Imaging techniques

Plain X-Ray

  • X-rays are high-energy radiation, which undergo differential absorption by tissues as they pass through the body.

X-ray Beam Generation: - A tungsten cathode is heated in a vacuum. - Generates high-velocity electrons directed toward a tungsten anode. - On hitting the anode, some electrons are knocked out of orbit to create X-rays (only 1% of these electrons are used to make the X-ray beam).

Quantity and Quality of X-rays Generated: - Quantity of X-rays is proportional to the number of moving electrons. - Quality of X-rays is proportional to the speed of the electrons (energy they carry).

X-ray Cassette: - Carbon fiber front, lead backing, and a film coated in photographic emulsion (silver iodobromide). - Carbon fiber and lead prevent X-ray scatter.

Image Processing: - X-rays captured on the film as black silver crystals. - Digital images have lower resolution than plain films due to a finite grey scale.

Types of X-ray Radiation: - Primary: Beam directed from the tube to the X-ray plate/cassette. - Secondary: Scatter of X-rays reflected from the patient or other objects, causing blur.

Parameters of X-ray Measurement: - kV: Penetration of X-rays (energy carried). - mA: Measurement of X-ray strength (volume of X-rays). - Total X-ray dose: Measured in Grey. - Effective dose: Measured in Sieverts.

Effective Dose Comparisons: - Bone scan = 5.0 mSv. - CT Spine = 3.6 mSv. - Chest X-ray = 0.05 mSv.

CT Scanning

  • X-rays are delivered by a fan-shaped rotating tube on a gantry, and sensitive detectors record the attenuated X-rays to form an image.
  • Each image is made up of pixels (each pixel has depth, termed a voxel).

Attenuation: - Describes the amount of X-rays absorbed by tissues. - Hounsfield Units (HU) describe the attenuation coefficient of tissues: - Bone: +1000 HU - Water: 0 HU - Air: -1000 HU

Advantages: - Reconstructions possible in any plane. - Excellent for surgical planning in complex fractures. - Superior resolution of cortical bone.

Disadvantages: - High radiation dose. - Claustrophobia.

MRI (Magnetic Resonance Imaging)

  • Uses superconducting magnets and radiofrequency (RF) coils to manipulate hydrogen ions (protons) to create high-contrast images.

Mechanism: - Protons spin around their own axis (nuclear spin). - A magnetic field aligns their spin longitudinally. - Energy delivered by RF pulse causes protons to spin transversely. - Once the RF pulse is turned off, protons realign and release energy, detected by RF receiver coils.

Imaging Terms: - T2: Time taken for protons to lose coherence. - T1: Time taken for 63% of protons to return to longitudinal spin. - TR: Time between repetition of pulses (T2 has higher TR times). - TE: Time to Echo (T2 has higher TE times).

Common Sequences: - T2 Weighted: Fluid and fat appear white. - T1 Weighted: Fat appears white. - STIR (fat suppression): T2 weighted with dark fat.

Gadolinium: Contrast agent that enhances imaging, showing pathologic fluid collections better (abscess).

Radionuclide Bone Scanning

  • Uses gamma rays emitted from a radioactive isotope (Technetium-99) bound to phosphate to map blood flow and osteoblast activity.

Technetium-99: - Unstable radioisotope derived from Molybdenum-99 decay. - Short half-life of 6 hours and is excreted via kidneys.

Mechanism: - Technetium-99 is attached to Methyl Diphosphonate (MDP) and injected intravenously. - MDP interacts with hydroxyapatite crystals in bone, directly reflecting osteoblastic activity. - Gamma rays emitted by Technetium-99 are detected by a gamma camera to create a digital image.

Phases of a Triple Phase Bone Scan: - Dynamic Phase (1-2 min): Shows arterial flow and hyperperfusion. - Equilibrium Phase (3-5 min): Shows bone and soft tissue hyperemia. - Static Phase (4 hours): Soft tissue activity clears, leaving only bone activity.

Uses: - Tumors, infections, stress fractures, prosthetic loosening, Paget’s disease.

Disadvantages: - Poor specificity but very sensitive. - False negatives in areas with low blood supply (e.g., avascular bone).

DEXA Scanning (Dual Energy X-ray Absorptiometry)

  • Utilizes X-rays of different energies absorbed differently by bone and soft tissue.
  • Used to assess bone mineral density (BMD).

Result Interpretation: - T Score: Compares BMD to peak young adult BMD. - Z Score: Compares BMD to an age-matched individual.

WHO Criteria for Osteoporosis: - 0 to -1: Normal. - -1 to -2.5: Osteopenia. - < -2.5: Osteoporosis. - < -2.5 + fragility fracture: Severe osteoporosis.

Ultrasound

  • Ultrasound waves are produced by a piezoelectric ceramic crystal within a transducer.
  • Voltage applied to the crystal alters its shape, generating a compression wave (ultrasound wave).
  • Reflected waves distort the piezoelectric crystal, generating an image.

Advantages: - Non-ionizing, cheap, portable, and dynamic imaging.

Disadvantages: - Limited to superficial structures and is operator-dependent.

This webpage provides a detailed overview of different imaging techniques used in medical practice, including X-ray, CT, MRI, radionuclide scanning, DEXA scanning, and ultrasound. Each technique has distinct advantages, limitations, and specific uses in clinical diagnostics.

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