Answer

Correct option: (iii) Mechanical

Ultrasound uses high-frequency mechanical sound waves, not electromagnetic radiation. These waves require a material medium to propagate.

Answer

Correct option: (iii) Blood flow velocity

The Doppler effect measures the frequency shift produced when ultrasound reflects from moving blood cells. The shift is related to the velocity and direction of blood flow, so Doppler ultrasound is widely used in vascular and cardiac imaging.

Answer

Correct options: (i) X-ray and (iv) CT-scan

Both conventional X-ray imaging and CT use ionizing X-ray radiation. MRI and ultrasound do not use ionizing radiation.

Exam note: If the paper expects only one option, CT-scan may be intended depending on the original wording, but scientifically both X-ray and CT are ionizing-radiation modalities.

Answer

Correct option: (iii) Radiofrequency waves

MRI uses a strong static magnetic field together with radiofrequency (RF) electromagnetic pulses to excite hydrogen nuclei. The detected RF signals are processed to form images.

Answer

Answer: Gamma camera / scintillation detector

In nuclear medicine, gamma photons are detected using a gamma camera, whose detector system commonly uses a scintillation crystal such as NaI(Tl) coupled to photodetectors. The detector converts gamma-ray energy into electrical signals for image formation.

The OCR of the paper has lost some answer choices, but CT and MRI do not directly detect gamma rays.

Pulse Height Analyser in a Gamma Camera

Definition

A Pulse Height Analyser (PHA) is an electronic energy-discrimination system used in nuclear medicine to select detector pulses corresponding to a desired range of gamma-photon energies.

Need for PHA

A gamma camera receives useful photons as well as scattered photons. Compton-scattered photons have lower energies than the original photopeak. If these scattered events are accepted, image contrast and spatial accuracy decrease.

Working

Gamma Photon
     ↓
Collimator
     ↓
Scintillation Crystal
     ↓
Photomultiplier Tubes
     ↓
Position + Energy Signals
     ↓
Pulse Height Analyser
     ↓
Accept / Reject
     ↓
Image Computer

1. A gamma photon enters through the collimator.
2. The scintillation crystal converts the gamma photon into visible light.
3. Photomultiplier tubes or solid-state photodetectors convert the light into electrical pulses.
4. Pulse amplitude is approximately related to deposited photon energy.
5. The PHA compares the pulse height with a selected energy window around the photopeak.
6. Pulses inside the window are accepted for image formation; others are rejected.

Energy Window

For a radionuclide with a known photopeak energy $E_0$, a window may be selected around $E_0$, for example:

$$E_0 \pm \Delta E$$

The exact window is chosen according to the radionuclide and camera protocol.

Importance

• Rejects a large fraction of scattered photons.
• Improves image contrast.
• Improves quantitative accuracy.
• Helps isolate the desired photopeak.

Conclusion

The PHA acts as an energy filter in a gamma-camera system, accepting pulses within the selected energy range and rejecting unwanted radiation.

Generations of CT Scanners

CT scanners evolved through several generations to improve speed, coverage, image quality and practicality.

Generation	Beam	Detector arrangement	Main feature
1st	Pencil beam	Single/few detectors	Translate-rotate; very slow
2nd	Narrow fan beam	Multiple detectors	Faster than 1st generation
3rd	Wide fan beam	Curved detector array	Rotate-rotate; common modern basis
4th	Wide fan beam	Fixed 360° detector ring	Rotating X-ray tube only
5th	Electron beam	Stationary target/anode arrangement	Very rapid cardiac imaging historically

1. First Generation

Used a narrow pencil beam and a single detector or very small detector array. The tube and detector translated across the patient and then rotated slightly. Many measurements were required, making scanning slow.

2. Second Generation

Used a narrow fan beam and multiple detectors. The increased number of detectors reduced the number of translation steps and shortened scan time.

3. Third Generation

Uses a broad fan beam with the X-ray tube and detector array rotating together around the patient. This design became the basis of most conventional modern CT systems.

4. Fourth Generation

Uses a stationary ring of detectors surrounding the patient while the X-ray tube rotates inside the ring. It can provide rapid acquisition but has different cost, calibration and dose considerations.

5. Fifth Generation

Electron-beam CT used an electron beam steered onto a stationary target ring to generate X-rays without mechanically rotating the X-ray tube. It was designed especially for very rapid imaging such as cardiac applications.

Modern CT

Modern scanners commonly use rotating gantries, multi-row detector arrays and helical/spiral scanning. Continuous rotation with slip-ring technology permits rapid volumetric acquisition, followed by computer reconstruction into cross-sectional and 3D images.

Conclusion

The progression from first to later generations primarily improved scan speed, detector coverage, spatial/temporal performance and clinical applicability.

Ultrasonography Modes and Preference for B-Mode

Ultrasound systems display returning echoes in different modes depending on how echo information is represented.

Major Modes

1. A-Mode (Amplitude Mode)

Displays echo amplitude as spikes against depth. It is mainly used in specialized applications such as ophthalmic measurements.

2. B-Mode (Brightness Mode)

Converts echo strength into brightness at the corresponding spatial location. Multiple scan lines form a two-dimensional grayscale image.

3. M-Mode (Motion Mode)

Displays movement of structures along a selected ultrasound line as a function of time. It is especially useful for cardiac valve and wall motion.

4. Doppler Mode

Uses frequency shifts caused by moving blood cells to estimate flow direction and velocity.

Common forms include spectral Doppler, color Doppler and power Doppler.

Why B-Mode is Preferred

B-mode is widely preferred for general diagnostic ultrasound because it provides a real-time two-dimensional anatomical image.

Advantages:

• Good visualization of anatomy.
• Real-time imaging.
• Grayscale representation is easy to interpret.
• No ionizing radiation.
• Suitable for abdomen, obstetrics, thyroid, breast and many other examinations.
• Can be combined with Doppler for simultaneous structural and flow assessment.

Conclusion

Although A-, M- and Doppler modes have specialized uses, B-mode is the principal general-purpose mode because it provides real-time anatomical imaging.

Cardiac Imaging Using Ultrasonography

Cardiac ultrasound is called echocardiography. It uses a transducer to send and receive ultrasound waves and produces real-time images of the heart.

Basic Arrangement

Ultrasound Probe
      ↓
Chest Wall
      ↓
Heart Structures
      ↓
Reflected Echoes
      ↓
Probe
      ↓
Beamformer / Processor
      ↓
B-mode + Doppler Image

Procedure

1. The patient is positioned, commonly in a left lateral decubitus position for transthoracic imaging.
2. Acoustic gel is applied to remove air between the probe and skin.
3. A phased-array transducer is placed at standard acoustic windows such as parasternal, apical, subcostal and suprasternal positions.
4. Short ultrasound pulses are transmitted into the chest.
5. Echoes from myocardium, valves and blood interfaces return to the probe.
6. B-mode creates real-time anatomical images.
7. M-mode can measure rapid valve and wall motion along a selected line.
8. Doppler measures blood-flow velocity and direction.
9. Color Doppler maps flow over the anatomical image.

Important Measurements

Echocardiography can assess:

• Chamber size.
• Wall thickness and motion.
• Valve structure and function.
• Blood-flow velocity.
• Ventricular function and ejection fraction.
• Abnormal flow such as regurgitation or stenosis.

Advantages

• Real-time imaging.
• No ionizing radiation.
• Portable and relatively inexpensive.
• Can assess both anatomy and blood flow.
• Useful for repeated examinations.

Limitations

Image quality can be reduced by obesity, lung air, ribs and other acoustic barriers. Operator skill and correct acoustic windows are important.

Conclusion

Echocardiography combines B-mode, M-mode and Doppler techniques to evaluate cardiac anatomy, motion and blood flow in real time.

Gamma Camera: Working Principle

A gamma camera detects gamma photons emitted by a radiopharmaceutical inside the patient and produces a two-dimensional distribution image.

Schematic

Patient / Radiopharmaceutical
          ↓ Gamma photons
      Collimator
          ↓
  NaI(Tl) Scintillation Crystal
          ↓ Light
 Photomultiplier Tubes
          ↓
 Position + Energy Circuits
          ↓
 Pulse Height Analyser
          ↓
 Computer / Image Processor
          ↓
       Gamma Image

Construction

Main components are:

1. Collimator – accepts photons traveling in selected directions and determines spatial localization.
2. Scintillation crystal – commonly sodium iodide activated with thallium, NaI(Tl), converts gamma photons into visible light.
3. Photodetectors – traditionally photomultiplier tubes convert light into electrical signals.
4. Positioning electronics – estimate the interaction position.
5. Pulse Height Analyser – selects photons in an appropriate energy window and rejects many scattered events.
6. Computer – converts accepted events into an image.

Working

1. A radiopharmaceutical accumulates in the organ of interest.
2. The radionuclide emits gamma photons.
3. The collimator allows selected photon trajectories to reach the detector.
4. The crystal produces flashes of light when gamma photons interact.
5. Photodetectors convert the light into electrical pulses.
6. The system estimates the position and energy of each event.
7. The PHA accepts events within the selected energy window.
8. Accepted events are accumulated to form the image.

Applications

• Bone scans.
• Thyroid imaging.
• Renal imaging.
• Cardiac perfusion studies.
• Hepatobiliary studies.

Conclusion

A gamma camera converts the spatial distribution of emitted gamma radiation into an image using collimation, scintillation, photodetection, energy selection and computer processing.

Construction of an Ultrasonic Probe

An ultrasonic probe (transducer) converts electrical energy into ultrasound and returning ultrasound echoes back into electrical signals.

Basic Construction

        Probe Face
   ┌──────────────────┐
   │ Acoustic Lens    │
   ├──────────────────┤
   │ Matching Layer   │
   ├──────────────────┤
   │ Piezoelectric    │
   │ Crystal / Array  │
   ├──────────────────┤
   │ Backing / Damping│
   ├──────────────────┤
   │ Electrodes       │
   └────────┬─────────┘
            │ Cable

Main Components

1. Piezoelectric Element

Usually made from a piezoelectric ceramic or other piezoelectric material. It converts electrical pulses into mechanical vibrations and converts returning echoes back into electrical signals.

2. Electrodes

Electrodes apply the excitation voltage and collect the electrical signal generated by received echoes.

3. Matching Layer

The acoustic impedance of the piezoelectric element is much higher than that of soft tissue. A matching layer reduces acoustic reflection at the probe-tissue interface and improves energy transmission.

4. Backing/Damping Material

The backing absorbs backward-directed acoustic energy and shortens the pulse duration. This improves axial resolution and controls ringing.

5. Acoustic Lens

The lens focuses the ultrasound beam and protects the probe face.

6. Housing and Cable

The housing mechanically supports the components, while the cable carries electrical excitation and received signals to the ultrasound system.

Working

1. The scanner applies a short electrical pulse to the piezoelectric element.
2. The element expands and contracts because of the piezoelectric effect, producing ultrasound.
3. The acoustic lens/matching layers transmit the sound efficiently into tissue.
4. Echoes return from tissue boundaries.
5. The piezoelectric element converts the echoes into electrical signals.
6. The scanner amplifies and processes these signals to form the image.

Array Probes

Modern probes often contain many small elements. Electronic beam steering and focusing can produce rapid scanning without mechanically moving the probe.

Conclusion

The probe's piezoelectric element, electrodes, matching layer, backing material, acoustic lens and housing work together to transmit and receive diagnostic ultrasound.