Background

• X rays are a form of high energy electromagnetic radiation.
• They belong to the upper end of the electromagnetic spectrum, with a wavelength of 0.01 to 10 nanometers (Figure 10
• Like other electromagnetic radiations, X-rays travel at the speed of light (300,000 km/s), travel in straight lines, are not affected by electric or magnetic fields  
• They pass through the human body, undergoing differential absorption by tissues.
• Bone, fat, muscle and other masses absorb X rays photons at different levels
• Tissues containing elements with a high atomic number e.g. calcium absorb a high proportion of X-rays (high beam attenuation)
• Tissues where little absorption of X-rays occurs e.g. fat, air low beam attenuation, give a black radiolucent appearance on x-ray film.

Figure 1. Electromagnetic spectrum and its uses

• The X-ray tube, where X-rays are produced, is made up of a tungsten cathode (electron source) and a anode, encased in an glass vacuum container which has a window for the X-rays to emerge but is otherwise encased in lead to prevent radiation escaping in all directions (Figure 2).
• The cathode is a filament that when heated by an electric current releases electrons, a process known as thermionic emission.
• These negatively charged electrons are drawn towards the positively charged anode (target).
• The cathode also has a focussing cup to better direct the emitted electrons across the vacuum to hit the target. 
• . The anode or positive electrode is a thick copper rod with a small tungsten target at the end. Tungsten is required as it has a high atomic number to improve the efficiency of bremsstrahlung x-ray production (see below), and a high melting point. 

On colliding with the tungsten, the electron may strike

• Outer orbit electrons-this generates heat
• Inner orbit electrons-X-rays are produced as electrons are knocked out of orbit
• Nucleus-the electrons slow and alter trajectory, releasing x-rays (bremsstrahlung)
• A cooling system is used to remove the large amount of heat generated where the electrons strike the target. T
• The anode is made to rotate so that the heat and continuous bombard-ment do not damage it by constantly striking it at one spot.

Figure 2. Schematic diagram of an X-ray tube assembly, showing the various components

X-rays

X-rays are produced by two main mechanisms - Characteristic and Bremsstrahlung x-rays

1. Characteristic x-rays

• When a fast moving electron collides with a K-shell electron, the electron in the K-shell is ejected (provided the energy of incident electron is greater than the binding energy of K-shell electron) leaving behind a 'hole'. This hole is filled by an outer shell electron (from the L-shell, M-shell etc) with an emission of a single X-ray photon, called characteristic radiation, with an energy level equivalent to the energy level difference between the outer and inner shell electron involved in the transition.

2. Bremsstrahlung x-rays

• Bremsstrahlung interactions, the primary source of x-ray photons from an x-ray tube (80%), are produced by the sudden stopping, breaking or slowing of high-speed electrons at the target. When the electrons from the filament strike the tungsten target, x-ray photons are created if they either hit a target nucleus directly (rare) or their path takes them close to the nucleus.
• If a high-speed electron hits the nucleus of a target atom, all its kinetic energy is transformed into a single x-ray photon.
• Most high-speed electrons have near or wide misses with the nuclei. In these interactions, a negatively charged high-speed electron is attracted toward the positively charged nucleus and loses some of its velocity. This deceleration causes the electron to lose some kinetic energy, which is given off in the form of a photon. The closer the high-speed electron approaches the nuclei, the greater is the electrostatic attraction on the electron, the braking effect, and the greater the energy of the resulting Bremsstrahlung photon.
• The principle of the conservation of energy states that in producing the X-ray photon, the electron has lost some of its kinetic energy (KE):
• Final KE of electron = initial KE of electron - energy of X-ray photon
• The 'lost' energy is emitted as X-ray photons, specifically bremsstrahlung radiation
• Bremsstrahlung X-rays can have any energy ranging from zero to the maximum KE of the bombarding electrons (i.e., 0 to Emax), depending on how much the electrons are influenced by the electric field, therefore forming a continuous spectrum. The 'peak' of the spectrum typically occurs at approximately one-third of Emax so for a bremsstrahlung spectra with an Emax value of say 120 keV, the peak of the spectrum would be at approximately 40 keV.
• The intensity of bremsstrahlung radiation is proportional to the square of the atomic number of the target (Z), the number of unit charges of the bombarding particle (z) and inversely with the mass of the bombarding particle (m): Z² z / m. It follows that light particles such as electrons and positrons bombarding targets of high atomic number are more efficient producers of bremsstrahlung radiation than heavier particles such as alpha particles or neutrons (which can also cause X-rays to be produced through bremsstrahlung, though it's much more unlikely than with electrons)

• Only 1% of the electrons kinetic energy is released as X rays. The rest is dissipated as heat.
• Primary radiation refers to X rays from tube to X ray plate. (Ideal image)
• Secondary radiation refers to scatter that can blur an image. This is reduced by lead or aluminum grids that absorb scatter radiation.

Parameters

• KV: penetration or extent of energy X rays carry.
• MA: exposure
• Time: exposure
• Resolution: ability to differentiate as separate forms objects placed very close to each other.
• Contrast: Distinguish different tissue types

• The main source of radiation for staff is the scattered radiation from the patient.
• Measures to minimize radiation exposure to staff can be summarized as TDS (Time/Distance/Shielding) 

(1)    Time: keep the radiation time exposure   As Low As Reasonably Achievable (ALARA principle).

(2)    Distance: The inverse square law: The amount of scatter radiation is inversely proportional to the square of the distance from the x-ray source. The distance between the x-ray source and the patient should be maximized i.e. keep the image-intensifier as close to the patient as possible. (Staff to stay 1 meter away from the x-ray source)

(3)    Shielding: Lead aprons (0.25mm thick), thyroid shields and protective goggles

All staff should be trained in radiation protection and staff exposure to radiation should be monitored

Use goadal shields on patients where appropriate

Image intensifier (Figure 3)

• Image intensifier is a device that amplifies or intensifies low light level images to images that can be seen by the human eye.
• X ray photons that exit the patient strike the input fluorescent screen of the image intensifier.
• The fluorescent screen absorbs X ray photons and emits light photons that are immediately passed to the photocathode.
• The photocathode absorbs the light photons and releases electrons.
• These electrons are accelerated towards the anode by a high voltage
• The electrons are then focused at the phosphor screen at the anode that is captured by film

Indications

• Diagnose fractures
• Osteoarthirits and rheumatoid arthritis
• Degenerative disease especially of the spine

Advantages

• Cheap
• Easily available
• Simple.

Disadvantages

• No soft tissue detail
• Tissues overlap
• Exposure to ionising radiation.

Contrast media

• These are liquids with a high atomic number that absorb X rays more effectively than surrounding body tissues
• They may cause allergic reactions and some are toxic to the kidneys.
• Particularly useful in enhancing vascular structures and tumour

CT SCANNER (Figure 4)

Background

•  scanning is a diagnostic imaging procedure that uses digital geometry processing to generate a 3-dimensional (3D) image
• It is a specialised form of plain x-ray still involving ionizing radiation, but whereas in plain x-rays the source of x-rays is fixed, in CT scanning, the x-ray tube is rotated around the patient
• A computer processes a series of X rays performed at different angles around a body to produce a cross sectional image of the body.
• Digital geometry processing is then used to create a three dimensional image of the object from a large series of two-dimensional radiographic images.

Figure 4. CT Scanner

How does it work

• A motorized table moves the patient through a circular opening in the CT imaging system. (Gantry)
• Gantry consists of a rotating X ray source and a stationary circle of detectors to detect the attenuated beam once it has passed through the patient.
• This data is then sent to a computer to create a cross sectional image. This data can be reformatted into any plane or converted into a 3D image.
• Spiral or helical CT (named because of the path of the X rays) allows faster scans, reduce motion blur and is perfect for 3D imaging (no motion blur)

The image by the CT scanner is a digital image and consists of square matrix of elements (pixel)each of which represents a(voxel)  (volume element) of the tissue of the patient

Uses in Orthopedics 

CT is a better imaging modality for bone than soft tissue.

• Image complex fractures, especially intra-articular.
• Assessment of fracture healing/union: scaphoid
• Image regions difficult to analyze with X rays like Cervicothoracic or sacroiliac.
• Total body CT as a primary survey in polytrauma patients.
• Combined with PET (PET-CT) for a three-dimensional image of the functional processes in the human body.
• CT spine: fracture/dislocation anatomy, if MRI contraindicated (can be combined with myelogram)
• Bony tumours
• Infection

Hounsfield Unit

• Numeric information contained in each pixel of a CT image.
• It portrays the density of a tissue and is related to density and composition of the tissue imaged.
• HU: Air: -1000,Fat: -60 to -120,Water:0, Bone: 1000
• Helps in predicting the nature of a tissue (E.g. tumor with HU -60 to -120 likely to contain fat.

Different tissues types have different attenuation coefficients (Hounsfield unit). A high atomic or molecular number confers a higher attenuation coefficient, giving a lighter appearance

There are a much wider range of attenuation coefficients than the shades of grey that the eye can differentiate between and therefore the CT image can be manipulated by changing the window levels and window depths to allow the whole range of CTattenuations to be displaced

Advantages

• Non-invasive
• Relatively quick procedure.

Disadvantages

• Expensive
• Exposure to ionizing radiation.

 Radiation dose

There is a significant radiation dose associated with CT as compared to radiography

• Natural background radiation: 2.4 mGy/year
• Chest X ray: 0.01-0.15mGy
• Chest CT: 13mGy
• Abdominal X ray: 0.7mGy
• Abdominal CT: 8mGy
• Head CT: 56mGy
• Pelvic CT: 6mGy

• Any nucleus with an odd number of protons or neutrons can produce a MR signal
• Hydrogen is used because its nucleus is made up of one unpaired proton that is positively charged. Hydrogen is abundant in the body in the form of water and fat.
• Every hydrogen nucleus is a tiny magnet that produces a small but detectable magnetic field
• When placed in a magnetic field, hydrogen atoms have a strong tendency to align in the direction of the magnetic field
• The static field causes the spinning proton to wobble from its original axis to the axis of the magnetic field –this is called precession
• The precession frequency is dependent on the strength of the external magnetic field. It is determined by the Larmor Equation

WO=yBo

Wo is the precession frequency in Hz

Bo is the magnetic field strength in Telsa

Y is the gyro-magnetic ratio

• The stronger the magnetic field the higher the precession frequency
• After the protons have aligned along the external magnetic field a radiofrequency (RF) pulse is applied that has the same frequency as the hydrogen precession frequency
• The protons pick up energy from the RF pulse and some of them are lifted to a higher energy level. After the RF pulse is switched off, protons go down from their higher to the lower state of energy and the higher energy gained by the protons is retransmitted (NMR signal)
• The original magnetization begins to recover (T1) and the excessive spine begins to diphase (T2)

The emitted energy is too small to convert to images and therefore the ON-OFF of RF pulses are required. The emitted energy is stored (k space), analysed and converted into images

An MRI has three main components

1 Static magnetic field coils

Super conducting magnets are normally used to generate the magnetic field. They are large and complex magnets generating a very strong magnetic field that improves anatomical resolution, reduces scan times with preservation of image quality. Under normal working conditions it is never switched off

2 Radiofrequency (RF) coils

RF coils act as a transmitter and receiver. RF coils are the “antenna” of the MRI system. They transmit the RF signal and receive the return signal

3 Gradient coils

These are used to produce deliberate variations in the min magnetic field. There are usually 3 sets of gradient coils, one for each direction

MRI is a diagnostic technique in which application of a strong magnetic field and excitation radiofrequency pulses to a patient result in emission of radiofrequency energy by tissues that is used to create an image.

It does not use ionizing radiation, but rather radio-waves and magnetic fields 

How does it work? 

• MRI detects hydrogen atoms (protons) in the body that are abundant in water and fat. Images reflect relative concentration of protons in tissues. Hydrogen proton nuclei are used as they are by far the most abundant nuclei in the body with the greatest concentration in water and lipid molecules.
• A proton is a positive charge and a proton within a nucleus moves in a particular way known as precession (like a spinning top) In the absence of a magnetic field, these axes are randomly orientated and cancel each other out
• Protons spin about their axis and since they carry a charge, they generate a small magnetic field. Outside a magnetic field, they are randomly oriented.
• When a strong magnetic field is applied, axis of rotation of protons aligns with the long axis of the magnet
• Next, a radiofrequency (RF) pulse is applied for a few milliseconds causing the protons to absorb some of its energy.
• When the RF pulse is switched off, the excited hydrogen atoms emit this energy as a radiofrequency signal that is measured by the receiving coil and sent to a computer to create an image.
• TR: time to repetition of radiofrequency pulse
• TE: time to echo i.e. from stoppage of radiofrequency pulse to signal measured

Image interpretation

The magnetic signal depends on the type of pulse applied, the number of free protons and the characteristics of different tissue types 

1) T1:

• Machine is programmed to look at longitudinal movement of protons.
• This is a short sequence
• Good anatomical detail
• Black: air, calcium, dense bone
• Dark: CSF, edema
• Bright: fat, fat bone marrow, blood, contrast
• T1 is good for looking at meniscal pathology and marrow. It is less good at detecting oedema when T2 is better

2) T2: 

• Machine is programmed to look at transverse movement of protons.
• This is a long sequence
• Good for pathology
• Black: air, calcium, dense bone
• Bright: CSF, blood, edema, fat.

3) STIR: 

• Short tau inversion recovery
• The contrast between fat and water is low on T2 sequences as both return a high signal and therefore fluid can be missed
• This technique nullifies fat signal
• Considered as fat suppressed T2 image
• Useful as fat signal suppressed and fluid (edema) is easily localized
• Excellent depiction of bone marrow edema that can be the only indication of an occult fracture.
• In arthropathies, detects marrow edema earlier in SI joint than erosions seen on CT scan

Advantages of MRI 

• Non invasive imaging modality and painless
• No radiation
• MRI provides a very clear picture of the complete injury/ries.
• Soft tissue damage is 'picked up' best with MRI
• Three dimensional views possible

Disadvantages of MRI

• Expensive procedure
• Can be claustrophobic
• Contraindicated in patients with pacemakers or joint replacements.
• Movement during scanning may cause blurred images

Contraindications of MRI 

• Intraocular foreign body
• Nerve stimulators
• Cochlear implants
• Cardiac pace maker
• Insulin pumps and nerve stimulators
• Lead wires or similar wires
• Aneurysmal clip in brain
• Metallic splints in the eye
• Severe claustrophobia (5-8%)
• Allergy to contrast media

Screening

Enquire about medical implants, prior surgery, tattoos, prior occupation (i.e metal work, welding, machining) or injury that may result in retained metallic fragments, particularly near the eye, pregnancy status and breastfeeding

 MARS MRI

• Metal artifact reduction sequence
• Variety of techniques used to reduce metal artifacts at MRI (e.g. thinner slices, STIR, etc.)

Used to follow up metal on metal hips to detect pseudotumours or soft tissue destruction

Background

• Ultrasound involves the use of high frequency sound waves that are reflected back from tissues and picked up by a probe to create an image. Unlike CT and plain radiographs ultrasound does not use ionising radiation

 How does it work?

• Sound waves are produces by a piezoelectric transducer encased in a plastic housing
• These are focused on a particular body part
• Sound waves then travel to the tissues and are reflected back to the probe.
• Receiving sound echo when hits the transducer probe produces vibration
• These vibrations are converted to electrical impulses by the transducer
• Ultrasound scanner then converts this electrical impulse to digital image.
• Ultrasound scanner takes into account the time taken to return back the signal and the strength of the receiving sound echo while creating an image

Uses in Orthopedics

• DDH diagnosis and follow up
• Collections in joints, muscles or other tissues
• Pathologies of tendons, ligaments, muscles or soft tissue masses
• FAST scan in polytrauma patients
• Aid in aspirations and injections

Advantages 

• Non invasive
• Non ionizing
• Less expensive than CT/MRI
• No radiation
• Dynamic (biggest advantage)
• Portable, can be performed at the bed side

 Disadvantages

• Inability to penetrate bone
• Highly operator dependent with a long learning curve
• Requires the operator to think in cross-section

Background

• Dual energy X ray absorptiometry
• Gold standard for detecting and assessing osteoporosis.

How does it work?

• Dexa machine sends low energy X rays at two energy peaks aimed at bones.
• One peak is mainly absorbed by the soft tissues and the other by bones.
• The soft tissue amount (mainly fat) is subtracted from the total to determine the bone mineral density.
• Measured from hip, lower spine or distal radius. 

False positive in spine 

• Previous laminectomy

False negative in spine 

Osteophytes due to arthritis

Background

• Bone scan involves the use of radioisotope tracers that depict changes in bone metabolism before they are evident on X rays.

 Types 

• Technetium 99
• Gallium 67m citrate
• Indium 111 labeled white cells

How does it work?

• The main aim of a bone scan is to pick up increased (mainly) or decreased osteoblastic activity in bone.
•  Radioactive tracer (Tc99) is injected intravenously along with carrier substance methylene diphosphonate (MDP)
•  MDP concentrates in the mineral phase of the bone, either in hydroxyapatite or in calcium phosphate.

Tc 99 being unstable emits gamma rays that are then picked up by a gamma camera

Increased uptake (hot spot)

• Increased vascularity
• Increased metabolic activity (osteoblasts).

Decreased uptake (cold spot) 

• Decreased metabolic activity (osteoblasts).

Phases

 1.      Vascular phase/Flow phase

• 60 seconds after exposure
• Depicts perfusion to area

 2.      Blood pool phase

• 5 minutes after the injection
• Depicts blood pool and not the blood flow.
• Bone and soft tissue inflammation.

 3.      Delayed bone phase

• 3 hours after the injection

Depicts bone turnove

Normal scan features 

• Symmetric
• Increased uptake in kidneys, bladder, ends of long bones, scapulae tips, SI joints and epiphysis in children.

False negative

• Overwhelming bone destruction (osteoblasts cant keep up) like myeloma or aggressive bony metastases (Ca breasts)
• Superscan: Intense symmetric activity in bone with reduced renal activity. (e.g. aggressive Ca Breasts)

Uses

• Trauma: occult and stress fractures
• Bony tumors: primary and metastases
• Acute osteomyelitis
• CRPS
• Metabolic bone disease such as active Paget’s
• Prosthetic joints: detect infection or loosening
• Painful pseudiarthrosis
• Avascular necrosis

Causes of a cold lesion on bone scan

Radiation therapylocal vascular compromise e.g. infarction, early aseptic necrosis

Early osteomylitis

Neoplasm e.g. Myeloma, plasmacytoma, brest carcinoma

Gallium scan

• Gallium binds to plasma protein (Transferrin)
• Images done 1-2 days after injection with a gamma camera
• Concentrates in areas of increased leukocytes (Gallium has affinity for leukocyte lactoferrin (more than Transferrin)
• Commonly used for detecting infection like vertebral osteomyelitis, fever of unknown origin, or patients not suitable for WBC scans (low WBC count)

Indium 111 white cell scan

• WBC’s are removed from patient and tagged with Indium-111.
• Cells injected back and body scanned after 6-24 hours.
• Accumulates in areas of infection
• More sensitive than Gallium in detecting osteomyelitis.

SPECT

• Tomography added to radionuclide scanning
• 3 D image
• Improves anatomical localisation

• Functional imaging technique
• Radioisotopes are tagged with a natural chemical

(Fluorodeoxyglucose-FDG)

• FDG: Fluorine 18 is radioisotope and glucose is natural chemical.
• Glucose goes to areas of body that need glucose as energy.
• Gamma rays emitted by positron emitting radionuclide are analyzed by a computer to create a 3D image.

Usually combined wit ha CT or MRI to give both anatomic and metabolic information