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Medical Imaging > 2. Basics of MRI > Flashcards

2. Basics of MRI Flashcards

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USMLE® Step 1 flashcards
1
Q

What are the characteristics of MRI?

A
  • emission imaging
  • non-ionizing radiation
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2
Q

How long does an average MRI take?

A

30-50 minutes (longer than CT)

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3
Q

How long is the MRI for acute stroke protocol?

A

approximately 12 minutes

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4
Q

How does MRI work?

A

hydrogen nuclei (containing one proton) are used due to the abundance in the human body

  1. protons are randomly oriented in the body without a magnetic field
  2. once on the scanner, the magnetic field aligns all protons to become parallel to the magnetic field along the longitudinal plane
  3. radiofrequency pulses are used to orient the protons in the same direction and excite them
  4. the protons relax once the RF pulse ends, releasing a radiofrequency signal
  5. the relaxation happens at varying speeds depending on the tissue, and this is detected by the scanner to produce an image

T1 relaxation is longer than or equal to T2 relaxation

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5
Q

What are the parts of an MRI scanner?

A
  • magnet: strong stable magnetic field
  • radiofrequency coils: provides RF pulses for excitation
  • gradient coils: produces variable fields that can be adjusted to the needs of the image
  • shim coils: makes the magnetic fielt homogenous
  • receiver coils: records the emitted RF signals

scanner bore is where the patient rests and all the coils and magnets are on the gantry

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6
Q

What are the characteristics of the MRI magnet?

A
  • superconducting magnet (cooled down to almost absolute zero, when there is almost no resistance to flow of electricity)
  • elcetric current flows continuosly, creating a permanent magnetic field – MRI magnet is ALWAYS on
  • magnetic field of clinical scanners is 0.2-3 Tesla (T) while the earth’s magnetic field is only 50 microT
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7
Q

How does the radiofrequency pulse excite the protons?

A

RF is sent at the protons’ resonance frequency (Larmor frequency) and this changes the orientation of the proton, which is referred to as excitation

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8
Q

What is the difference between T1 and T2 relaxation?

A

T1 and T2 relaxation happen at the same time but have different durations

T1: also referred to as recovery; the time it takes for the proton to recover to the longitudinal state, parallel to the external magnetic field

T2: also referred to as decay; the time it takes for the proton to gain transverse orientation (perpendicular to the external magnetic field)

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9
Q

How do the gradient coils work in an MRI scanner?

A
  • gradient coils are loops of wire or thin conductive sheets on a cylindrical shell in the gantry
  • when current passes through it a secondary magnetic field is produced to slightly distort the main magnetic field in a predictable patter
  • allows for spatial decoding of the MR signal
  • each electrical pulse makes the coils vibrate, which produces a loud noise
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10
Q

What are the scanner-related factors determining MR intensity?

A
  • magnetic field strength
  • T1 weighting
  • T2 weighting
  • time of repetition
  • echo time
  • flip angle
  • presaturation
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11
Q

What are the patient-related factors that determine MR signal intensity?

A
  • proton density
  • intercellular water content
  • intercellular structure
  • fat content
  • protein content
  • cytoplasm
  • fibrotic structure
  • motion
  • cellularity
  • nuclear/cytoplasmic ratio
  • paramagnetic materials
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12
Q

What is time of repetition in MRI?

A

the repetition between two RF excitations, which influences the amount of T1 weighting

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13
Q

What is echo time in MRI?

A

time in between the pulse and its resultant echo, which influences the amount of T2 weighting

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14
Q

What is T1 weighting?

A
  • RF pulses have short/intermediate TR and short TE, to create a T1 weighted image
  • tissues with short T1 appear bright (hyperintense)
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15
Q

What is T2 weighting?

A
  • RF pulses have a long TR and long TE, to create a T2 weighted image
  • tissues with a long T2 will appear bright (hyperintense)
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16
Q

How does fluid appear in T1 and T2 MRI images?

A

T1: dark
T2: bright

find fluid (CSF, water, urine etc.) on an MRI image to distinguish between T1W and T2W

17
Q

What appears bright on T1W MRI?

A
  • fat
  • hemorrhage (2-28 days)
  • melanin
  • proteinaceaous structures
  • gadolinium and other paramagnetic substances (manganese, copper)
18
Q

What appears bright on a T2W MRI?

A
  • fat
  • water, edema, infection, cysts
  • hemorrhage
19
Q

Appearance of brain scans on T1W MRI?

A

CSF: very black
fat: white
grey matter: dark grey
white matter: light grey

20
Q

Appearance of brain scans on T2W MRI?

A

CSF: very white
fat: grey
grey matter: light grey
white matter: dark grey

21
Q

What is the flow-void phenomenon?

A
  • flow void is when there is a signal loss in vessels containing vigorously flowing blood
  • protons in flowing fluid move out of the plane, so it appears black on MRIs
22
Q

Determining the stages of hemorrhages

A
  • MRIs can precisely determine the age/stages of hemorrhages
  • this is important in forensic cases (ie. suspected child abuse)
  • immediate: (oxy-Hb) appears grey on T1W and T2W
  • 1-2 days: (deoxy-Hb) appears dark on T2W, and grey on T1W
  • 2-7 days: (intracellular MetHb) appears dark on T2W and bright on T1W
  • 7-28 days: (extracellular MetHb) appears bright on T1W and T2W
  • hemosiderin appears dark on both T1W and T2W
23
Q

What are the types of pulse sequences?

A
  • pulse sequences are preselected according to the specific disease or body parts
  • spin echo and gradient recalled echo are the two main types
    - SE has high signal to noise ratio (better image quality)
    - GRE is faster for rapid imaging
24
Q

What is fat suppression in MRI?

A
  • fat suppression is the suppression of signals from fatty tissues (fat appears dark after suppresion), which helps visualize pathologies better
  • fat suppression also helps in identifying fat-containing lesions (ie. ovarian dermoid cysts, adrenal adenomas, myelolipomas, liposarcomas)
  • easier to visualize hyperintense structures on a darker background, especially in the abdominal region
25
Q

What is fluid suppresion in MRI?

A
  • fluid suppression is used to produce FLAIR (fluid attenuated inversion recovery) images.
  • free fluid is usually bright on T2W MRI but appears dark in case of FLAIR
26
Q

What is diffusion weighted imaging?

A
  • DWI produces an image by measuring the diffusivity of water due to Brownian motion in the tissue
  • restricted diffusivity appears as hyperintense
27
Q

What is an ADC map?

A
  • ADC (apparent diffusion coefficient) is a quantitative measure of diffusion. ADC map uses DWI images to identify diffusion restriction.
  • restricted diffusivity appears as hypointense
28
Q

Imaging of acute stroke with DWI and ADC

A
  • the cytoxic edema leads to less brownian motion and thus appears as restricted diffusion on the DWI
  • DWI: area of stroke is hyperintense
  • ADC map: area of stroke is hypointense
29
Q

Imaging of brain malignancy on DWI and ADC

A
  • tumor tissue is more cellular and thus has restricted diffusion
  • solid mass is hyperintense on DWI
  • solid mass is hyopointense on ADC
30
Q

What is MR spectroscopy?

A
  • functional technique
  • most commonly used in examination of bran lesions
  • provides quantitative information of the various metabolites
  • doesn’t add much information but increases specificity and ability to determine the histological grade accurately
31
Q

What is 3D TOF imaging?

A
  • 3D time of flight angiography
  • used to visualize flow within vessels w/o administration of iv. contrast agent (MR technique)
  • subsitute for CTA or DSA since it doesn’t require any iodine-based contrast agent
32
Q

What is gadolinium and its effect?

A
  • most common iv. contrast used in MRI
  • gadolinium is a rare earth metal that is chelated with different compounds to form the contrast agent
  • once injected intravenously, it enhances organ parenchyma and then is excreted by the kidneys
  • shortens T1 relaxation time, so T1W images appear brighter with gadolinium than without (postgadolinium images are always T1W)
  • brighter structures are described as enhancing
33
Q

What are the precautions to take for claustrophobia, in case of MRI?

A
  • the MRI bore is narrower than CT but also takes longer, so someone struggling of sever claustrophobia might be unable to complete the examination
  • it is possible to treat the patient with sedative or use an open magnet imaging machine
  • communication to claustrophobic patients is crucial!!
34
Q

What are the precautions to take for ferromagnetic objects, in case of MRI?

A
  • ferromagnetic objects can move and cause damage to the surrounding tissue, or get heated and burn surrounding tissue
  • examples within the body: ferromagnetic cerebral aneurysm repair clip, vascular clip, surgical staples (these are now made to be MRI-compatible); bullets, shrapnel or metal in the eyes (common in metal workers)
  • patients with possibility of a foreign ferromagnetic object in their body must undergo X-ray before MRI
  • In case of metal present, CT is used
  • ferromagnetic objects around the MRI machine (ie. oxygen tanks, scissors, scalpels, metallic tools) are strictly forbidden in an MRI scanning room
35
Q

What are the precautions to take for mechanical or electrical devices, in case of MRI?

A
  • pacemakers, pain stimulator implants, insulin pumps, implantable drug infusion pumps or cochlear implants
  • these devices may not be MRI safe, or may need to be reprogrammed to be MRI compatible
36
Q

What are the precautions to take for pregnancy, in case of MRI?

A
  • MRI is not contraindicated
  • MRI is used if the risk-to-benefit ratio weighs in favor of performing the MRI
  • there is no evidence of MRI harmful effects on the developing fetus
  • gadolinium is contraindicated because it crosses the placenta and has to be excreted by the fetal kidneys (unknown effects on the fetus)
37
Q

What is nephrogenic systemic fibrosis?

A
  • gadolinium can lead to nephrogenic systemic fibrosis, in case of renal insufficiency
  • NSF produces fibrosis of the skin, eyes, joints and internal organs, resembling scleroderma
  • high-risk iv. contrast agents have been removed from the market
  • patients with end stage or chronic kidney disease (especially when on dialysis) are at higher risk, in this case gadolinium is not administered
  • best to administer gadolinium when GFR>30
38
Q

What are the strengths of MRI?

A
  • 3D imaging
  • creates images on different anatomical planes
  • non-ionizing radiation
  • multiparametric imaging
  • excellent soft tissue resolution (even w/o iv. contrast)
  • good spatial resolution
  • whole-body MRI can be used in oncology
  • safe during pregnancy
39
Q

What are the weaknesses of MRI?

A
  • high cost of equipment
  • availability can be a problem
  • spatial resolution is inferior to CT
  • signal void phenomenon
  • image acquisition is relatively slow
  • more motion artifacts than CT
  • NSF
  • patient co-operation is necessary
  • contraindications: aneurysm clip, claustrophobia, pacemaker etc.

Medical Imaging (16 decks)

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