EXAM #7 — MODULE 7

Question 1

Patient placed in magnetic bore: longitudinal axis of body parallel to longitudinal magnetic field

1. _____ axis: from right to left
2. _____ axis: from anterior to posterior
3. _____ axis: along longitudinal axis (from head to toe)
   * Static magnetic field strength is fairly _____: all protons in body precess w/ _____ frequency

Answer 1

Patient placed in magnetic bore: longitudinal axis of body parallel to longitudinal magnetic field

1. X axis: from right to left
2. Y axis: from anterior to posterior
3. Z axis: along longitudinal axis (from head to toe)
   * Static magnetic field strength is fairly homogeneous: all protons in body precess w/ Larmor frequency

Question 2

To examine a specific slice:

Magnetic field strength can be varied along any axis to create a magnetic field _____: a progression of magnetic field strengths through the patient. Magnetic gradient is _____ over external magnetic field

1. By using the Larmor equation: ωo=γβ it can be determined from which location (slice) within the patient the signal arises (fig 2.7 mic)
   a. γ = _____ MHZ for hydrogen protons (constant)
   b. β : magnetic field _____ and location (slice) within patient is established and known
   c. ω : we know the _____ frequencies of protons in each slice since we know the location of each magnetic field strength in the gradient

Answer 2

To examine a specific slice: Magnetic field strength can be varied along any axis to create a magnetic field gradient: a progression of magnetic field strengths through the patient. Magnetic gradient is superimposed over external magnetic field

1. By using the Larmor equation: ωo=γβ it can be determined from which location (slice) within the patient the signal arises (fig 2.7 mic)
   a. γ = 42.5 MHZ for hydrogen protons (constant)
   b. β : magnetic field strength and location (slice) within patient is established and known
   c. ω : we know the precessional frequencies of protons in each slice since we know the location of each magnetic field strength in the gradient

Question 3

To examine a specific slice:

2. By turning on a gradient (slice selecting gradient) simultaneously to the RF pulse, we can match the RF pulse frequency to the equal proton precessional frequency: only protons in the slice that are precessing at the _____ frequency as the RF pulse will be excited to flip 90° or 180° and generate a _____ (fig 57, pg87).

Slice selecting gradient is turned _____ only during application of RF pulse

Answer 3

To examine a specific slice:

2. By turning on a gradient (slice selecting gradient) simultaneously to the RF pulse, we can match the RF pulse frequency to the equal proton precessional frequency: only protons in the slice that are precessing at the same frequency as the RF pulse will be excited to flip 90° or 180° and generate a signal (fig 57, pg87).

Slice selecting gradient is turned on only during application of RF pulse

Question 4

To examine a specific slice:

By knowing the _____ frequency in each location (slice) across the gradient and by knowing the frequency of the _____, we therefore know the slice which the RF pulse is exciting and which is generating the signal

Answer 4

To examine a specific slice:

By knowing the precessional frequency in each location (slice) across the gradient and by knowing the frequency of the RF pulse, we therefore know the slice which the RF pulse is exciting and which is generating the signal

Question 5

Orientation of magnetic field gradient, selected by technologist, determines in which orthogonal plane the excited slice will lie and in which plane the image will be

1. _____ axis: sagittal slice and image
2. _____ axis: coronal slice and image
3. _____ axis: transverse (axial) slice and image

Answer 5

Orientation of magnetic field gradient, selected by technologist, determines in which orthogonal plane the excited slice will lie and in which plane the image will be

1. X axis: sagittal slice and image
2. Y axis: coronal slice and image
3. Z axis: transverse (axial) slice and image

Question 6

Determining Slice Thickness - 2 methods:

1. The wider the range of RF frequencies (band width) introduced concurrently with the slice selecting magnetic gradient, the _____ the slice (thinner RF bandwidth-thinner slice)
   * RF frequencies will match precessional frequencies of protons over a broader thickness of the magnetic gradient, thus excite a _____ slice of protons

Answer 6

Determining Slice Thickness - 2 methods:

1. The wider the range of RF frequencies (band width) introduced concurrently with the slice selecting magnetic gradient, the wider the slice (thinner RF bandwidth-thinner slice)
   * RF frequencies will match precessional frequencies of protons over a broader thickness of the magnetic gradient, thus excite a thicker slice of protons

Question 7

Determining Slice Thickness - 2 methods:

2. The steeper the slice selecting magnetic gradient (ex: the more difference in magnetic field strength between the feet and the head), the _____ the slice (transverse slice) if RF bandwidth remains constant
   * this occurs because in a steeper magnetic gradient over the same length of anatomy many more intervals of magnetic field strengths must fit into a fixed length, thus each magnetic field strength width (each precessional width) must be _____

Answer 7

Determining Slice Thickness - 2 methods:

2. The steeper the slice selecting magnetic gradient (ex: the more difference in magnetic field strength between the feet and the head), the thinner the slice (transverse slice) if RF bandwidth remains constant
   * this occurs because in a steeper magnetic gradient over the same length of anatomy many more intervals of magnetic field strengths must fit into a fixed length, thus each magnetic field strength width (each precessional width) must be thinner

Question 8

Localizing a Signal from a point within a slice:

1. RF pulse applied concurrently with slice selecting gradient (ex: along z-axis) gets all protons in one (transverse) slice precessing in phase at the _____ frequency

Answer 8

Localizing a Signal from a point within a slice:

1. RF pulse applied concurrently with slice selecting gradient (ex: along z-axis) gets all protons in one (transverse) slice precessing in phase at the same frequency

Question 9

Localizing a Signal from a point within a slice:

2. After RF pulse turned off, another magnetic gradient, the _____ encoding magnetic gradient (readout gradient), is _____ on the external magnetic field (after the slice selection gradient) and _____ during signal collection (ex: along x-axis of the slice) - causes protons in different sagittal columns from R to L across the slice to precess with different decreasing frequencies and give off signals with different decreasing frequencies proportional to magnetic field strength of that column (fig 59)

Determines origin of signal from a certain _____ (specific location in column unknown at this point: all protons in one column in same magnetic field strength & precess w/ same frequency)

Answer 9

Localizing a Signal from a point within a slice:

2. After RF pulse turned off, another magnetic gradient, the frequency encoding magnetic gradient (readout gradient), is superimposed on the external magnetic field (after the slice selection gradient) and maintained during signal collection (ex: along x-axis of the slice) - causes protons in different sagittal columns from R to L across the slice to precess with different decreasing frequencies and give off signals with different decreasing frequencies proportional to magnetic field strength of that column (fig 59)

Determines origin of signal from a certain column (specific location in column unknown at this point: all protons in one column in same magnetic field strength & precess w/ same frequency)

Question 10

Localizing a Signal from a point within a slice:

3. Also after the RF pulse is turned off, another magnetic gradient, the _____ encoding magnetic gradient, is turned on for a very short time (ex: along y-axis of the slice), then turned off - causes protons and their signals at each vertical point along the column to precess out of phase (i.e.: to be at a different point in the phase cycle at each point in the column in which the phase encoding magnetic gradient field differs) , but at the same frequency in any one phase enc _____ (in proportion to the frequency encoding magnetic field) (fig 60, pg 93)
   - Thus signal frequencies from column to column vary, but are the same in any one column; signals from different points (heights) in the same column have different phases
   * This determines the specific point of origin of a signal within a certain column in a slice: system can differentiate each pixel’s different unique frequency and phase (each pixel has its own unique (x,y) address)

Answer 10

Localizing a Signal from a point within a slice:

3. Also after the RF pulse is turned off, another magnetic gradient, the phase encoding magnetic gradient, is turned on for a very short time (ex: along y-axis of the slice), then turned off - causes protons and their signals at each vertical point along the column to precess out of phase (i.e.: to be at a different point in the phase cycle at each point in the column in which the phase encoding magnetic gradient field differs) , but at the same frequency in any one phase enc row (in proportion to the frequency encoding magnetic field) (fig 60, pg 93)
   - Thus signal frequencies from column to column vary, but are the same in any one column; signals from different points (heights) in the same column have different phases
   * This determines the specific point of origin of a signal within a certain column in a slice: system can differentiate each pixel’s different unique frequency and phase (each pixel has its own unique (x,y) address)

Question 11

4. Using the Fourier transformation, a mathematical process, a computer analyzes how much signal (intensity) of a specific _____ (column) and _____ (row or point in the column) is generated
   - a signal of a certain intensity can be assigned to a specific location (column and row) within the slice and an image can thus be reconstructed through the Fourier transform

Answer 11

4. Using the Fourier transformation, a mathematical process, a computer analyzes how much signal (intensity) of a specific frequency (column) and phase (row or point in the column) is generated
   - a signal of a certain intensity can be assigned to a specific location (column and row) within the slice and an image can thus be reconstructed through the Fourier transform

Question 12

Hydrogen Nucleii: preferred for MR imaging

1. consist of _____ proton
2. occur in large abundance throughout the body
3. give the _____ intense (strongest) signal compared to other nucleii in a magnetic field
4. have an _____ number of protons: an extra will always be left in parallel orientation, creating a net magnetization vector (a magnetic moment)

Answer 12

Hydrogen Nucleii: preferred for MR imaging

1. consist of 1 proton
2. occur in large abundance throughout the body
3. give the most intense (strongest) signal compared to other nucleii in a magnetic field
4. have an odd number of protons: an extra will always be left in parallel orientation, creating a net magnetization vector (a magnetic moment)

Question 13

MRI Hardware

1. magnetic field strength measured in Tesla: 1 Tesla = _____ Gauss (1T = 1G)
2. field strength range for clinical application: typically _____ T to _____ T

Answer 13

MRI Hardware

1. magnetic field strength measured in Tesla: 1 Tesla = 10,000 Gauss (1T = 1G)
2. field strength range for clinical application: typically .5 T to 2.0 T

Question 14

magnet types:

a. permanent magnets

• _____ loses magnetism
• requires no energy/power to _____ magnetic field
• _____ field strength
• extremely _____

Answer 14

magnet types:

a. permanent magnets

• never loses magnetism
• requires no energy/power to maintain magnetic field
• limited field strength
• extremely heavy

Question 15

magnet types:

resistive magnets:

• _____ current passed through wire loops: induces magnetic field
• thus also called _____
• achieve _____ magnetic field strength than permanent magnets
• require electrical power to _____ magnetic field
• resistance in conductive wire _____ generates heat: require cooling

Answer 15

magnet types:

resistive magnets:

• electrical current passed through wire loops: induces magnetic field
• thus also called electromagnets
• achieve higher magnetic field strength than permanent magnets
• require electrical power to maintain magnetic field
• resistance in conductive wire loops generates heat: require cooling

Question 16

magnet types:

superconducting magnets:

• are electromagnets: current through conductive _____ induces magn field
• conductor super cooled to _____° K or _____° C
• resistance in conductor _____

*_____ of current requires no power, current is permanent

*_____ (helium/nitrogen): coolants; expensive refilling needed

• have _____ magnetic field strength
• excellent magnetic field _____
• high _____ to operate
• quench: loss of _____ due to temperature rise above superconducting level
• sudden resistance to flow of electricity
• cryogens _____ off

Answer 16

magnet types:

superconducting magnets:

• are electromagnets: current through conductive loops induces magn field
• conductor super cooled to 4° K or -269° C
• resistance in conductor lost
• maintenance of current requires no power, current is permanent
• cryogens (helium/nitrogen): coolants; expensive refilling needed
• have high magnetic field strength
• excellent magnetic field homogeneity
• high cost to operate
• quench: loss of superconductivity due to temperature rise above superconducting level
• sudden resistance to flow of electricity
• cryogens boil off

Question 17

magnet types:

coils:

• _____ coils: send RF pulses to excite protons
• _____ coils: receive resulting signals
• volume coils:
• part of the _____
• completely surrounds _____: close to size of subject
• is an _____ transmitter for all types of exams
• is signal _____ for large body part exams
• head coil is _____, body coil is transmitter to head
• _____ coils: induce magnetic fields superimposed over main magnetic field to improve homogeneity

Answer 17

magnet types:

coils:

• transmitter coils: send RF pulses to excite protons
• receiver coils: receive resulting signals
• volume coils:
• part of the scanner
• completely surrounds body: close to size of subject
• is an RF pulse transmitter for all types of exams
• is signal receiver for large body part exams
• head coil is receiver, body coil is transmitter to head
• shim coils: induce magnetic fields superimposed over main magnetic field to improve homogeneity

Question 18

magnet types:

coils:

• gradient coils:
• created linearly _____ magnetic fields for slice selection and spacial pinpointing information
• 3 sets of gradient coils; 1 for each of 3 spacial dimension
• source of _____ noise from scannner
• surface coils:
• are _____ coils only for specific body parts
• RF pulse transmitted by body _____ to body _____
• shape corresponds to _____ of interest
• receive signals from nearby body part surfaces
• cannot receive signals from _____ body areas

Answer 18

magnet types:

coils:

• gradient coils:
• created linearly increasing magnetic fields for slice selection and spacial pinpointing information
• 3 sets of gradient coils; 1 for each of 3 spacial dimension
• source of banging noise from scannner
• surface coils:
• are receiver coils only for specific body parts
• RF pulse transmitted by body coil to body parts
• shape corresponds to anatomy of interest
• receive signals from nearby body part surfaces
• cannot receive signals from deeper body areas

Question 19

MRI facilities

1. static (main) magnetic field creates fringe field: extends outside of imager
   - must be contained within _____ room
   - can attract _____ objects
   - can influence operation of electrical/mechanical devices

*_____

*_____

*_____

2. outside _____ _____ can interfere with received MR signal: Faraday cage shields magnet

Answer 19

MRI facilities

1. static (main) magnetic field creates fringe field: extends outside of imager
   - must be contained within scanning room
   - can attract metallic objects
   - can influence operation of electrical/mechanical devices
   * monitors
   * pacemakers
   * computers
2. outside radio waves can interfere with received MR signal: Faraday cage shields magnet