L11 MRI, US and NMI

Mri Ultrasound And Nuclear Medicine Imaging¹

Dr Dayea Oh OMF Radiologist

Learning Objectives²

• To understand the basic theory of:
  • Magnetic Resonance Imaging (MRI)
  • Ultrasound Imaging / Ultrasonography (US)
  • Nuclear Medicine Imaging (NMI)
    • Scintigraphy
    • Positron Emission Tomography (PET)
    • Single-Photon Emission Computed Tomography (SPECT)
• To know applications / indications for each of the above advanced imaging modalities

Magnetic Resonance Imaging³

I. Magnetic Resonance Imaging (MRI)

Siemens Envision Medical Imaging

Introduction To Mri Principles⁴

Magnetic Resonance Imaging (MRI)

• Magnetic field
  • = “Magnetic”
• Radiofrequency waves
  • = “Resonance”
• NO ionising radiation
• Soft tissue imaging
• Bore

Patient Safety And Contraindications⁵

MRI Scan

• Long scanning time
• ✅ Claustrophobia
• Noisy
• Heat in the area being examined
• ✅ Ferromagnetic metals

Pre-MRI Check⁶

• Pacemaker / Defibrillator (ICD)
• Neurostimulator
• Cochlear Implant
• Stapedectomy (middle ear prosthesis)
• Aneurysm Clip
• Morphine / Insulin Pump
• Glucose Monitor
• Programmable VP Shunt
• Shrapnel
• Retained Metal fragment in eye (eg. from welding / grinding)

Mri Physics And Theory

Step 1. Magnetisation⁷

Step One Magnetisation

• STRONG external magnetic field of 1.5T or higher (MRI machine)
  • 1 T can lift a car!
• Hydrogen atoms (in water “H2O” and fat)
  • Single proton (& 1 electron & zero neutron)
  • Naturally spinning and wobbling particles with +ve charge
  • Charge with motion creates a magnetic field

Magnetisation & z-axis

Under external magnetic field (MRI scanner)

Step Two Resonance And Excitation⁸

Step 2. Resonance; precession, excitation & relaxation

• Hydrogen spin & wobble motion = precession
• Larmor’s equation $\Rightarrow \omega =\gamma B$
  • $\omega$ is the frequency in MHz,
  • $\gamma$ is the gyromagnetic ratio in MHz/Tesla,
  • $B$ is the strength of the magnetic field in Tesla
• H protons precess at the rate of 63.87MHz at 1.5T
• Matching radiofrequency (RF) waves create RESONANCE
• Resonance results in ‘Excitation’
• ‘Relaxation’ releases MR Signals captured by receiver coil
  • Different body tissues ‘relax’ at different rates
  • The faster the tissue relaxes, the more the MR signals released in a given time

Secondary Magnetic Field (Gradient)

• Distorting / tilting the main magnetic field to acquire slices from different sections of the body
• Different resonance frequencies along the gradient
• Localisation of different anatomical structures

Step Three Signal Acquisition And Processing

Step 3. Signal Acquisition and Processing

• MR Signal received by receiver coil is converted into a digital form and transmitted to computer
• The computer processes the data and an image is created

Understanding Mri Signal Sequences

T1 Weighted Sequence⁹

• (black): air, calcium, cortical bone, rapidly flowing blood
• low SI: fluid, ligaments/muscles/tendons, abdominal organs, cartilage
• intermediate SI: high-protein tissue (abscess, complex cysts, synovial fluid)
• high SI: fat, blood, gadolinium (= contrast), melanin, protein

T2 Weighted Image

• (black): air, calcium, cortical bone, rapidly flowing blood
• low SI: ligaments, tendons, liver, pancreas, adrenals, cartilage
• intermediate SI: fat, liver, pancreas, adrenals, muscles, cartilage
• high SI: fluid, CSF, bladder, bile/gallbladder, kidneys

Mri Applications In Dental And Maxillofacial Radiology¹⁰

Applications¹¹

• TMJ disorders
• Tumours and cysts in the head and neck
  • Jaw lesions
  • Salivary gland lesions
  • Paranasal sinus lesions
• Assessment of IDN
• Infection / inflammation eg. osteomyelitis
• Vascular lesions

Diagrams

Mouth Closed

• Glenoid fossa
• Disc
• Condyle
• Bilaminar zone
• EAC

Mouth Open

• Articular eminence
• A, P (Anterior, Posterior)
• ILZ (Interalveolar Ligament Zone - inferred)
• Disc

MRI scans showing:

• Disc, C (Condyle), EAC (External Auditory Canal)

Ultrasound Imaging¹²

• Non-ionising
  • Sound waves
• Soft tissue imaging
• Compact
• Inexpensive
• Real-time imaging
• Technique sensitive

Principles Of Ultrasonography¹³

Ultrasound (US) = sound wave with high frequency

• Diagnostic frequency = 2MHz to 30MHz
• Human hearing is 16Hz – 20kHz

Piezoelectric transducer:

1. Send US waves into the body
2. Receive reflected waves “echoes” from the body
3. Convert echoes into electric signal
4. Signal sent to computer for image display

US transmission is improved with ultrasound gel “jelly”

Understanding Ultrasound Echogenicity

Understanding US Signals “Echogenecity”¹⁴

Echogenicity

Echogenicity Level	Label
Leftmost (light)	hyperechoic
Light-grey	isoechoic
Grey	hypoechoic
Dark grey/black	anechoic

Tissue/Structure	Classification
metal	hyperechoic
bone	hyperechoic
soft tissue	isoechoic
fat	hypoechoic
air	anechoic

EUS TERMINOLOGY¹⁵

’Anechoic’ (black)	‘Hypoechoic’ (dark)	‘Hyperechoic’ (bright)
Anything fluid-filled (cyst, blood vessel…)	Tumor, lymph node…	Fat tissue, stone…

Ultrasound Applications In Dental And Maxillofacial Radiology¹⁶

• Salivary gland disease
• Soft tissue swelling of the head and neck
  • Tumours, Cysts, Infection, etc.
• Vascular diseases and anomalies of the head and neck
  • Carotid artery stenosis (blockage or narrowing)
  • Hematoma (collection of blood in tissues from damaged vessels)
  • Abnormalities in the blood vessels, lymph node, etc.
• Ultrasound-guided procedures
  • Biopsy (Fine needle aspirational / FNA)
  • Corticosteroid injection (TM joint)
• TMJ Imaging
  • Real-time imaging (disc-condyle relationship on mouth opening)

Nuclear Medicine Imaging¹⁷

III. Nuclear Medicine Imaging (NMI)

Principles Of Radiopharmaceuticals

What is Nuclear Medicine Imaging?¹⁸

• Radiology “done inside out”
• Radiopharmaceuticals
  • 99mTc MDP
    • Technetium99m + Methyl diphosphate
    • Half life of 6 hrs
• “Hot spots”
• Three types:
  • Scintigraphy (2D imaging eg. Bone scan)
  • Single photon emission computed tomography (SPECT)
  • Positron emission tomography (PET)

Three Dimensional Imaging Modalities¹⁹²⁰

SPECT (single photon emission computed tomography)²¹

• 3D $\to$ localisation & estimation of the size & extend of the disease
• Single gamma rays
• Two receptors

PET (positron emission tomography)

• Like SPECT, but more dimensionally accurate
• Paired gamma rays
• Multiple receptors

Nuclear Medicine Applications In Dental And Maxillofacial Radiology

Nuclear Medicine Imaging in DMFR²²

• Progressive or spontaneous growth of the jaws
  • Eg. condylar hyperactivity, neoplastic growth
• Osteonecrosis of the Jaws
  • Eg. Medication-related osteonecrosis of the jaws (MRONJ), Osteoradionecrosis of the jaws (ORNJ)
• Osteomyelitis
  • Eg. infectious (secondary) osteomyelitis, primary osteomyelitis
• Bone fractures
• Bone disorders inc. primary bone tumours
• Head and neck cancer $\to$ PET
  • Eg. oral squamous cell carcinoma (OSCC), lymphoma

Further Reading

Footnotes

1. Original PDF page 1: L11 MRI, US and NMI, p.1 ↩

2. Original PDF page 2: L11 MRI, US and NMI, p.2 ↩

3. Original PDF page 3: L11 MRI, US and NMI, p.3 ↩

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