---
title: "MRI components, image detection, MRI vs CT"
subject: "Biomedical Instrumentation"
chapter: 5
key: "semester_7/biomedical_instrumentation:mri-components-vs-ct"
source: https://www.bctengineer.com/q/semester_7/biomedical_instrumentation/mri-components-vs-ct
site: BCT Engineering
---

# MRI components, image detection, MRI vs CT

Biomedical Instrumentation · Chapter 5: Non- Invasive Diagnostic Instruments

- Asked in: 2075 Chaitra, 8 marks
- Video: [The Insane Engineering of MRI Machines](https://www.youtube.com/watch?v=NlYXqRG7lus) — Real Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/biomedical_instrumentation/mri-components-vs-ct
- Whole chapter: https://www.bctengineer.com/md/semester_7/biomedical_instrumentation/chapter-5

## Question

What are the system components of MRI? Explain Image Detection system with its function and application. Give four differences of MRI with CT-scan machine.

## Short answer

MRI system includes a magnet, gradient and RF coils, receiver, and computer. The receiver detects proton relaxation signals, digitizes them, and constructs images via 2D Fourier transform, yielding high soft-tissue contrast for brain/spine. MRI is non-ionizing, offers excellent soft-tissue contrast and any-plane imaging, but is slower and costlier than CT, which uses X-rays and excels at bone/lung.

## Answer

**MRI (Magnetic Resonance Imaging)** produces images from the radio signals emitted by hydrogen nuclei (protons) of the body when placed in a strong magnetic field and excited by RF pulses.

#### System components
```text
 +--------------------------------------+
 | Main magnet (0.5-3 T) + shim coils   |
 |  gradient coils (X,Y,Z)  RF coil     |
 |       [ patient on table ]           |
 +--------------------------------------+
   ^ gradient amps ^ RF transmitter   | signal
   |               |                   v
 [Computer / control] <- [RF receiver, ADC]
        |
 [Display, storage]
```
1. **Main magnet:** superconducting (or permanent/resistive) magnet giving a strong uniform field B0, to align the proton spins.
2. **Shim coils:** make the field homogeneous.
3. **Gradient coils (X, Y, Z):** give a linear field variation for slice selection and position (frequency and phase) encoding.
4. **RF coils and transmitter:** send an RF pulse at the Larmor frequency (f = 42.58 MHz/T × B0) to flip the spins.
5. **RF receiver and image detection system:** picks up the signal.
6. **Computer, console, display and storage.**

#### Image detection system
- **Function:** after the RF pulse, the protons return to equilibrium and emit a weak RF signal (FID or echo) which induces a voltage in the receiver coil. The signal is amplified by a low-noise preamplifier, demodulated, filtered and digitised by an ADC. The digital data fill k-space; a **2D Fourier transform** converts it into the image, with brightness depending on proton density, T1 and T2.
- **Application:** gives high-contrast images of soft tissue: brain, spinal cord, joints, heart, tumours, and with contrast agents for blood vessels (MR angiography).

#### Four differences between MRI and CT
| Point | MRI | CT |
|---|---|---|
| Radiation | Non-ionising (magnetic field and RF) | Ionising X-rays |
| Soft tissue contrast | Excellent | Moderate |
| Planes | Any plane directly | Mainly axial (reformatted) |
| Scan time and cost | Longer, costly | Shorter, cheaper |
| Best for | Brain, spine, joints | Bone, lung, emergency |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the function of the X, Y, Z gradient coils in MRI?
   A. To produce the main strong field B0
   B. To give a linear field variation for slice selection and position encoding
   C. To digitise the signal
   D. To cool the magnet
   Answer: B. Gradient coils vary the field linearly so frequency and phase encode position and select slices. (Key point: Gradient coils)

2. What does the RF pulse frequency depend on?
   A. It is fixed at 1 MHz
   B. It is the Larmor frequency, f = 42.58 MHz/T × B0
   C. It depends only on T2
   D. It is proportional to the patient's weight
   Answer: B. The RF pulse must be at the Larmor frequency of protons, proportional to B0. (Key point: Larmor frequency)

3. How does the image detection system produce an image?
   A. The coil signal is amplified, demodulated and digitised, filling k-space, then a 2D Fourier transform gives the image
   B. The film is exposed by the signal
   C. The X-rays are filtered back-projected
   D. The signal is displayed directly on an oscilloscope
   Answer: A. Signal from the receiver coil goes through preamp, demodulator, filter and ADC; 2D FT of k-space gives the image. (Key point: Image detection)

4. Which is a true difference between MRI and CT?
   A. MRI uses ionising X-rays, CT uses RF
   B. MRI is non-ionising with excellent soft-tissue contrast and any plane directly; CT is ionising and faster
   C. MRI is cheaper and faster than CT
   D. CT gives any plane directly
   Answer: B. MRI uses magnetic field and RF (non-ionising) and excels in soft tissue, but is longer and costly. (Key point: MRI vs CT)

## Related in Biomedical Instrumentation

- [Computed tomography, CT vs X-ray, biological effects](https://www.bctengineer.com/question.md?key=semester_7%2Fbiomedical_instrumentation%3Acomputed-tomography-ct-xray) — Biomedical Instrumentation, chapter 5

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