EXAMINER: What are X-rays?

CANDIDATE: X-rays are electromagnetic radiations of wavelength 15–0.01 nm.

EXAMINER: How are X-rays generated?

CANDIDATE: X-rays are released on heating a fine tungsten filament to around 2200°C in a vacuum.¹ Electrons travelling from the filament (cathode) to the target (anode) convert a small percentage (1%) of their kinetic energy into X-ray photons (by the formation of Bremsstrahlung and characteristic radiation).

Bremsstrahlung interactions, the primary source of X-ray photons from an X-ray tube, 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.

EXAMINER: How does digital radiography work?

CANDIDATE: Digital radiography uses a phosphor compound detector plate instead of the conventional photographic emulsion film. The detector generates a digital image that can either be printed or sent to PACS (picture archiving and communication system).

EXAMINER: What measures will you take to minimize radiation exposure to staff while using an image intensifier?

CANDIDATE: The main source of radiation for the surgeon and the team is the scattered radiation from the patient. Measures to minimize the radiation exposure to staff are:

EXAMINER: What is collimation?

CANDIDATE: Reduction in the size of the window through which the X-rays are emitted leads to sharper radiographs as well as reduction in radiation dose.

EXAMINER: What is the maximum safe dose of occupation-related radiation exposure?

CANDIDATE: The whole-body exposure of 20 mSv over a year is the maximum acceptable radiation dose (averaged over 5 years) (UNSCEAR, 2010).

EXAMINER: What does the picture show and what is its use in theatre (Figure 28.1b)?

CANDIDATE: This shows a dosimeter. This is a device that measures exposure to ionizing radiation. It is used to estimate the radiation dose deposited in an individual wearing the device.

EXAMINER: Anything else?

CANDIDATE: Ideally all orthopaedic surgeons should wear a personal radiation dosimeter. This should be in a constant position beneath the protective lead gown to record any personal dose.

If dose readings are too high then a review of theatre practice must take place. Dosimeters must be safely stored away from radiation sources and should never be shared.

It is important to also take proper care of the lead apron. Crumpling of the lead apron will break the integrity of the lead fibre shielding. Lead aprons should be properly hung up after use and their integrity checked regularly.

COMMENT: The answer could have been structured into discussion of radiation protection covering five areas: (1) minimization of radiation use, (2) maximizing the distance between the individual and the X-ray source, beam and scatter, (3) use of lead screens, (4) personal protective garments and (5) monitoring personal exposure dose.

EXAMINER: What is ultrasound?

CANDIDATE: Ultrasound is a form of imaging that utilizes high-frequency sound waves to image interfaces between tissues with different acoustic properties (described as echogenic or hypoechoic). Fluid-filled tissues have low echogenicity while fat is highly echogenic.

EXAMINER: What are the advantages of ultrasound?

CANDIDATE: Ultrasound is cheap, easily available, portable, non-invasive and dynamic. It is safer than CT as it does not emit ionizing radiation and unlike MRI can be used in patients with cardiac pacemakers or metal clips.

EXAMINER: And the disadvantages?

CANDIDATE: Its main disadvantage is that it is operator-dependent.

EXAMINER: What is the physics behind ultrasound imaging?

CANDIDATE: The passage of electric current through a piezoelectric crystal causes deformation of the crystal surface, in turn producing sound waves. When the transducer is then applied to the patient’s skin using a lubricating jelly, these waves are then transmitted into the patient’s body. The reflected waves when received back by the transducer cause distortion of the crystal surface, producing a voltage, which is then converted to an image. The duration between the sound wave emission and detection reflects the depth of the tissue being studied.¹

The amount of energy reflected at the interface between tissues depends on the difference in acoustic impedance of those tissues. The acoustic impedance of a tissue is mainly determined by its density. Air has a much lower density than water or soft tissue, which in turn have a much lower density than bone. The larger the difference in acoustic impedance, the more energy will be reflected, and the brighter the resulting image.³

However, at the interface between soft tissue and air or bone, nearly all the wave’s energy is reflected. No energy is transmitted, and hence no information can be gained about tissues which lie deep to this point. This explains why ultrasound is generally not useful for assessment of bone, bowel or lung. It also explains why a coupling gel is required between the probe and patient’s skin to eliminate air.³

As a sound wave passes through the body it gradually loses its energy in a process called attenuation. The causes of attenuation are: absorption, reflection, diffraction and refraction.

Refraction causes a transmitted wave to be deflected from its original course. Diffraction is scattering of the wave which occurs particularly when a wave interacts with small structures. Most of attenuation, however, occurs due to absorption. The energy of the sound wave is converted into friction between oscillating tissue particles and is lost in the form of heat. To compensate for this loss of energy, the ultrasound machine uses a process called time gain compensation. This gives greater amplification to those echoes which take longer to return to the transducer, producing an even image.³

Frequency 3–50 MHz with a high-frequency probe used for deep tissues and a low-frequency probe for superficial tissues.

EXAMINER: Tell me a few of the practical applications of ultrasound in orthopaedics.

CANDIDATE: A few uses of ultrasound in orthopaedics include:

EXAMINER: How does a CT scanner work? What are the principles of a CT scanner?

CANDIDATE: The X-rays are liberated from the axially rotating X-ray tube. After passing through the patient, they are received by a circle of stationary detectors. The data collected are processed by the computer and digital images are reconstructed. The scanning gantry rotates helically around the patient, allowing continuous acquisition of data.

With modern multidetector CT transverse (or axial) anatomical sections can be produced with high resolution and reformatted to create reconstructions in any plane. The 3D CT scans allow better visualization of complex intra-articular fracture/spinal problems but at the cost of slight loss of definition (subtle fractures can fade away/be created).

A recent additional feature is the ability to ‘ghost out’ structures, for example: a ghost outline of the femoral head can be maintained in cases of fractures of the acetabulum to show its relationship to the fracture and at the same time the acetabular fracture can be visualized more clearly.

EXAMINER: Give some orthopaedic indications for CT scan.

CANDIDATE:

1. 
   

   (1) Complex peri-articular fractures – for example, fractures of the acetabulum, tibial plafond, tibial plateau, proximal humerus, Lisfranc’s, talus, calcaneum and vertebrae. CT aids in planning incisions to minimize additional trauma to soft tissues (Figure 28.3).

   
   
2. 
   

   (2) Polytrauma patients – head, chest and abdominal scanning (in about 30 seconds).

   
   
3. 
   

   (3) CT arthrography/myelography (if MRI is contraindicated).

   
   
4. 
   

   (4) Assessment of choice for non-union when radiographs are inconclusive: scaphoid.

   
   
5. 
   

   (5) Drilling/ablation of osteoid osteoma.

   
   
6. 
   

   (6) Customized implants: CT scans are increasingly used for patient-specific implants and instrumentation, especially for complex arthroplasty.

EXAMINER: Please describe the findings (Figure 26.4a,b)?

CANDIDATE: This is an MRI scan of the lumbo-sacral spine of … Dated … Age …

EXAMINER: Is it a T1-weighted image or T2?

CANDIDATE: The image to the left is a T1 image as it has a TR (time to repetition) value of (< 1000), whereas the image on the left is a T2 image (TR > 1000).⁴

(Don’t rely on fluid/fat signals for judging the type, as they can be confusing in STIR or fat-suppressed images.)

Most imaging protocols will use a combination of different sequences to optimally evaluate the structures and pathologies in question by providing both anatomical and pathological information.

MRI can be utilized in any plane, and this is sometimes crucial in providing additional information.

EXAMINER: What are the differences between T1 and T2 images?

CANDIDATE:

EXAMINER: What contrast is generally used with MRI?

CANDIDATE: Chelated gadolinium. It enhances the oedematous tissues on T1 image.

An important but rare complication of gadolinium is nephrogenic systemic fibrosis (a fibrosing dermopathy), which has been reported to occur rarely in patients with Stage 4 renal failure.

Current guidance is that if the eGFR < 30, gadolinium should not be administered.

EXAMINER: What are the contraindications for MRI scan?

CANDIDATE:

1. 
   

   (1) Implanted cardiac pacemaker and/or defibrillators.

   
   
2. 
   

   (2) Internal hearing aids/cochlear implants.

   
   
3. 
   

   (3) Implanted nerve stimulators/dorsal column stimulators.

   
   
4. 
   

   (4) Metal objects in orbit of the eye.

   
   
5. 
   

   (5) Intracranial metal aneurysm clips.

   
   
6. 
   

   (6) Aortic stent graft.

   
   
7. 
   

   (7) Mechanical heart valve.

Many coils, filters, coronary stents and grafts are made from non-ferromagnetic materials and MRI can be safely performed. They will require a pre-MRI check regarding their compatibility as some will demonstrate magnetic field interactions and are not safe with MRI use.

MRI is usually used with extreme caution within the first 6 weeks postoperatively when any metallic clip (including skin clips) or implant has been utilized as they will not have become incorporated securely in tissues.

EXAMINER: What else?

CANDIDATE: Certain types of intracranial aneurysm clips are an absolute contraindication to the use of MRI because excessive, magnetically induced forces can displace these clips and cause serious injury or death.

Exposure of internal hearing aids to MRI may damage these components.

In the case of cardiac pacemakers and defibrillators the MR environment can cause movement and heating of pacing leads, temporary or permanent modification of the device, and deactivation of the device.

EXAMINER: What are the advantages of MRI use?

CANDIDATE: MRI provides excellent soft tissue contast. It is particularly good for imaging tumours and occult fractures and does not involve using ionizing radiation.

EXAMINER: What are the disadvantages of MRI use?

CANDIDATE: Patients may be claustrophobic, and it is not as good as CT for cortical bone imaging.

EXAMINER: What are the indications for MR arthrograms?

CANDIDATE:

1. 
   

   (1) Shoulder: Suspected capsular/labral tears (Figure 28.4c).

   
   
2. 
   

   (2) Hip: Labral tears/Impingement.

   
   
3. 
   

   (3) Knee: Post-meniscectomy for recurrent tear.

   
   
4. 
   

   (4) Wrist: Scapholunate dissociation, TFCC tear, occult scaphoid fracture.

   
   
5. 
   

   (5) Ankle: Undisplaced osteochondral lesion of the talus.

EXAMINER: What is the basic principle of an MRI scanner?

CANDIDATE: The MRI scan involves exploiting the magnetic moment/nuclear spin property of the hydrogen nucleus (in the tissues) when placed in a strong magnetic field. Radiowaves (64 MHz) are then applied using the transmitter coil. Following the cessation of the radiowaves, the individual magnetic moments then precess and the response is recorded in the receiver coil.

The frequency of precession depends on the strength of the external magnetic field.

EXAMINER: What is bone densitometry?

CANDIDATE: Bone densitometry, also called dual-energy X-ray absorptiometry or DEXA scan, uses simultaneous measurement of the passage through the body of X-rays with two different energies. It uses a low dose of ionizing radiation and is accurate in diagnosing osteoporosis.

The WHO guidelines for interpretation of bone densitometry results (T-score):

EXAMINER: What are its uses?

CANDIDATE: The clinical applications of bone densitometry include:

1. 
   

   (1) Assessment of bone status in primary and secondary osteoporosis.

   
   
2. 
   

   (2) Assessment of the effect of treatment for osteoporosis.

   
   
3. 
   

   (3) Fracture risk assessment (what is FRAX score?).

   
   
4. 
   

   (4) Evaluation of preventive measure for bone loss associated with ageing/metabolic disorders.

   
   
5. 
   

   (5) Measure periprosthetic bone loss (particularly cementless hip arthroplasty).

EXAMINER: OK. You have now started bisphosphonates on this patient for osteoporosis, how will you find out whether your treatment has been effective? Will you repeat the bone density scan? When?

CANDIDATE: Repeated dual-energy X-ray absorptiometry (DXA) has a high within- and between-patient variability. The average annual increase in BMD in patients treated with alendronate is about 0.0085 g/cm². This change is smaller than the typical year-to-year (within-person) BMD variation of 0.013 g/cm². It would therefore be difficult to differentiate the medication’s effect from the random variation inherent in DXA scans.

Response is generally favourable after 3 years of treatment. While there is variation in test results from year to year, longer-term findings are more reliable. After 3 years of treatment, 97.5% of patients taking alendronate had an increase in hip BMD of at least 0.019 g/cm², with a strong correlation between hip and spine measurements.

Hence, if needed I would repeat the bone density scan after 3 years, to evaluate efficacy of the treatment monitored in low–medium-risk patients. The treatment of high-risk patients would need to be considered on an individual case basis.^{6, 7}

EXAMINER: Name some techniques used to measure bone density in the axial skeleton? (The same question can be asked regarding appendicular skeleton.)

CANDIDATE: Techniques that can be used to measure bone density in axial skeleton are:

1. 
   

   (1) DEXA. Dual energy X-ray absorptiometry.

   
   
2. 
   

   (2) Quantitative CT (rarely used).

   
   
3. 
   

   (3) Quantitative MRI (rarely used).

EXAMINER: Does DEXA measure true bone density or apparent? (A rather pointed question!)

CANDIDATE: DEXA measures apparent density (obviously) as it is a two-dimensional measurement quantified in g/cm². Quantitative CT gives a volumetric (three-dimensional) true representation of bone density in g/cm³. Even quantitative CT can give false low readings as it also measures intravertebral fat.

EXAMINER: Which of the bone density measurement techniques can differentiate between cortical and trabecular bone?

CANDIDATE: Quantitative CT and quantitative MRI.

EXAMINER: What is FRAX score?

CANDIDATE: FRAX (Fracture Risk Assessment Tool) is the tool used to assess 10-year probability of hip/major osteoporotic fracture of patients. It integrates clinical risk factors with bone mineral densitometry for femoral neck. Treatment for osteoporosis is recommended if the FRAX score for hip fracture is more than 3% or that for major osteoporosis-related fracture is more than 20% (National Osteoporosis Foundation 2008).^{8, 9}

EXAMINER: What is the Singh and Maini index for osteoporosis?¹⁰

CANDIDATE: The Singh index has been described on the basis of the presence/attenuation of various compressile/tensile trabeculae on a plain anteroposterior(AP) pelvis radiograph. Grade 6 being normal and Grade 1 being severe osteoporosis.

EXAMINER: Any concerns with using the Singh index for measuring osteoporosis?

CANDIDATE: The Singh index traditionally was used in the diagnosis and classification of osteoporosis. Although it is a simple, cheap method for giving a rough measurement of bone mass, it has been shown to be an inaccurate method of estimating the degree of osteoporosis and the technique is not relevant with the current use of DEXA scans. DEXA scan provides a more precise estimate of bone mineral density and is considered the gold standard for diagnosis and quantification of osteoporosis (Figure 28.5).

EXAMINER: What are the principles of bone scanning? How does a bone scan work?

CANDIDATE: Bone scan involves the intravenous injection of 99mT-MDP (technetium methylene diphosphonate compound). The technetium gets adsorbed onto the hydroxyapatite crystals in bone and emits gamma rays, which are then received using a scintillation gamma camera. The gamma camera contains sodium iodide crystals, which absorb 99mT gamma rays. The received signal is amplified using photomultiplier tubes and processed using a computer to produce an image. It reflects blood flow and osteoblastic activity.

COMMENT: Some features of 99mT-MDP that could be asked by the examiners:

1. 
   

   (1) Half-life: 6 hours.

   
   
2. 
   

   (2) Excretion: urine – 70% of the administered dose is excreted within 24 hours.

   
   
3. 
   

   (3) Dose: 500–600 MBq.

   
   
4. 
   

   (4) Emits only gamma rays (not alpha or beta).

EXAMINER: What is a SPECT scan and give any uses.

CANDIDATE: Single photon emission tomography involves obtaining tomographic images on a bone scan. The scans are obtained with an arc of 360° around the patient. The images can then be reconstructed in axial, coronal and sagittal planes. The SPECT scan is especially useful to study the posterior spinal elements and to look for areas of decreased uptake in osteonecrosis. Sequential imaging with SPECT and CT with the patient in the same position can provide increased diagnostic accuracy by allowing fusion of the anatomical images.¹¹