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2 spatial localizati
Spatial localization
| Question | Answer |
|---|---|
| Rate an event repeats itself? | Frequency |
| Refers to weather certain events are synchronized with each other. | Phase. |
| Cartesian Coordinate System | X - L to R Y - A -P Z - H - F |
| A hardware component of the system, consisting of many coils of wire which, when current is passed through them, are capable of caring the Main Magnetic Field. | The Magnetic Field Gradients. |
| The specific selection of the slice plane is made possible with the use of | Magnetic field gradients |
| Causes the magnetic field to increase in one direction and decrease in the opposite direction in a linear fashion. | A magnetic field gradient |
| Magnetic field gradient is expressed in terms of? | The effect that gradient has on the magnetic field at a given distance from the center of the magnet. |
| To create an image how many magnetic field gradients are needed? | All three physical gradients are needed (X,Y,Z) to be applied along perpendicular geometric axes in the magnet, and vary the field along each direction accordingly |
| Physical Gradients ? | xyz |
| B= Bo + (D x G / 1000) | How to calculate the mag field at any point along that gradient. |
| Using a gradient to vary the mag field is important because of what relationship ? | The relationship between the mag field and the precessional frequency |
| Net magnetization precesses at a frequency determined by the magnetic field expressed by the protons | Larmor equation |
| Gradients cause the precessional frequency to vary depending upon the .... | The position of the protons that generate the magnetization. |
| The _______ of the net magnetization corresponds to a specific location on the axis along which the gradient is applied. | Percessinal Frequency |
| The concept of using a gradient to map precessional frequencies is the basis of what ? | Spatial Localization of the signals in MRI |
| The principle of resonance. | The transfer of energy only occurs if the frequency of the transmitting system is matched to that of the receiving system |
| The process of sliced selection | Turning on a gradient at the same time as applying an RF pulse of the same resonant frequency of tissue/area desired. |
| Selecting a Transverse slice | Applying RF pulse and Z gradient at the same time. |
| Selecting a Coronal slice | Applying RF pulse and Y gradient at the same time. |
| Selecting a Sagittal slice. | Applying RF pulse and X gradient at the same time. |
| To ensure the appropriate proton receive energy, what must happen? | The gradient responsible for slice selection must turn on at the same time as the RF energy. |
| The slice position is specified by the? | Center frequency of the RF pulse for a given gradient amplitude. |
| The slice thickness can be specified by ? | The transmitter bandwidth, (or range of frequencies), at which RF pules is applied. |
| The broader the range of frequencies the ____? | The wider the slice. |
| A higher (narrower) gradient amplitude corresponds to ? | A thinner slice, or meaning that a given range of frequencies corresponds to a smaller distance. |
| The simultaneous application of an RF pulse and the slice select gradient will determine ? | The Orientation, Position, and Thickness of an Slice. |
| A wide range of frequencies will select ____ slice ? | Thick slice |
| A narrow range of frequencies will select ___ slice ? | Thin slice |
| Every point in an image can be located by specifying what? | Frequency and Phase encoding gradients |
| The gradient turned on at the same time of the ECHO Collection? | Frequency encoding gradient |
| Also called the readout gradient ? | Frequency gradient |
| Linearly varies the mag field, and therefore the processional frequencies of the protons, as each echo is being read from the patient? | Frequency encoding gradient |
| Frequency gradient is applied on what gradient? | Can be applied at any of the remaining two gradients not used for slice selection. |
| Frequency direction moves in what direction ? | if horizontal, frequency process different from left to right if vertical, frequency precess different top to bottom. |
| Collectively know as the Logical Gradients ? | Phase encoding , Frequency encoding, and Slice encoding gradients |
| Each Logical gradient corresponds to a ? | Physical gradient |
| Every time a gradient is switched on their is a build up of ? | Phase |
| The amount of phase will accumulate linearly over time, if what is changed ? | Frequency |
| The part of the MR signal used in the localizing origin of contrast information in the image plane ? | Phase in the MR signal |
| Is unwanted, and must be cancelled or balanced by the time the echo is generated ? | The phase information from the frequency gradient and slice select gradient. |
| The relationship used to balance phase ? | The fourth relationship: A positive change in frequency followed by an equal opposite change in frequency will have no net effect on the phase. |
| Phase encoding requires many repetitions in order to ? | Spatially encode contrast information along this last direction of the image plane. |
| After each TR, the _____ of the phase encoding gradient is incrementally increased in order to uniquely phase encode each acquired echo. | The Amplitude |
| As each echo is collected, it is placed in a data file called? | The Raw Data |
| Raw data, represents a mathematical domain called? | K-Space |
| TR period is repeated and the phase encoding gradient is incremented after ? | After the acquisition of each echo; |
| A mathematical calculation applied to the raw data | Fourier transform |
| breaks out the frequency and phase components, therefore the anatomical detail and tissue contrast information, corresponding to each point of anatomy in the image plane. | Fourier transform |
| Responsible for converting the raw data (k-space) into an image ? | Fourier tranform |
| Motion artifacts appear along what direction ? | Phase direction |
| Chemical shift artifacts always appear along what direction ? | Frequency direction |
| How many phase encoding lines are generated from single TR period ? | Only a single phase encoding line is generated |
| The size of the image matrix along the frequency encoding direction is independent of what process ? | Phase encoding process. |
| If a population of protons moves through some distance during the time the echo is being sampled, what will the appearance of the image have? | Artifacts along the in phase encoding direction of the image |
| 180 degree pulse can only recover phase errors which are ? | Stationary (not moving) within the mag, and which are equal before and after the 180 pulse. such as mag inhomogeneities |
| Proton which move along the direction of the gradient during the process of measurement, result in what ? | phase errors in the protons signals, thus appear as motion in phase direction |
| Each raw data point contributes to what part of the final image ? | every data point in the raw data contributes to the entire image. |
| true / false All protons at a given x coordinate are at the same precessional frequency regardless of their y and z coordinate | true |
| creates a spectrum of pressional frequency | Applying a gradient, creates a linear magnetic gradient field |
| Any motion that occurs after the middle of the 90 RF pulse contributes phase, and thus ? | a motion artifact in the phase direction. 180 degree pulse can only recover phase errors that are stationary |
| What is contained within a single spin echo? | Many frequencies |
| Creates a spectrum of precessional frequency along the slice select direction ? | the slice select gradient |
| A given range of frequencies is spread across a lager distance, as the gradient strength (amplitude) is ? | decreased |
| Decreasing the slice select gradient amplitude, Will have what effect on the resulting slice if the RF transmitter bandwidth is held constant? | Increase the slice thickness |
| Applied while the echo is recorded ? | The frequency encoding gradient aka readout gradient |
| The primary role of the frequency encoding gradient ? | Is to create a spectrum of precessional frequencies within protons which varies linearly with distance along that gradient while the echo is recorded |
| It separates the phase and frequency components corresponding to each point of anatomy in the image plane | Fourier transform |
| Enables us to view the contrast information which corresponds to each point of the anatomy in the image | Fourier transform |
| What effect will an applied gradient have on a population of stationary, in phase protons within a 10cm sphere at the center of the magnet ? | It will increase the phase dispersion... protons will become phase advance /or phase lagged, with respect to the center of the magnet. |
| True or False. A 1 to 1 relationship does not exist between the position of the echo in the raw data and the position of a line within the image | TRUE. |
| To make an oblique image which is orientated at an angel between traverse and coronal, which of the following physical gradients must be applied for slice selection? a.) X and Y together b.) Y and Z together c.) Z d.) Y | b.) Y and Z together |
| The phase encoding gradient must be turned on during which of the following events? a.) prior to echo collection, but the exact timing isn't critical b.) readout of the echo. c.) application of RF energy. d.)frequency encoding. | a.) prior to echo collection, but the exact timing isn't critical. |
| During the readout of the echo, only ______ , gradient may be applied. | The frequency encoding (readout) gradient |
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tomvilla21
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