Term
| What are the primary advantages to MRI. |
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Definition
| High contrast sensitivity to soft tissue and inherent patient safety due to nonionizing radiation. |
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Term
| SI unit for magnetic field strength? |
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Definition
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Term
| How many Gauss are in one Tesla? |
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Definition
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Term
| Strength of the Earth's magnetic field? |
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Definition
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Term
| Common magnetic field strengths in clinical MRIs? |
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Definition
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Term
| Give 3 types of coils and what are they used for? |
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Definition
| Shim coils - improve main magnetic field homogeneity, RF coils - transmit energy and receive signals, Gradient coils - produce a linear variation in field strength |
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Term
| What is required for a nucleus to have a magnetic moment? |
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Definition
| The sum of the protons and neutrons must be odd. |
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Term
| What element is most commonly used in MR and why? |
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Definition
| Hydrogen because it has the largest magnetic moment and the greatest abundance. |
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Term
| The orientation of "free" protons in a applied magnetic field. |
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Definition
| Parallel and anti-parallel to the applied field. There will be more in the lower energy parallel direction and this creates a measurable magnetic moment. |
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Term
| Torque applied in the direction perpendicular to the magnetic field results in? |
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Definition
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Term
| What is the Larmor equation? What does it describe? |
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Definition
ω0 = γ * Bo
It express the relationship between the external magnetic field and the angular precessional frequency. |
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Term
| What is the Larmor equation in terms of linear frequency? |
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Definition
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Term
| What is the gyromagnetic ratio for hydrogen? |
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Definition
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Term
| At equilibrium, Mo is at a ________ and Mxy is at a _______. |
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Definition
| Mo - maximum, Mxy - minimum |
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Term
| The RF excitation pulse is known as the ________ field. It is applied at the _______ frequency. |
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Definition
| B1 field, applied at the Larmor frequency. |
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Term
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Definition
| The degree of Mz rotation by the B1 field into the xy plane. |
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Term
| The flip angle displacement is determined by |
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Definition
Θ = ω1t = γB1t
per the Larmor equation
be careful: linear vs angular frequency |
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Term
| What flip angle provides the largest Mxy and thus the largest detectable signal? |
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Definition
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Term
| What is the advantage of flip angles that are less than 90 degrees? |
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Definition
| Results in a larger transverse magnetization per unit excitation time. This is used in fast MRI techniques. |
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Term
| A damped sinusoidal signal from the transverse magnetic field is known as |
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Definition
| FID - free induction decay |
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Term
| Decay of the transverse magnetic moment is called _____ - ______ decay and is governed by the ____ relaxation time. |
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Definition
| spin-spin decay - T2 relaxation time |
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Term
| T2 is the time required for the transverse magnetic moment to drop to ____% of its peak signal. |
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Definition
| Drop to 37% of the peak signal. |
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Term
| The mathmatical expression for T2 decay. |
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Definition
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Term
| Return of the longitudinal magnetic moment is called _____ - ______ decay and is governed by the ____ relaxation time. |
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Definition
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Term
| T1 is the time required to recover ____% of MZ. |
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Definition
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Term
| The mathematical expression for regrowth of the Mz. |
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Definition
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Term
| T2* includes the loss of phase coherence from both |
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Definition
| intrinsic and extrinsic magnetic field variations. |
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Term
| Extrinsic field variations are caused by ____. |
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Definition
| Main magnetic field inhomogeneities and susceptibility agents in the tissue (contrast material) |
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Term
| List the relaxation time constants in order of size. |
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Definition
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Term
| T1 is typically ____ times longer than T2? |
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Definition
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Term
| Fundamental properties of the different tissues are given by |
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Definition
| T1, T2, T2*, and proton density |
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Term
| How does increasing the main magnetic field affect T1 and T2. Why? |
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Definition
| T1 increases and T2 is unaffected. T1 increases because there is less overlap between the molecular vibration frequency and the Larmor frequency. |
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Term
| Since most tissues of clinical interest are smaller-sized molecules, tissues with a longer T1 usually have a _____ T2 and tissues with a shorter T1 usually have a _____ T2. |
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Definition
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Term
| For brain tissues, which has the shortest T1? The longest? |
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Definition
| Shortest T1 - fat, Longest T1 - CSF |
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Term
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Definition
0.1 to 1 sec for soft tissues,
1-4 seconds for aqueous tissues (CSF) |
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Term
| Define time of repetition, give a typical range of values. |
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Definition
Period of time between B1 excitation pulses.
milliseconds to several seconds |
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Term
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Definition
| Period of time between the excitation pulse and the appearance of the peak amplitude of an induced echo. |
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Term
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Definition
| By applying a 180 degree RF inversion pulse or a gradient polarity reversal at a time of TE/2. |
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Term
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Definition
| TI is the time between the initial inversion/excitation (180 degrees) RF pulse and a 90-degree readout pulse. |
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Term
| Why does partial saturation occur? |
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Definition
| The repetition time (TR) between excitation pulses does not allow for a full return to equilibrium, so Mz is reduced for the next excitation pulse. |
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Term
| Greater partial saturation occurs in tissues with ____ _____. |
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Definition
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Term
| What are the three basic pulse sequences? |
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Definition
Spin Echo (SE)
Inversion Recovery (IR)
Gradient Echo (GE) |
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Term
| Briefly describe the Spin Echo sequence. |
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Definition
| A 90-degree RF pulse excites the protons and induces a FID. A 180-degree RF pulse at TE/2 inverts and refocuses the spins so phase coherence occurs at TE. Data is then read out at TE. |
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Term
| Signal intensity for a SE sequence is given by |
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Definition
| S ≈ ρH (1-e-TR/T1)e-TE/T2 |
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Term
| What parameters can the user control? Which can they not control? |
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Definition
User controls TR, TE and TI.
They can't control T1, T2, T2*. |
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Term
| A T1 weighted SE sequence has a ____ TR and a ____ TE |
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Definition
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Term
| T2 differences are minimized by using a ____ ____? |
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Definition
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Term
| T1 differences are minimized by using a ______ ______. |
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Definition
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Term
| A proton weighted SE sequence has a ____ TR and a ____ TE |
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Definition
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Term
| In a T1 weighted image, which tissues will be brightest and why. |
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Definition
| Fat, because it has the shortest T1, so it will recover the most Mz during the TR period. |
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Term
| What tissue has the highest signal in a SE proton weighted image. |
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Definition
CSF - it has the greatest proton density
(CSF > fat > gray matter > white matter) |
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Term
| A T2 weighted SE sequence has a ____ TR and a ____ TE |
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Definition
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Term
| What tissue appears brightest in a SE T2 weighted image? |
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Definition
| CSF because it has the largest T2. |
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Term
| During conventional SE sequences, two differently weighted signals are acquired. Which ones and how does it work. |
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Definition
| Proton density and T2 weighted signals are acquired during the same TR. Both use a long TR to minimize T1. A short TE is used to get the Proton density weighted signal followed by a second 180 degree RF pulse at a longer TE/2 to get a T2 weighted signal. |
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Term
| Inversion recovery emphasizes ___ relaxation times. |
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Definition
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Term
| TI - the time of inversion is |
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Definition
| the time from the initial 180-degree inversion pulse to a 90-degree excitation pulse. |
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Term
| Describe Inversion Recovery Spin Echo |
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Definition
| An initial 180 degree RF pulse is used to invert Mz to -MZ. After a short time - the time of inversion, TI - a 90-degree RF (readout) pulse rotates the recovered MZ into the transverse plan to generate a FID. A second 180-degree pulse at TE/2 produces an echo at TE. |
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Term
| What is STIR. Why is it used. |
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Definition
| STIR - Short Tau Inversion Recovery - it uses a very short TI and magnitude signal processing where Mz is always positive. It is used to Null out fat signals. It works because fat has the shortest T1. |
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Term
| Bounce point or tissue null |
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Definition
| During inversion recovery, the Mz signal will go through zero, effectively creating no signal for that tissue if a readout pulse is applied. |
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Term
| During Inversion Recovery (or Short Tau Inversion Recovery) when does the signal null (bounce point) occur? |
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Definition
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Term
| What is a FLAIR sequence. |
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Definition
| Fluid Attenuated Inversion Recovery - TI is selected at the bounce point for CSF. |
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Term
| What are the advantages and disadvantages of the SE and IR SE sequences? |
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Definition
Advantages - less sensitive to magnetic field inhomogeneities, high SNR and CNR
Disadvantage - Relatively long TR and thus long acquisition times. |
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Term
| What is the GE technique? |
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Definition
| GE - Gradient Echo - uses a magnetic field gradient reversal to induce the formation of the echo instead of the 180-degree inversion pulse. |
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Term
| Gradient Echo does not cancel out _________ like SE does. |
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Definition
| extrinisic inhomogeneities - (Bo inhomogeneities and contrast agents) |
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Term
| Gradient Echo tends to use a _____ flip angle. |
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Definition
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Term
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Definition
The rate at which the gradient can be created.
T/m per sec or T/sm |
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Term
| What are the three gradients used for localizing the MR signal? |
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Definition
| Slice select, frequency encode and phase encode. |
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Term
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Definition
| Currents that are induced in nearby conductors when the gradient field is turned on. |
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Term
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Definition
| The range of frequencies over the field of view. |
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Term
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Definition
| BW divided by the number of discrete samples. |
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Term
| Slice thickness is determined by |
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Definition
| 1) the BW of the RF pulse and 2) the gradient strength across the FOV. |
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Term
| How can a thin slice be excited. |
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Definition
| Use a narrow BW or a high Gradient strength. |
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Term
| How can a thick slice be excited? |
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Definition
| Use a wide BW or a lower gradient strength. |
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Term
| The FEG is also known as the |
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Definition
| Frequency Encode Gradient - aka the readout gradient. |
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Term
| The FEG is applied ______ to the SSG and it varies from ___ to ___ over the field of view. |
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Definition
| orthogonally, from -f to f. |
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Term
| The PEG is applied before the _____ and after the _____. |
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Definition
| before the FEG and after the SSG. |
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Term
| The PEG induces a _____ change in the proton spins that ______ as you go ____ from the null. |
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Definition
| phase change, increases, farther |
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Term
| For a SE sequence, each TR interval is repeated with a different ______ strength. |
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Definition
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Term
| The K-space matrix describes the ________ domain. |
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Definition
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Term
| The FEG induced frequency variations are mapped along the _____ axis and the PEG induced phase variations are mapped along the _____ axis. |
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Definition
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Term
| The center of k-space represents the _____ spatial frequencies and the periphery of k-space represents the ______ spatial frequencies. |
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Definition
center - lower frequencies
periphery - higher frequencies |
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Term
| GRE is more sensitive to magnetic field inhomogeneity. When can this be an advantage? |
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Definition
| Identifying a hemorrhage. |
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Term
| What are the advantages and disadvantage of GRE over SE? |
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Definition
Advantages: short TR, faster image acquisition time, lower SAR
Disadvantages: decreased SNR |
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Term
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Definition
| After each echo, the residual transverse magnitization is destroyed either with a spoiling gradient or RF spoiling pulse. |
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Term
What are the units for gradient strength?
Typical values? |
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Definition
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