Imaging
Bone scans
Definition
Technetium 99m labelled MDP (Mono - diphosphonate)
Advantages
1. Pure gamma emitter
2. Half life only 6 hours
- limits radiation exposure
3. Localises in bone
- chemical interaction on the surface of the hydroxyapatite crystal of bone
- phosphorous component interacts with the endogenous calcium
- produces insoluble technetium calcium phosphate complexes.
Pharmocokinetics
50% goes to bone
50% in equilibrium throughout soft tissues
Excreted in urine
Uptake occurs in areas of
1. Increased blood flow
- e.g. hypervascular tumours, fractures, inflammatory process
2. Increased cellular activity and mineral turnover
- osteoblastic activity produces immature osteoid
- this has numerous binding sites for developing apatite crystals
- i.e. healing fractures, inflammatory foci, growth plates
- remodelling of trabeculae in response to stress
3. Metabolic bone disease
- where there may be abundant immature unmineralised collagen
Imaging protocol
IV injection of 550 to 740 MBq of Tc 99 MDP
Patient encouraged to drink several glasses of water and micturate frequently
- dilute radiation dose to bladder wall
- accumulates in bladder
- a full bladder may obscure posterior abdominal wall
Gamma camera picks up emitted gamma rays
Anterior and posterior
- if just take anterior picture, posterior gamma rays will be absorbed by anterior body wall
3 phase scan
1. Flow phase
- patient positioned under camera
- images taken at 5 second intervals after injection into antecubital vein
2. Blood pool phase
- within next 5 - 10 minutes
- shows extent of soft tissue and bone hyperaemia
- shows soft tissue component of lesions
- i.e. cellulitis around osteomyelitis, soft tissue extent of bony tumours
- synovial hyperaemia in inflammatory arthritis
3. Delayed scan
- 2-4 hours later
- soft tissue activity has cleared
- skeletal structures demonstrated
- separate anterior + posterior scans obtained
- takes 15-30 minutes
SPECT (Single photon emission CT)
Tomographic examination
- rotate gamma camera around the patient
- creates CT like slice
- useful in spine i.e. spondylysis
Normal scan
Children
- overall very active
- hot symmetrical epiphyseal growth zones
Adults
- slightly hot at ends of long bones, SI joints, tips of scapulae, nasal cavity
- age related changes (ACJ, DDD)
Hot Lesions on Bone Scan
Metastasis
Primary Malignant Bone Tumour
Osteomyelitis
Trauma / Stress Fracture
Osteoid Osteoma
Paget's
Fibrous dysplasia
Arthritis
Locally increased blood flow
Primary hyperparathyroidism
Renal osteodystrophy
Specific Conditions
1. Investigation for bony metastases
95% sensitivity
- multiple scattered focal hot spots in axial skeleton
'Superscan'
- can occur if metastases coalesce
False Negative Bone Scan
- osteolytic and osteoblastic components are balanced
- multiple myeloma, melanoma, renal cell carcinoma
2. Primary malignant bone tumours
Indication
- detects metastasis
- detects extent of lesion for resection / skip lesions
3. Benign bone tumours
Most show low grade uptake
Giant cell tumours / osteoid osteomas
- have intense uptake
4. Fractures
Uses
- detection of stress fractures
- scaphoid fractures
- myositis ossificans
Findings
- initially blood flow & blood pool phases hot
- then only delayed scan positive which remains hot for several months
- when a fracture fails to unite, blood pool phase negative with delayed scan mildly positive
Sacral insufficiency fracture
- H / Honda sign
- bilateral linear uptake in sacral alar
- transverse uptake in mid sacrum
5. Infection
Cellulitis
- Hot flow and pool phases, negative delayed scan
Osteomyelitis
- All 3 phases positive
6. Investigation of pain around prostheses
Bone scan becomes normal at 12 months
- persistently positive scan means loosening or infection
7. Arthritis
Can differentiate between degenerative + inflammatory arthritis
Degenerative
- negative blood pool phase, positive delayed scan
Inflammatory
- all three phases positive
8. Avascular necrosis
Ischaemic bone cold, surrounding bone hot
- doughnut appearance in hip
9. Paget's disease
Intensely hot on all three
10. Fibrous dysplasia
11. Superscan
Metabolic bone disease
- osteomalacia
- hyperparathyroidism
- enal osteodystrophy
Myelofibrosis
Disseminating coalescing metastasis
12. Undiagnosed bone pain
May reveal an osteoid osteoma / unsuspected AVN / microfractures / low grade osteomyelitis
Gallium Scan
Gallium
Gallium 67 citrate
- localises in areas of inflammation and neoplasia
- due to exudation of labelled serum proteins
Technique
Delayed imaging at 24-48 hrs
Frequently used in combination with a technetium bone scan
- a double tracer technique
Less dependent on vascular flow than technetium
Difficulty in distinguishing between cellulitis and osteomyelitis
Technetium or Indium 111-Labelled White cell scan
Technique
Label patients own WBC's with radioactive tracer
Labelled white cells accumulate in areas of inflammation but not in areas of neoplasia
Useful in diagnosing osteomyelitis or infection around joint replacement
Unlike gallium also useful in the presence of pseudarthrosis
CT Scan
Principle
Irradiate a slice of tissue from multiple angles
Measure the output from different sides
Tissues have different densities
- with denser tissue fewer x-rays reach the detectors
Hounsfield scale
Bone 2000
ST 40
Water 0
Fat -100
Air -1000
Settings
Grey scale
- eye can only see 11 different shades
Adjust settings to bone or soft tissue
Multislice
Multiple detectors at any one time
- increase amount of data acquired in a single slice
- increased speed
- reduces artefact
Slice thickness
Decreased slice thickness
- increased info
- increased radiation
Reconstruction
Axial
Sagittal
Coronal
3D
Radiation dose
CXR 1
Pelvis 35
CT Chest 400
CT Abdo/pelvis 500
MRI
Creation of images
Placing patient into a strong magnetic field
- 30 000 x stronger than the earth's magnetic field
Stronger magnets, better images, shorter times
- 1.5 Tesla
- 3 Telsa
The nuclei of elements with odd numbers of protons line up
- i.e. hydrogen atoms
- hydrogen is plentiful in fat and water
A radiofrequency is then applied, exciting the protons
- as the excited protons relax back into equilibrium, a RF signal is emitted
- a receiver coil or antenna listens for an emitted radiofrequency signal
- the method and timing of the application of the radiofrequency signal can be varied
- T1 / T2 weighted, fast spin echo, fat suppressed or a gradient echo sequences
TE / time echo
- time for 90o RF to echo from tissue
- vary the time to detect the signal
TR / time repetition
- time between 90o RF
The hydrogen atoms return to a relaxed state by two mechanisms
- T1 relaxation
- T2 relaxation
- these are dependent on molecule size and binding to larger macromolecules
- all tissues have different T1 and T2 relaxation times
Liquids
- long T1 and T2 values
Fat
- short T1 and T2 values
By varying TE and TR can weight the sequences as T1 or T2
- if increase TE and TR
- produce T2 weighting
- sensitive for fluid i.e. oedema and inflammation
Contraindications
Absolute
Intracerebral aneurysm clip
Cardiac pacemakers
Automatic defibrillators
Implanted infusion devices
Internal hearing aids
Metallic orbital foreign bodies
Dorsal column stimulators
Vascular clips anywhere less than 2 weeks after insertion unless proven to be MRI compatible
Relative
- 1st + 2nd trimester of pregnancy
- middle ear prosthesis
- penile prosthesis
- internal orthopaedic hardware is safe but can create local artefact
- Claustrophobia
Disadvantages
Expensive
Can be claustrophobic
Very loud
- difficult for young children to cooperate, need sedation
Advantages
No radiation used
When to use which sequences in the musculoskeletal system
Types of images / Sequences
T1
Low TE/TR
- TE < 60 ms
- TR < 1000ms
- T1 relaxation - 1s
T1 weighted films
- fat has a bright signal e.g. bone marrow
- those tissues with little fat or water e.g. cortical bone, tendons, ligaments are dark in both T1 & T2
Standard workhorse for anatomy
Options
- post gadolinium
- spin echo
- gradient echo
- fat saturation (important to improve contrast when using gadolinium)
Gadolinium usually performed in T1 with STIR to determine if patient has abscess
T2
High TE/TR
- TE > 60 ms
- TR > 1000 ms
- T2 relaxation - 40 ms
T2 weighted films
- fluid has a bright signal
- Highlights pathology / fluid
Options
- spin echo (SE)
- gradient echo (GE)
- turbo / fast spin echo (TSE/FSE)
STIR
A method for fat suppression
- very important for TI and gadolinium
- changing the appearance of fat from white to black
- important for T2 to highlight fluid
Proton density
Intermediate between T1 and T2
- fat is high signal intensity
- oedema is high signal intensity
Usually done as part of a standard T2 spin echo image
Long TR / Short TE
- TR > 1000 ms
- TE < 60 ms
Can be useful on its own to look at the anatomy of tendons and ligaments
- good for menisci
- good for cartilage
Gradient echo
Accelerated T2 sequence
- very good for ligaments and articular cartilage
Images are fast but very susceptible to chemical shifts which can produce artefacts
Shows cancellous bone as black which can be helpful
Spin echo (SE)
A spin echo is a 90o RF followed by 180o RF
Turbo spin echo or fast spin echo
- faster than standard spin echo
- an accelerated way of acquiring T2 and PD images
Fat remains bright
- cannot differentiate between water and fat
- therefore fat suppression is required & can be performed using STIR
Can reduce metal artefact
OOPS (Out of Phase Sequence)
A technique for separating water and fat
- useful if there is watery fat or fatty water in two adjacent structures
Magic angle effect
When collagen bundles are 55o to the magnetic field
- artifactual high signal on T2
- reduce with STIR
- i.e. PD show increased peroneal signal, but not seen on T2
- therefore is due to magic angle
Radiation Exposure
Radiation Units
Exposure
- is a measure of quantity of ionisation, produced in air, by x-ray, or gamma radiation per unit mass
- SI unit is the Coulomb per kg (C/kg)
Absorbed dose
- is a measure of the amount of energy imparted to matter by ionising radiation per unit mass of irradiated material
- SI unit is the Gray(Gy)
Dose equivalent
- is a quantity introduced for radiation protection purposes
- correlates better with the harmful effects caused by exposure to the various types of ionising radiation
- SI unit is the Sievert (Sv)
- the subunit, millisievert (mSv), one thousandth of a Sv, is used more often because of the large size of a Sievert
- Dose equivalent = Absorbed dose X quantity factor X modifying factor
- for xrays, dose equivalent and absorbed dose are the same in magnitude
Background radiation
Depends on where you live
- 2 mSv / yr = 5 micro SV/day
- flying = 3-4 micro Sv/hr
Xray Doses
AP CXR 20microSv (4 days)
Lat CXR 50microSv
AXR 100microSv (20 days)
Lumbar Spine
- AP&Lat 500microSv (100 days)
- 5 view series 1milliSv
Pelvic XR 100 microSv
Wrist XR 4 microSv
Bone Scan
6mSV = 200 CXR = 2.5 years
CT
CT Chest/Abdo/ Pelvis
- 8-12 mSV each (5 years)
- whole series is 30 mSV (15 years)
CT Spine 8-10 milliSv
CT Head 20 milliSv (10 years)
CT Hand 4 milliSv
Ratios of exposure
CXR = 20 microSV (4d)
Pelvis XR = 5 CXR
Spine XR (AP/L) = 25 CXR
NMBS = 200 CXR
CT chest / abdo / pelvis = 400 CXR
CT head = 800 CXR
Exposure
Dose
- directly related to trauma load
- use of Image Intensifier
Average Orthopaedic Trainee
- exposed to 1.6 mSv per year on outside lead gown
- normal background radiation 2.5 milliSieverts / yr
- thyroid shield can reduce radiation by X13
- most commonly with NOF pin and plate / IM Nailing / MUA
- highest dose during IM Nail Femur
(Long procedure, increased scatter, close proximity during distal locking)
Exposure reduced by
- increased distance from source > 50 cm
- wearing lead apron
- minimise screening times
- with lead apron, maximum exposure to head & neck & hands
Effects
Lymphatic depression > 100mSv
Radiation sickness & increased solid neoplasia ~ 1000mSv
NHMRC Recommended Maximum Dose in addition to background radiation
- Occupational < 5mSv over 3/12
- General population < 0.25mSv over 3/12
- Average orthopedic trainee 0.4 mSv over 3/12
Primary concern is malignancy
- minimum safe dose unknown
- ? > 25 Gy
Areas in question are
- Eyes / Thyroid / RES / Gonads / Hands (Skin)
Absolute risk from low-dose radiation not determined
Protection
Wear protective lead apron at all times
- check regularly for cracks in lead
- thyroid protector & lead-lined glasses
Safety Procedures
- minimise exposure time
- use sparingly
- avoid live screening
- avoid cavalier operating
- do not handle tube
- do not place hand in tube
- don't operate II without radiographer
"HINTS TO MINIMISE EXPOSURE"
As low as reasonably achievable (ALARA)
- Operative planning
- Inform all staff
- Consider set-up and positioning of equipment
- Operating surgeon to set the example
- Lead gown - 0.5mm / thyroid protector / lead glasses
- Gloves in high exposure procedure
- Advise all staff to wear appropriate protection
- Wear a radiation monitor (beneath lead gown)
- Never stand behind someone for protection
- Sign on door - ionizing radiating in use
- Maintain distance from beam
- Lead shield if possible
- Don't use II as table
- Single exposure (not continuous)
- Minimise exposure time
- Clear warning when to be used
- Consider altering surgical technique to avoid excess exposure
- Minimise II distance from pt
- Minimise field size
- Exposure tube side > II side
- Don't use saline bags
Ultrasound
Physics
Very high frequency sound waves
- 3 - 15 MHz
Production
Piezoelectric effect
- run electric current through a crystal
- produces ultrasound waves
Theory
Density of substance determines velocity of the US
- reflected back to the probe at different velocities
- converted into 2D image on monitor