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

 

Bone Scan Blood Pool PhaseBone Scan Blood Pool Phase 2

 

3. Delayed scan

- 2-4 hours later

- soft tissue activity has cleared

- skeletal structures demonstrated

- separate anterior + posterior scans obtained

- takes 15-30 minutes

 

Bone Scan Static Bone Images

 

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)

 

Bone Scan Normal Adult

 

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

 

Bone Scan MetastasisBone scan Metastasis0001Bone scan Metastasis0002

 

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

 

Bone Scan TKRBone Scan Loose Revision TKR

 

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

Pagets Bone Scan

 

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 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

 

Leukocyte ScanBone scan and Leucocyte Scan Infected TKR

 

 

 

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

 

Osteoblastoma Soft Tissue CTOsteoblastoma Bone Window

 

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

 

Axial Spine CTSagittal CTCoronal CT

 

3D

 

3D CT Reconstruction3D CT Hip

 

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

 

MRI T1Ankle T1 MRI

 

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

 

MRI T2Spine T2 MRI

 

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

 

MRI Proton DensityMRI Spine Proton DensityMRI PD Hip

 

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