TOPIC: MAGNETIC RESONANCE IMAGING
🌟 IMPORTANT POINTS TO REMEMBER
🔹It is formally called Nuclear Magnetic Resonance.
🔹Felix Bloch is called the father of MRI.
🔹Bloch proposed that the nucleus behaves like a small magnet as it spins on an imaging axis, and has an associated magnetic field.
🔹This field is called the magnetic field.
🔹This equation is called as Bloch equation.
🔹MRI uses non-ionising radiation from the RF band of the electromagnetic spectrum (109–1011 nm of wavelength).
🔹Net Magnetism: Net magnetism is zero because randomly oriented hydrogen nuclei in the field cancel each other.
🔹Hydrogen nuclei behave like the bar magnet.
🔹Precession: Hydrogen nuclei, in addition to aligning parallel or antiparallel to the magnetic field lines after the application of an external magnetic field, also move around in a certain way. This additional motion (wobbling or tilting) of the axis about the direction of magnetic flux is termed precession.
🔹Larmor Frequency: The rate (frequency) of precession is termed the Larmor frequency.
🔹Longitudinal Magnetisation: This vector is a sum vector made up by adding the magnetic vectors of nuclei pointing upwards which are not cancelled by opposite nuclei. As this magnetisation is in a direction along /longitudinal to the external magnetic field, it is also called longitudinal magnetisation.
🔹RF Pulse: You send into the patient not a wave of long duration, but a short burst of some electromagnetic wave, which is called an RF pulse.
🔹Resonance: When RF pulse and the hydrogen protons have the same frequency, they can take energy from the radio wave, a phenomenon called resonance (this is where the ‘resonance’ in magnetic resonance comes from).
🔹Transverse Magnetisation: After application of RF pulse, the protons do not point in random directions any more, but move in step, and point in the same direction at the same time, and thus their magnetic vectors add up in this direction. This results in a magnetic vector pointing to the side to which the precessing protons point. This is in a transverse direction, and it is called transversal magnetisation.
🔹Spin–Lattice Relaxation: The energy that protons had picked up from the RF pulse is handed over to their surroundings, the so-called lattice. And this is why this process is called not only longitudinal relaxation but also spin-lattice relaxation.
🔹Longitudinal Relaxation Time Or T1 Relaxation Time: The time that it takes for the longitudinal magnetisation to recover, to go back to its original value, is described by the longitudinal relaxation time, also called T1.
🔹Transverse Relaxation Time: After the RF pulse is switched off, the protons are no longer forced to stay in step, and as they have different precession frequencies, they will be soon out of phase. The time taken to go out of phase is called as transverse relaxation time or T2 relaxation time or spin-spin relaxation time.
🔹FID Signal: The further the vector goes away from the microphone, the less loud is the sound. The frequency of the sound remains the same because the sum vector spins with the precessing frequency. This type of signal is called a FID signal.
🔹TR: TR-long, i.e. time to repetition long.
🔹Pulse Sequence: When you use more than one RF pulse – a succession of RF pulses – it is called as pulse sequence.
🔹T1- Weighted Image: When we used shorter TR, there was a difference in signal intensity between the tissues, determined by their difference in T1. The resulting picture is called a T1-weighted picture.
🔹Proton Density (-weighted) Images: Where there are no hydrogen protons, there will be no signal; where there are many hydrogen protons, we will have ‘lots’ of signals.
🔹Spin-Echo Sequence: The 180° pulse acts like a wall, from which the protons bounce back, like a mountain reflecting sound waves as echoes. So, this strong signal is also called an echo, or spin echo. This type of pulse sequence is called a spin-echo sequence, consisting of a 90° pulse and a 180° pulse (causing the echo).
🔹Void: The number of hydrogen nuclei within the region also strongly affects the signal magnitude (magnetisation vector). For example, as there are few hydrogen nuclei within bone, the MR signal from bone tissue is undetectable and is termed ‘void’.
🔹MRI System: The MRI system consists of a large magnet, RF coils, gradient coils and the operator’s control station. A low-field magnet system uses static-magnetic fields of <0.2 T, a midfield system uses a field of 0.2–1 T and a high-field magnet uses field strengths in excess of 1 T.
🔹Precaution: MRI is contraindicated for patients with cardiac pacemakers, cerebral aneurysm clips and internal drug infusion pumps; to scan such patients could result in death.
🔹Signal Intensity: The intensity of signal from each tissue on MR images is termed the ‘signal intensity’. Commonly, if the signal intensity from tissue is lower than that of muscle on T1- or T2-weighted images, it is referred to as ‘low-signal intensity’. If the signal intensity from tissue is the same or higher than that from fat tissue on T1- or T2-weighted images, it is referred to as ‘high-signal intensity’.
🔹Fat Tissue: Fat tissue appears as high-signal intensity on T1-weighted images and low signal intensity on T2-weighted images with fat suppression.
🔹Muscle tissue: Muscle commonly appears as low-signal intensity on both T1- and T2-weighted images with fat suppression.
🔹Cortical Bone Tissue: Cortical bone tissue is indicated as a signal intensity void on T1- and T2-weighted images; however, cancellous bone tissue demonstrates high-signal intensity on T1-weighted images and low-signal intensity on T2-weighted images with fat suppression.
🔹Lymph Nodes And Tonsils: Lymph nodes and tonsils have low-signal intensity on T1-weighted images and intermediate-high-signal intensity on T2-weighted images with fat suppression.
🔹Teeth: The teeth, except pulp tissue, appear as a signal void on T1- and T2-weighted images; pulp tissue has intermediate-signal intensity on T1-weighted images and high-signal intensity on T2-weighted images with fat suppression.
🔹Parotid Gland: Signal intensities differ among the tissues of the salivary glands. The parotid glands have relatively high-signal intensity on T1-weighted images and low-signal intensity on T2-weighted images with fat suppression.
🔹Submandibular Gland: The submandibular glands have intermediate-signal intensity on T1-weighted images and low-signal intensity on T2-weighted images with fat suppression.
🔹Sublingual Gland: The sublingual glands have intermediate-signal intensity on T1-weighted images and relatively high-signal intensity on T2-weighted images with fat suppression.
🔹TMJ: The discs of the TMJ have low-signal intensity on T1- and T2-weighted images.
🔹Cavities: The cavities (maxillary sinus and nasal cavities) appear as void signal on T1- and T2-weighted images.
🔹Blood vessels: Blood vessels usually have void signal intensity due to blood flow, termed ‘signal void’, on both T1- and T2-weighted images.
🔹MR Images Using Contrast Medium: The contrast material used for MRI is Gd-DTPA, which belongs to the lanthanide group of elements.
🔹Identification Of Vessels In Oral and Maxillofacial Regions Using MRA: MRA, without injection of contrast media, is a technique for the detection of vessel systems. Vessels with flow are indicated as bright homogenous linear structures on MRA.
🔹Identification Of Salivary Gland Ducts Using MR Sialography: MR sialography is non-invasive and does not require cannulation of the duct, injection of contrast medium or the use of ionising radiation. On MR sialographic images, the extraglandular and intraglandular portions of the parotid and/or submandibular gland and the paths of the duct branches are indicated from multiple angles as bright homogenous linear structures.
🔹Identification Of Trigeminal Nerve In REZ Using MR Cisternography: Trigeminal nerves in the REZ are identified as slightly clearer homogenous linear structures on MR cisternographic images.
🔹fMRI: This technique also enables the identification of motor and sensory areas of the brain in relation to oral-related functions, including occlusion.
📌 The Simple Steps In MRI Are As Follows:
🔸 The patient is placed in a magnet.
🔸A radio wave is sent in the patient.
🔸The radio wave is turned off.
🔸The patient emits a signal.
🔸The picture is reconstructed.
📌MULTIPLE CHOICE QUESTION (MCQs)
💡Felix Bloch is the father of
a. MRI
b. CT scan
c. USG
d. Sialography
Answer: A
💡MRI uses
a. Non-ionising radiation
b. Ultraviolet radiations
c. X-rays
d. g rays
Answer: A
💡Which of the following features is true about MRI?
a. Non-invasive
b. Uses radio wave/RF pulse
c. Strong magnetic field
d. All of the above
Answer : D
💡The direction of magnetic field can be determined by
a. Left-hand rule
b. Right-hand rule
c. Thumb rule
d. Rule of isometry
Answer: A
💡MRI is not indicated in
a. Tumour staging
b. TMJ investigation
c. Assessment of intracranial lesion
d. Patients with insulin pumps
Answer : D
💡MRI is contraindicated for patients with
a. Cardiac pacemakers
b. Cerebral aneurysm clips
c. Internal drug infusion pumps
d. All of the above
Answer : D
💡MRI system consists of
a. Large magnet
b. RF coils
c. Gradient coils
d. Control station
e. All of the above
Answer: E
💡External magnetic field applied to the body in modern MRI system is
a. 1.5 T
b. 10 T
c. 0.1 T
d. 10–15 T
Answer : A
💡 Phenomenon of energy transmission from the RF pulse to hydrogen nuclei is termed as
a. Resonance
b. Magnetism
c. MDM
d. Magnetising vector
Answer : A
💡 To obtain MR images of oral region, patient is aligned along
a. Z-axis
b. X-axis
c. Y-axis
d. None of the above
Answer : A
💡Greater contrast resolution of images is in
a. MRI
b. CT
c. USG
d. Conventional tomography
Answer : A