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ECVN study > Thrall Ch 5 > Flashcards

Thrall Ch 5 Flashcards

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1
Q

What is the primary advantage of CT and MR imaging over conventional radiography?

A

Tomographic nature and increased contrast resolution

CT and MR imaging allow for examination of tissues in thin sections, eliminating superimposition.

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2
Q

What is contrast resolution?

A

The capacity of a system to accurately represent differences in tissue characteristics

It is linked to x-ray attenuation (CT) or signal intensity (MR).

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3
Q

How does the spatial resolution of CT and MR imaging compare to radiography?

A

More limited

Spatial resolution corresponds to the minimum resolvable separation between high-contrast objects.

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4
Q

What is the term for the small cubical sections that make up each CT or MR image?

A

Voxels

Each voxel is associated with an electronic current that is processed to form the image.

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5
Q

What is the significance of slice thickness in CT and MR imaging?

A

It is typically the largest dimension of each voxel and limits resolving capabilities

Advances aim for isotropic resolution with equal dimensions in x, y, and z.

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6
Q

What is multiplanar reformatting?

A

The process of rearranging images from a volume dataset to display voxels along a different plane

It aids in structure recognition and assessment of tissues.

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7
Q

What is the principle behind the generation of a magnetic field in MR scanners?

A

When electronic current travels along a loop of wire, a magnetic field is generated

This is based on Faraday’s law of induction.

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8
Q

What is precession in the context of MR imaging?

A

The wobbling motion of protons about their axis when placed in a magnetic field

This behavior is controlled by the application of an external force.

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9
Q

What happens during the excitation of protons in MR imaging?

A

Protons absorb energy and jump to a higher energy state

This process causes a shift in the net magnetization vector.

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10
Q

What are the two distinct processes that occur during relaxation in MR imaging?

A

Longitudinal (T1) and transverse (T2) relaxations

T1 relaxation is related to protons returning to low-energy states, while T2 involves dephasing.

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11
Q

What are spin echo sequences in MR imaging?

A

Technical methods used to excite and receive signals using RF and gradient pulses

They help in evaluating differences in T1 and T2 relaxation times between tissues.

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12
Q

What does the term ‘flip angle’ refer to in MR imaging?

A

The angle that the net magnetization vector flips away from the z-axis

It is influenced by the strength and duration of the RF pulse.

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13
Q

What is isotropic resolution?

A

Imaging sections of tissue with perfectly cubical voxels

Achieved with advances in multidetector row scanners.

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14
Q

What is the relationship between the electronic current induced in the coil and the transverse magnetic field?

A

The strength of the electronic current induced in the coil is proportional to the transverse magnetic field.

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15
Q

What phenomenon occurs when RF pulse is stopped, leading to dephasing of protons?

A

T2 relaxation.

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16
Q

What accelerates T2 relaxation in the body?

A

Inhomogeneities in the magnetic field due to metallic objects, air, calcium, and imperfections in the MR system.

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17
Q

What is the term for the rapid dephasing of protons due to magnetic field inhomogeneities?

A

T2*.

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18
Q

What technique is developed to eliminate the effect of T2*?

A

Spin-echo sequences.

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19
Q

What is the purpose of adding a 180-degree RF pulse after the initial 90-degree pulse in spin-echo sequences?

A

To allow protons affected by inhomogeneities to become coherent again, increasing transverse magnetization.

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20
Q

What does TE represent in MR imaging?

A

The time between the peak of echo and the initial 90-degree RF pulse.

TE = time of echo

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21
Q

What is TR in the context of MR imaging?

A

The time it takes for a sequence to run one time.

TR = time to repetition

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22
Q

How does adjusting TE and TR affect tissue contrast in MR imaging?

A

It highlights differences in T1 and T2 relaxation characteristics among tissues.

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23
Q

What happens to the MR signal when transverse magnetization begins to dephase?

A

The measured MR signal decreases and becomes null when completely dephased.

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24
Q

What is required to detect T2 differences between tissues effectively?

A

Time must be allowed for short-T2 tissues to decay enough so that long-T2 tissues are highlighted.

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25
What are the implications of optimizing T1 and T2 effects in MR imaging?
Optimizing one necessitates inhibiting the other.
26
What is proton-density (PD) weighting?
A condition where long TR and short TE inhibit both T1 and T2 effects, and signal intensities depend mainly on tissue proton density.
27
What is the role of magnetic gradients in MR imaging?
They determine the precise anatomic location of signals from tissues.
28
What does the slice selection gradient (GSS) do?
Produces a linear gradation of magnetic field strength along the direction of the main magnetic field to target a specific slice.
29
What is the purpose of the phase-encoding gradient (GPE)?
To cause each row of protons in the slice to have a different phase.
30
What does the frequency-encoding gradient (GFE) achieve?
Alters the Larmor frequencies for each column within the slice.
31
What is a fast spin echo sequence?
A spin echo sequence with more than one 180-degree rephasing pulse applied during the same TR.
32
What is the significance of inversion recovery sequences?
They are used to null the signal from specific tissues or substances.
33
What is the effect of a short inversion time recovery (STIR) sequence?
Suppresses the signal from fat and enhances the conspicuity of soft tissue lesions.
34
What does a fluid-attenuated inversion recovery (FLAIR) sequence help differentiate?
Brain parenchymal lesions from cerebrospinal fluid (CSF).
35
How do gradient echo sequences differ from spin echo sequences?
They use smaller flip angles and do not use a 180-degree refocusing RF pulse.
36
What is the impact of magnetic susceptibility on gradient echo sequences?
It leads to signal loss and misregistration due to local field inhomogeneities.
37
What is the primary purpose of contrast media in MR imaging?
To assess vascular network and tissue perfusion.
38
What effect do gadolinium-based contrast agents have on T1 relaxation time?
They shorten the T1 relaxation time, enhancing signal on T1-weighted images.
39
What is the purpose of chemical fat saturation in MR imaging?
To selectively attenuate fat signal intensity.
40
What is an example of a preparatory pulse used in MR imaging?
Spatially selective saturation pulse.
41
What can be achieved by using fat-suppressed T1-weighted sequences?
Maximized visibility of contrast-enhancing lesions.
42
What is the purpose of a spatially selective saturation pulse in MRI?
To selectively cancel out signal from a region whose transverse magnetization is not desired ## Footnote This technique helps to remove signals from flowing blood or moving tissue to reduce artifacts.
43
What type of imaging is very sensitive to cytotoxic edema in early ischemia?
Diffusion-weighted imaging ## Footnote This imaging reflects Brownian motion or microscopic movement of water, which is restricted in ischemic tissue.
44
What does perfusion-weighted imaging use to support a diagnosis of brain infarct?
A T2*-weighted pulse sequence following a bolus injection of contrast medium ## Footnote Gadolinium is commonly used as the contrast medium.
45
What imaging technique can detect shunts and portal vascular anomalies?
Contrast-enhanced MR angiography ## Footnote Other techniques include time-of-flight and phase-contrast MR angiography, which can be used without contrast medium.
46
What is a major challenge when imaging animals under anesthesia?
Balancing image quality and acquisition time ## Footnote Achieving the most diagnostic examination as quickly as possible is crucial.
47
What factors influence spatial resolution in MRI?
Slice thickness, field of view (FOV), and matrix size ## Footnote The height and width of each voxel are determined by the number of rows and columns in the matrix divided by the image FOV.
48
What is the signal-to-noise ratio (SNR) in MRI?
The amount of signal arising from protons forming each voxel ## Footnote High SNR is crucial for image quality.
49
How can SNR be improved in MRI?
By averaging signal from sequential excitations ## Footnote This process unfortunately increases acquisition time.
50
What is parallel imaging in MRI?
A technique that exploits multiple elements of a multi-channel phased array coil ## Footnote It improves SNR and reduces acquisition time compared to standard coils.
51
What are artifacts in MRI?
Factors that can degrade image quality or lead to misdiagnosis ## Footnote Understanding these artifacts is essential for accurate interpretation.
52
What causes magnetic susceptibility artifacts in MRI?
Local magnetic field inhomogeneity due to structures affecting homogeneity ## Footnote Metallic objects, whether ferrous or not, can lead to these artifacts.
53
What is the magic angle effect in MRI?
An artifact that alters the signal intensity of tendons and ligaments ## Footnote This effect occurs when tendons or ligaments are positioned at specific angles in the magnetic field.
54
What are the advantages of high-field MR systems compared to low-field systems?
Superior SNR, spatial resolution, and advanced imaging protocols ## Footnote High-field systems typically offer better image quality.
55
What is a disadvantage of low-field MR systems?
Limited SNR associated with longer scan times and reduced spatial resolution ## Footnote They often require moving the patient for complete anatomic coverage.
56
What is a benefit of low-field MR imaging?
Reduced susceptibility artifact and easier patient access ## Footnote The open design of low-field systems allows for easier imaging of standing animals.
57
How are MR images created?
Signals arising from excited tissues are detected as echoes by receiving coils, then localized spatially, and processed.
58
Describe the concept of voxel to pixel translation
Although several different values of signal intensity can coexist in each voxel, especially if the voxel is large and if composed of heterogeneous tissue components, only the mean value is expressed and converted to pixel brightness
59
What is the source of image formation in an MRI
Energy transfer to and from protons (hydrogen nuclei) with electromagnetic properties which can be localized spatially (i.e., in individual voxels). MRI is essentially mapping the distribution of H+ protons in the water (H2O) and lipid (CH2/CH3) component of tissues.
60
What is Faraday’s law of induction
When electronic current travels along a loop of wire, a magnetic field is generated perpendicularly in proportion to the strength of the current. ## Footnote To generate and maintain such high magnetic fields, superconducting wires are immersed in liquid helium, which serves as cooling agent.
61
Describe the characteristics of hydrogen protons in MRI
The positively charged nucleus spins about its axis, generating a very small, local magnetic field, and thus acting as tiny magnet. These protons, or spins, are oriented randomly in tissues under normal conditions so that their magnetic fields cancel out. When placed inside a strong, external magnetic field with linear orientation (B0), these spins are realigned and orient themselves either parallel or anti-parallel to this field. A small excess of these spins (proportional to the strength of B0) are oriented in the parallel direction, thus generating a net magnetization of the tissue. This net magnetization, which at equilibrium is oriented longitudinally, is targeted during imaging sequences.
62
What is an echo?
An echo is the emission of energy in form of an electromagnetic resonance signal of a nuclei after its excitation
63
What are the two main groups of sequences?
Spin echo and Gradient recalled sequences
64
What is the purpose of the receiving coil?
By placing one or several loops of wire (i.e., receiving coil) around the patient, in a plane perpendicular to the transverse axis, the strength of the electronic current induced in the coil is proportional to the transverse magnetic field.
65
Describe the concept of T2-star relaxation
The first step of a spin echo sequence consists of a radiofrequency (RF) pulse that flips the magnetization moment of protons toward the transverse plane. This pulse is sometimes referred to as a 90-degree pulse. At this point, protons precess synchronously, which causes strong transverse magnetization in the xy-axis. As soon as the RF pulse is terminated, protons start to dephase because of imperfections in the magnetic field, which is never completely uniform. These slight changes in field uniformity, or field strength, cause protons to precess either more rapidly or less rapidly. This results in rapid loss of transverse net magnetization—called T2* (T2-star) decay, or relaxation.
66
What is the purpose of a spin echo sequence?
To eliminate the effect of T2-star

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