work?

A
  • Sound waves produced from PIZOELECTRIC CRYSTAL.
  • A DC VOLTAGE is applied & then reversed across the crystal which expands/ contracts- changes shape -> compression sound waves
  • When Coupled to the skin using lubricating gel sound waves are transmitted into the body and reflected back.
  • When sound waves arrive back at the transducer-> distort shape of PIZOELECTRIC crystal -> voltage
  • The more sound waves reflected the greater the voltage -> brighter the image
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14
Q

How does higher frequency of the wave effect the overall image?

A
  • The higher the frequency the higher spatial resolution , the attenuation within the tissue is higher- so limiting visualisation to specific structures
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15
Q

What is uss used for?

A
  • Assessing tendons
  • Assessing masses
    • cystic= hypoechoic appearance
    • solid= hyperechoic appearance
  • Confirming joint effusions
    • with guidance of injections
  • Screening and evaluation of DDH
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16
Q

What are the advantages?

A
  • NO IONISING RADIATION
  • No side effects
  • Painfree
  • Non invasive
  • Machines
    • SMALL
    • ** INEXPENSIVE**
    • PORTABLE DOPPLER added to observe Vascularity and blood supply- dynamic
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17
Q

What are its disadvantages?

A
  • High operator dependance for acquision and intrepretation
  • Field of view is often limited
  • Difficult to characterise the imaged tissue
  • Cannot penetrate cortical bone
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18
Q

What do the additional letters A,B,M mode stand for ?

A
  • A-mode= amplitude mode
  • B- mode= Brightness mode
  • M-mode= time motion mode
  • B mode mostly commonly used in clinical practice -> 2 d images
  • A mode-> 1 dimensional image used in opthalmolgy
  • M- used to asses cardiac valve motion
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19
Q

What does CT stand for?

A

Computerised Tomography

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20
Q

What is CT?

A
  • Is an imaging modality that uses x-rays to create pictures of cross-sections of the body.
  • X-rays are within a fan shaped x ray beam which rotates around the patient and sensitive detectors opposite which record the attenuated X-rays as they pass thru the body.
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21
Q

What are ct scans made up of?

A
  • Comprise of Scanning gantry - X-ray generator and curvlinear detector - opposite each other
  • Pt couch
  • Computer processor
  • Display system
  • Each rotation of the gantry produces an axial slice thru the patient
  • . Modern scanners use helical rotation to allow continuous acquisition of the data
  • The tissues that attenuate the beam to a higher degree, such as bone, appears denser on CT image than tissues such as muscle
  • EAch tissue is assigned an attenuation coefficient ( in Hounsfield units, HU)
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22
Q

What are the CT images made up of?

A
  • Pixels- it not only presents a 2d image but a thickness (in VOXELS)
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23
Q

What happens to the ct image before we see it?

A
  • It has to be manipulated by changing the window levels & widths so that the variable areas of attenuation in the grey scale that we cannot determine by the naked eye can be seen
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24
Q

What is ct used for ?

A
  • Assessment of fractures
  • With contrast to assess joints
  • Bone densitometry
25
Q

What tissues have a higher attenuation? How are this graded?

A
  • Bone - appear denser on ct image- white cf areas of low attenuation- fat appear dark
  • Each tissue is given a ATTENUTATION COEFFICIENT in HOUNSFIELD UNITS relative to the Value of WATER ( zero HU)
26
Q

What are CT advantages ?

A
  • 3D imaging and x sectional anatomical data
  • Better than xrays for soft tissue imaging but not as gd as MRI
  • Quantiative data on composition
  • Good for assessing cortical bone
  • Guide interventional proceedures- biopsy
27
Q

What are its disadvantages ?

A
  • Significant use of ionizing radiation
  • Inferior soft tissue contrast resolution to MRI
  • Claustrophic for pt
28
Q

What is the advantage of spiral CT?

A
  • Allow rapid multi slicing as the gantry rotates continuously
29
Q

What are the advantages of multidetector Ct?

A
  • Multiple rows of detectors opposite side of X-rays source- so one rotation allow multiple slices thru the body to be imaged at once
30
Q

What does MRI stand for?

A
  • Magnetic resonance imaging
31
Q

What is MRI?

A
  • An image modality using STRONG MAGNETIC FIELDS and EXCITATION RADIOFREQUENCY PULSES to the patient which results in emission of RADIOFREQUENCY signals by the tissues of interest which are used to build up an image.
32
Q

What do MRI DETECT?

A
  • Hydrogen nuclei- PROTONS in water
  • Images reflect the relative concentration of protons in tissues by measuring the strength of signals from individual VOXELS In a slice of the patient.
33
Q

How do MRI work?

A
  • Each proton spins like a top- aka nuclear spin
  • Without a magnetic field the axes are RANDOMLY ORIENTATED so produced no net magnetic field.
  • The MRI applies a STRONG STATIC MAGNETIC FIELD -> Protons AXIS ALIGN WITH LONG AXIS OF THE MAGNET/SCANNER -> net magnetic field
  • The protons also wobble around their axis w a fixed frequency= PRECESSION
  • A RADIOFREQUENCY PULSE is applied for a few MILLISECONDS-> protons realign at an angle to the longitudinal axis of the scanner & also causes precession of each proton to be pulled into step = phase with each other
  • They induce currents in the recieving coils
  • the changes in the currents/ signals are used to characterise the tissue
  • When the pulse stops-
    • the protons realign with the long axis of the scanner-> long magnetization vector, it increases to max
    • The precessional then fall out of step (dephasing)-> the transverse magnetization vector to decrease from its max
34
Q

Define T1 in an MRI image?

A
  • The time taken for the LONGITUDINAL MAGNETIZATION VECTOR to recover to 63% of its max value
35
Q

Define t2?

A
  • The time taken for the TRANSVERSE MAGNETIZATION VECTOR to fall to 37% of its max value.
36
Q

How are t1 and t2 images used?

A
  • T1 and t2 vary for different tissues and therefore are used to form the image
  • T1- fat bright signal - gd for anatomy
  • T2- fluid has bright signal- gd for pathology
37
Q

What is MRI used for?

A
  • In orthopaedics
  • Osteonecrosis
  • Infection
  • Trauma - occult fractures
  • Tumours
  • Regional disease- Rc pathology/ Intervertebral disc disease
38
Q

What are its advantages?

A
  • Image in any plane- multi planar
  • High contrast resolution- gd soft tissue, bone marrow imaging
  • Non ionising radiation
  • Non invasive imaging of blood vessels
39
Q

What are the disadvantages of MRI?

A
  • Limited by contraindications- think clinic SHEET!!
  • Absolute
    • pacemaker
    • defribrillators
    • cerebral aneurysmal clips
    • vascular clips(<2 wks old)
    • metal in orbit
    • internal hearing aids
    • dorsal column stabilisers
    • Relative - 1st/2nd trimester of pregnancy , claustrophia, obesity, penile prostheis, children to sit within scanner
    • Other
      • Not as good as Ct for imaging bone
      • High cost of equipment
      • Image artefact - motion and ferromagnetic objects
      • Claustrophia
40
Q

What is a bone scan?

A
  • A image modality involving the intravenous injection of technetium 99m phosphonates, which localises at the sites of OSTEOBLASTIC Activity
41
Q

How does bone scanning work?

A
  • Radionuclides are UNSTABLE nuclei with neutrons excess or deficit.
  • The nuclei distingretate to form other atoms with the release of alpha, beta or gamma radiation.
  • Technetium 99m is a radionuclide with a short half life and pure GAMMA emitter
  • It is coupled with methylene disphosphonate and is Administered iv.
  • It is chemical absorbed onto hydroxyapitite crystals in bone.
  • It’s uptake is reflection of OSTEOBLASTIC activity.
  • within 24 hrs around 70% of administered dose is excreted thru kidneys hence bladder exposure to radiation must be minimised by pt hydration adn frequent micturition
42
Q

How is the gamma radiation detected ?

A
  • Photo emission recorded by Scintillation gamma camera ( contains crystals of sodium iodide that absorb Tc 99 gamma rays
  • Signal is enhanced by PHOTOMULTIPLIER TUBES and processed by computer
43
Q

Describe the 3 phases of a bone scan?

A
  • 1)Flow/ Dynamic phase- immediate images 1-2mins= arterial blood flow and hyper fusion
  • 2) Blood pool/ Equilibrium images- images at 3-5mins showing EC FLUID VOL- so extent of bone and soft tissue hyperaemia
  • 3) STATIC/ Delayed images- approx 4 hours after injection- when soft tissue activity has cleared, highlighting skeletal activity.anterior and post scans available.
44
Q

What are bone scans used for?

A
  • Assessment of occult bone pain
  • Assessment of metastatic disease
  • Infection - flow and blood pool- hot in cellulitis
  • blood flow, pool and static phases all hot osteomyelitis
  • Trauma- stress facture
  • Tumour- osteoid osteoma osteoblastoma especially spine
  • Arthritis- activity and extent
  • Post-op- paeudoaethrothesis, painful prosthesis
  • Assessment of Paget’s disease
45
Q

What are the advantages of bone scan?

A
  • Very sensitive to increases in bone turnover
  • Good general survey for skeletal pathology
  • Helps to localise local bone lesions after which further imaging can occur
46
Q

What are the disadvantages of bone scans ?

A
  • Poor spatial resolution
  • Non specific appearance for elucidating causes if increased activity
  • may give False negative in myeloma and lytic metastasis due to decreased blood flow or lack of osteoblastic response
  • Relatively high radiation dose - ESP to bone marrow and in children
47
Q

What is SPECT?

A
  • Single photon emission computer tomography
  • Applies tomography technology to radionuclide scanning- enabling x sectional images to be obtained with enhances the conspicity of the lesion and hleos in their localisation
  • the gamma head rotates 360 degrees around the body obtaining multiple planar images which are then reconstructed in sagittal, coronal and axial planes
48
Q

What use is INDUM 111?

A
  • It used to label white cells and platelets and can be used to LOCALISE AREAS OF INFECTION AND INFLAMMATION
49
Q

What is the use of gallium 67?

A
  • It binds to **plasma proteins **
  • can be used to localise areas of inflammation e.g abscess and neoplasia
  • frequently used in combo with technetium bone scan and is less dependent on vascular flow than a technetium scan
50
Q

What is dexa?

A
  • Dual energy X-ray absorptiometry
51
Q

What is used for?

A
  • To measure the BONE MINERAL DENSITY in the central skeleton -FEMUR AND LUMBAR SPINE
52
Q

How does it work?

A
  • It used TWO X-RAYS OF DIFFERENT ENERGIES which are absorbed in different proportions by bone and soft tissues
  • The pt lies on the table for 2 mins while **ap of femur **pt supine, ties together heels separated 25cm) and lateral of spine
  • lower ribs cover L2 and iliac crests cover L4 on lateral so only L3 maybe the only vertebra scanned.
53
Q

What is a dexa scan used for?

A
  • Assessing bone mineral density in primary and secondary osteoporosis- for diagnosis and monitoring - allows for fracture risk
  • Evaluating the effect of preventive interventions on metabolic diseases- primary hyperparathyroidism
  • Measuring periprosthetic bone loss- in pt with cementless thr- measure magnitude and rate of change in bm content
54
Q

How are the results interpretated?

A
  • T score when bone density values (g/cm3) are matched FOR SEX AND RACE and COMPARED TO PEAK BONE MASS OF NORMAL ADULTS =
  • pt bmd - population bmd/ standard deviation of population bmd
55
Q

What is the z score?

A
  • When matched for SEX, RACE AND AGE =
  • (pts bmd- pop age related bmd)/ Standard deviation of population age related bmd
56
Q

Define osteoporosis ?

A
  • is a progressive bone disease that is characterized by a decrease in bone mineral density which can lead to an increased risk of fracture.
  • diagnosed If the T score IS OVER -2.5 sd according to the WHO criteria - kanis et al 1994
  • Nb- normal t score is >/** -1 **
  • Osteopenia is a score -2.5 to <-1
  • osteporosis \<-2.5
  • severe oestoporosis = <-2/5 and presence of one or more fragility fx
  • _In premenopausal women Z scores are used _
57
Q

What is z score used for?

A
  • Compare pts BMD with the mean value for individuals of the same age
  • A LOW Z SCORE -= an aetiology other than age related bone loss
  • z score used in premenopausal women
58
Q

What are the advantages of dexa scan?

A
  • Assessment of fracture risk in osteoporosis and the need for treatment
59
Q

What are the disadvantages of dexa scan?

A
  • Doesn’t distinguish between CORTICAL and CANCELLOUS bone density
  • In the spine -AP PROJECTION fasley increase bmd by intervertebral oa- so prefer LATERAL
  • False high density values maybe seen with fractured vertebra and calcification
  • False low density values found if posterior element surgical resection

Basic Science (31 decks)

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