Lab Location: MRI, B2, Cardinal Carter Wing, St. Michael s Hospital, 30 Bond Street

Transcription

1 Lab Location: MRI, B2, Cardinal Carter Wing, St. Michael s Hospital, 30 Bond Street MRI is located in the sub basement of CC wing. From Queen or Victoria, follow the baby blue arrows and ride the CC south elevators to B2. A B OBJECTIVE: Acquire a high resolution, 2D coronal, T2 weighted, image of the Pituitary Gland You need to modify the parameters of a pulse sequence given to you on the scanner and take an image of the Sella Turcica, as pictured in A. Unfortunately, the pulse sequence you ve been given to start with is not that good (B). By the end of this lab you will: - Have hands on experience using a state of the art MRI system - Have an understanding of SNR - Experienced the relationship between - pulse sequence parameters, - acquisition timing and - SNR

2 Lab Day Events - Tour of MRI - Explore Magnet and various entry ports and peripherals - Magnetic Field - Coils - Faraday Cage - Set up volunteer for scan - You will be given 40 minutes to improve the base sequence given to you - Acquire a 3 Plane Localizer - Acquire the baseline image of the sequence given to you - Change ONE pulse sequence parameter - Acquire an image after changing each parameter - Six parameters were changed from A to B - Marks awarded - 10% each parameter change, correct parameter change and correctness of new parameter - 40% image quality - Final image will be judged by 2 radiology fellows as unacceptable, clinically diagnostic, excellent image quality, outstanding. - Review the advantages and disadvantages of changing the pulse sequence parameters you ve chosen during the lab. The remainder of this guide will walk you through the interface of the Siemens Skyra 3 T scanner that you will be using on your lab day. Parameters you MIGHT want to change are described. To help you think through the problem, questions are italicized and right justified, e.g: What happens if I sleep in for the lab?

3 You will be working on a Siemens Skyra 3 T system. Staff on site will help you place your volunteer in the MRI coil and put them into the MRI scanner. We will also guide you in a bit of the user interface to plan a scan. SCANNER INTERFACE OVERVIEW: As time is limited, we will not be going through all the functionality of the MRI scanner, but doing the most basic slice positioning and parameter modification The scanner interface can be roughly divided into three areas. A: Is the area where new images will appear B: Is the list of sequences that are upcoming Blue highlight indicates currently selected sequence C: Are where we will find the sequence parameters. This is where we ll spend most of the lab Note the tabs across the bottom of section C (Program, Routine, Contrast etc) We will discuss the tabs you need in detail. A B C

4 Let s look at Box C Outlined in the blue box at the top are a few pulse sequence statistics. TA: Acquisition Time, the time it will take to acquire the image with the parameters you ve currently got set. Keep your eye on this number. What Parameters Change TA? PM: Patient Motion beyond the scope of this lab PAT: Parallel Acquisition Technique, 1 or OFF indicates that the technique is not accelerated using advanced reconstruction techniques to reduce imaging time (e.g. SENSE, SMASH, GRAPPA). The higher this number, the higher the acceleration. What Happens to TA When You Increase PAT? What Happens to SNR when you Increase PAT? Voxel Size: Gives the reconstructed size of each voxel What Happens to TA when you Decrease the Voxel Size? What Happens to SNR when you Decrease the Voxel Size? Rel. SNR: Relative Signal to Noise Ratio. When you first open a pulse sequence, this number starts at 1.00, changes to the pulse sequence parameters gives you an estimation of what happens to your SNR. This number resets to 1.0 if you save the sequence and re-open it :tse : This area tells you the type of pulse sequence being used (here Turbo Spin Echo). We re not going to change this during the lab.

5 ROUTINE TAB Orientation: Slice Angulation. Sagittal, Transverse, Coronal, oblique angulations will show some strange text. Phase Enc. Dir: Direction of the Phase Encoding Gradient. Foot Head, Left Right or Anterior Posterior. Phase Oversampling: extra phase encode lines to acquire the image is cropped to the original Phase Field of View. Can you think of 2 reasons why would we acquire extra phase encode lines? FoV Read: Field of View in the Readout Direction (perpendicular to Phase Enc Dir) FoV Phase: Field of View in the Phase Direction, as a percentage of Fov Read What will Decreasing FoV do to Voxel Size? What will Decreasing FoV do to Rel. SNR? Slice Thickness: The size of the slice in mm Averages: How many times to acquire the exact same phase encoding data Parameters Beyond the Scope of the Lab: Slice Group: Sets of slices that can be in different orientation Slices: The number of 2D Slices to acquire Dist Factor: The gap between the slices, expressed as a percentage of slice thickness Position: Center of the slice in Left/Right, Anterior/Posterior, Foot/Head coordinates AutoAlign: A Siemens feature that automatically computes anatomical planes TR / TE: These can be changed on the Contrast Tab Concatenations: The number of times the pulse sequence will be stopped during acquisition. Allows for breaking up a long acquisition into a series of breath holds.

6 CONTRAST TAB In this tab, important parameters that effect the contrast of the image are shown. The pull downs allow you to select parameters that will work with the current sequence The bar (denoted by yellow arrow) shows the range of the highlighted parameter. Green is good. Orange is the limit between good and infeasible. Red identifies the range where a second parameter will have to be modified to make this value feasible. There will be a pop up that will suggest values in the case of a red selection. TR: Time of Repetition Observe Changes to your Pulse Sequence Statistics by changing TR TE: Echo Time, there is no signal model in the pulse sequence statistics so you won t see them change. However: What will happen to the Contrast when you increase TE? Other Parameters on this page beyond the scope of the lab Flip Angle: For a Turbo Spin Echo, this is the flip angle of the refocusing pulse (the 180 ). Often these numbers have been optimized to avoid increasing RF deposition and SAR. MTC: Magnetization Transfer Contrast an off resonance RF pulse is used to partially saturate macromolecules. Can be used to enhance contrast of simple substances (e.g. blood for MR Angiography), Restore magn: A flip up pulse at the end of an echo train. Can be used in conjunction with lower TR. Fat and Water Suppr: Chemical Excitation Selective Saturation pulses tuned to Water & Fat

7 RESOLUTION TAB FoV: Field of View (in millimeters). In both the readout direction and phase What happens to TA / SNR when you change TR TE: Echo Time, there is no signal model in the pulse sequence statistics so you won t see them change. However: What will happen to the Contrast when you increase TE? Other Parameters on this page beyond the scope of the lab Flip Angle: For a Turbo Spin Echo, this is the flip angle of the refocusing pulse (the 180 ). Often these numbers have been optimized to avoid increasing RF deposition and SAR. MTC: Magnetization Transfer Contrast an off resonance RF pulse is used to partially saturate macromolecules. Can be used to enhance contrast of simple substances (e.g. blood for MR Angiography), Restore magn: A flip up pulse at the end of an echo train. Can be used in conjunction with lower TR. Fat and Water Suppr: Chemical Excitation Selective Saturation pulses tuned to Water & Fat

8 ipat TAB (Image Parallel Acquisition Technique) This is a sub-tab of the Resolution page. There are some important parameters on this page which are becoming very widely used in MR pulse sequences. PAT mode: There are a few different reconstruction techniques to speed up an image acquisition, GRAPPA, SENSE, msense, CAIPIRAHNA or none. Accel Factor PE: This number determines how much data is skipped and reconstructed using the PAT technique. Here 5 means that if the number of phase encodes was set to 100, only 20 phase encodes would be acquired (1/5 th ) What happens to SNR / TA when you increase PAT?

9 SEQUENCE TAB We ve skipped over the System, Physio and Inline Tabs. These deal with pre-scan options, cardiac and physiological monitoring, and post processing. These are well beyond the scope of what we have time to cover. The last important tab is Part 1 of the Sequence Tab Bandwidth: The rate at which data is digitized, in terms of Hz / pixel. The larger this number, the faster the readout portion of the sequence. What does reducing Bandwidth do to your SNR? Flow Comp: Flow Compensation, gradient moments are nulled to reduce artifacts from moving blood What will gradient moment nulling do to your TE? Beyond the scope of this lab: Contrasts: This number determines the number of different images reconstructed, useful to acquire a PD and a T2 weighted image in one. Introduction: A knocking noise that alerts the patient that the scan is starting Compensate T2 Decay: A post processing technique that tries to eliminate blur from T2 decay on long TSE readout Averaging Mode: Changes the order the phase encodes are acquired (Long term acquires all the data first, then repeats the acquisition, Short Term acquires data in order, but repeats each phase encode line right away Multislice mode: Interleaving acquires slices 1, 3, 5, 7 then 2,4,6,8 Reduces slice cross contamination