M R I Physics Course

Transcription

1 M R I Physics Course Some Body Techniques/Protocols Nathan Yanasak, Ph.D. Jerry Allison, Ph.D. Tom Lavin, M.S. Department of Radiology Medical College of Georgia

2 References: 1) The Physics of Clinical MR, Focusing on the Abdomen, Taught Through Images AUTHORS: VAL M. RUNGE1 MD, WOLFGANG R. NITZ2 PHD, STUART H. SCHMEETS2 BS, RT, WILLIAM H. FAULKNER, JR.3 BS, RT, NILESH K. DESAI1 MD 2

3 Some Techniques Relevant to Body Imaging The trick of Body Imaging usually is reduce motion. Specific Absorption Rate (SAR) can also be an issue in the large volume of abdominal tissue. Two different ways to go fast: HASTE rapid abdominal imaging (spin echo) TrueFISP rapid abdominal imaging (gradient echo) Motion Correction patient restlessness/chest movement Spectral Fat Suppresion differentiation, delineation of tissues. 3

4 HASTE (Half Acquisition Single-Shot Fast Spin Echo) Advancements in gradient efficiency and RF systems assist sequences that decrease scan time and minimize the impact of patient motion. Example: HASTE (Half Acquisition Single-shot Turbo spin Echo). This approach combines half- Fourier and fast spin echo imaging. GE name: SSFSE single shot fast spin echo 4

5 HASTE (Half Acquisition Single-Shot Fast Spin Echo) Component #1: With HASTE, each slice is acquired (and often reconstructed) before acquiring the next slice. How? The echo train will consist of the required number of phase encoding steps for one slice. So, somewhat like a spin-echo EPI sequence. This differs from normal turbo (fast) spin echo in which phase encoding lines from multiple slices are acquired throughout the examination. 5

6 HASTE (Half Acquisition Single-Shot Fast Spin Echo) Component #2: The HASTE method employs the half Fourier technique: the symmetry of k-space is used to synthesize approximately 50% of the data for each slice. Long echo trains for spin echo = many 180 o pulses ( SAR). Filling ½ of k-space reduces SAR issue. Reduced SNR, but same spatial resolution. Images with HASTE are usually acquired in 2 seconds or less per slice very good for reducing patient motion artifacts. 6

7 HASTE (Half Acquisition Single-Shot Fast Spin Echo) Sampling bandwidth issues: Low longer times between echoes T2 image blurring (A,C). High smaller echo spacing lower SNR (B,D). However, the SNR reduction is typically less of an effect than the image blurring (i.e. choose high bandwidth). HASTE finds clinical applicability in particular for the upper abdomen (sometimes, for rapid brain imaging), with excellent depiction of tissue morphology. 7

8 Steady State Free Precession Imaging (TrueFISP) Fully coherent steady-state free precession imaging (SSFP) is a fast, gradient echo based imaging method. Unlike many GRE sequences, SSFP sequences rephase, instead of spoiling, the transverse magnetization after multiple, rapid excitations. Examples: true fast imaging with steady state precession (truefisp) balanced fast field encoding (balanced-ffe) fast imaging employing steady-state acquisition (FIESTA)) 8

9 Steady State Free Precession Imaging (TrueFISP) In fast imaging, some residual amount of transverse magnetization remains before the next repetition of rf pulses is applied (i.e., before the next TR). What should be done? If nothing is done, residual magnetization (with residual coherence) from one TR period can cause spurious echoes occurring at a later times, unpredictably. With grad echo, these can both be observed during later readouts. (from Carl s Roost of artifacts: chickscope.beckman.uiuc.edu/roosts/carl/artifacts.html) 9

10 Steady State Free Precession Imaging What can we do? (TrueFISP) Spoiled-gradient echo destroy the transverse magnetization before the next repetition. (e.g., SPGR spoiled gradient recall). A.k.a., Steady-state incoherent. Recalled-gradient echo rewind (or rephase) the magnetization before the next repetition. truefisp belongs to this group. A.k.a., Steady-state coherent. Both achieve a long-term steady-state magnetization state. 10

11 Steady State Free Precession Imaging (TrueFISP) Ordinary 2D gradient echo sequence (note that the gradient pulse shapes are simplified for this discussion): Echo location rf Slice select Phase-encode Readout grad ADC Readout 1. Spins are excited in plane. 2. Phase-encode gradient increments phase of spins as a function of space. 3. Readout gradient encodes spins with different frequency, and data is read out. 4. Spoiling (not shown) with large gradient before next 11 excitation.

12 Steady State Free Precession Imaging (TrueFISP) Example of a simple Steady- state Coherent sequence : 1. Spins are excited in plane. 2. Phase-encode gradient increments phase of spins as a function of space. 3. Readout gradient applied during readout of data. 4. Equal but opposite phase- encode gradient rewinds the spins. Although longitudinal mag. increases slightly during a TR, this sequence attempts to wind the phases back to 12 what they were at the start.

13 Steady State Free Precession Imaging (TrueFISP) Motion will affect this technique. The echo times are kept short (via larger bandwidths, etc), decreasing sensitivity to artifacts from moving spins such as blood and CSF. SSFP is excellent for abdominal and cardiac applications. The SSFP technique has increased sensitivity to off-resonance effects ruin the ability to rewind the phases. Therefore, shimming, similar to that done prior to spectral fat- saturation, is carried out before the measurement to improve overall B0 field homogeneity. 13

14 Steady State Free Precession Imaging (TrueFISP) TrueFISP provides the highest signal intensity of all steady state sequences. Tissue contrast for longer TR values (~.5 2 sec) is a complicated function of the T 1 /T 2 ratio. When acquired with short TR and short TE, truefisp images are primarily T 2 weighted, with very high signal intensity for all types of fluid (including CSF, flowing blood). 14

15 Steady State Free Precession Imaging (TrueFISP) Coronal truefisp imaging of the upper abdomen (A) without and (B) with fat suppression. Bowel, liver, and, in particular, the vascular system are well defined. Scan time: ~ 1 second per image. 15

16 Steady State Free Precession Imaging (TrueFISP) Example: (A) Axial truefisp with fat suppression at the kidneys, with high signal intensity from CSF, fluid within the small bowel, the vascular structures, and urine within the collecting system. (B) Sagittal truefisp. The gall bladder and hepatic vasculature are depicted with high signal intensity. 16

17 HASTE vs TrueFISP TrueFISP is more technically complicated Poor scanner performance can ruin quality (B homogeneity, gradients) Motion is a problem TrueFISP has higher SNR, but more complicated T 1 /T 2 contrast. Somewhat like T1, but not exactly. HASTE is typically T2-weighted, due to readout of multiple echoes. HASTE will always be SAR-limited. MCG: Contrast-enhanced image: TrueFISP, for T1-like image T2W image: HASTE 17

18 Abdomen Motion Correction Magnetic resonance provides an excellent tool for imaging of the internal structures of the thorax and abdomen. But physiologic motion (e.g., respiration and cardiac pulsation, squirming) can lead to artifacts reducing the diagnostic quality of the final image. Most MR scanners offer hardware, software, and sequence-based options designed to minimize or eliminate the effects of physiologic motion during scan acquisition. 18

19 Abdomen Motion Correction Motion artifacts: caused by translations or shifts in the position of the imaged structure during the acquisition of data. Routine spin echo and gradient echo sequences encode and acquire data to fill k-space in a line-by-line fashion: each line collected at a different time (TR). In fast spin echo imaging, several lines are collected with each TR. 19

20 Abdomen Motion Correction In-plane motion (2D or 3D): Changes in the location of anatomic structures between TR periods can lead to misalignment in spatial encoding. Fourier transform of the lines of k space data acquired during motion results in blurring or ghosting in the final image. Often, duplicates occurs along the phase-encoded direction. Phase Encoding 20

21 Abdomen Motion Correction Through-plane motion (for 2D imaging): Motion along the slice-select direction result in the anatomy changing in a slice for each TR. For T1W imaging, steady state transverse magnetization relies on repeated excitation of the SAME slice at periodic intervals (i.e., TR). Through-plane motion disrupts this steady state, resulting in unexpected changes in signal intensity between TRs. Image is generally blurred. 21

22 Abdomen Motion Correction How to reduce motion (non-abdomen, e.g., brain): 1) Tell patient to lie still. 2) Use restraints of some sort. Usually not much of a problem for standard patients. Children, patients with pain, seizures, Parkinson s, etc can create problems even for imaging other than the abdomen. 22

23 Abdomen Motion Correction Respiratory method #1: breath holds (simplest). Rapidly acquired T 1 - and T 2 -weighted gradient echo sequences are used to collect data during suspended respiration, thereby minimizing motion artifacts. Problem: scan time < 25 seconds for the average patient, limiting spatial resolution and number of slices. The use of this approach is also restricted by the patients health and mental status, as well as their age. 23

24 Abdomen Motion Correction Method #2: Single-slice techniques such as HASTE, truefisp, and echo planar imaging (EPI) acquire the data for one entire slice before beginning the next slice. Parallel imaging techniques may also help. In most cases, the acquisition is rapid enough to freeze respiratory motion, greatly minimizing artifacts within each slice. These don t account for through-plane motion. The addition of techniques such as gating or breath-holding is necessary to assure consistent and complete anatomic coverage. 24

25 Abdomen Motion Correction Method #3: Special sequences allow for reconstruction of images with movement (e.g., PROPELLER). Acquire strips in k-space, in short periods of time. Each strip will sample very little motion, and strips can be aligned with each other in k-space. (from Pipe, J., MRM 1999, 42: ) Each strip stores complete image info along a particular direction in the 25 image.

26 Propeller Example (from Pipe, J., MRM 1999, 42: ) 26

27 Abdomen Motion Correction Method #4: Respiratory gating incorporates the use of a bellows device placed around the patient s chest to track respiratory motion. Data is acquired during a specific portion of the respiratory cycle defined in the sequence parameters during set up. This method can reduce respiratory artifacts (most of the time); however, scan times are lengthened (significantly at times: ~3x increase), sometimes to the point of futility. Not frequently used for humans o.k. for mice. 27

28 Abdomen Motion Correction Method #5: Navigator echoes do not require either additional hardware or patient cooperation Act as a measure of the respiratory cycle, using MRI info as a bellows. A simple, 1D navigator: additional RF pulses in the sequence track superior to inferior translational motion of abdominal structures based on the diaphragm position. Information from the navigator can be used to trigger data ( gate ) acquisition during a specified portion of the respiratory cycle. Higher order navigator echoes also exist to adjust for 2D translation in cardiac imaging and 3D translation in the brain during functional MRI studies. 28

29 Abdomen Motion Correction Advantages: Incorporating navigators to reduce the time required for breath-holding will break up the large, multi-slice measurements into smaller groups of slices, reducing the breath-hold duration of each group by a corresponding amount. Differences in the diaphragmatic position between measurements due to varying levels of inspiratory volume are corrected by gradient system adjustments, reducing the chance for overlapping of slices or large gaps in slice coverage. Some vendors call this approach PACE (prospective acquisition correction). 29

30 Abdomen Motion Correction For the navigator echo, a "rod of tissue" is excited, placed through the dome of the liver with a 1D craniocaudal extension (A). The 1D information is read out in parallel to the imaging sequence with the motion of the liver- lung interface serving as an indicator for breathing (B). One defines whether data acquisition should be activated close to the inspiratory or expiratory portion of the respiratory cycle. A tolerance in mm of liver excursion is usually taken as indication whether the data acquisition is to be switched on or off. 30

31 Fat Suppression (Spectral Saturation) The suppression of signal produced by adipose tissue in MR imaging can be helpful in the delineation and differentiation of certain tissues and pathologies. One method: spectral fat saturation (or fat sat ). Axial T1-weighted images of the upper abdomen, (A) without and (B) with spectral fat sat. Note that the pancreas (arrow) is well delineated on the image with fat sat, leading to widespread use of this technique for pancreatic imaging. 31

32 Fat Suppression (Spectral Saturation) What is Spectral Fat Saturation? Water and fat protons resonate at slightly different frequencies in a magnetic field. Frequency separation increases with field strength: At 1.5 T, ω~220 Hz. A special 90 o RF pulse is applied prior to ordinary excitation, at the specific resonance frequency of fat fat is excited. Then, a spoiler gradient is applied to dephase this signal fat tissue remains saturated yet dephased during the spin excitation of water fat does not contribute to the resulting echo and image formation. 32

33 Fat Suppression (Spectral Saturation) Differences in magnetic susceptibility (e.g., metal objects in tissue, or variations in tissue shape in neck/chest) can lead to a change in the specific resonant frequency of fat in localized areas ( ω would be different, and ω fat pulse would be wrong). So, shimming can be critical. Greater uniformity of the magnetic field gives less change for incomplete or inconsistent fat saturation. 33

34 Fat Suppression (Spectral Saturation) Consequently, users should make an extra effort to assure that all metal objects (e.g., buttons and jewelry), are removed prior to the exam to improve the final spectral fat saturation. In general, we only allow rings to be left on, for any protocol. For abdomen, we usually specify that patients are in a gown. Body piercings can be a problem. Other than for safety (ferrous material), metal can distort an image depending on how massive it is, how far it is from the isocenter, and motion of the object. Moral of the story: less metal is better. 34

35 Spectral vs. Spatial Saturation Spatial Saturation can also saturate materials in a particular region by applying a 90 o RF pulse prior to the spin preparation excitation. When applied with the proper gradients switched on, spins in a localized area can be saturated prior to excitation of desired region. This is useful for eliminating flow effects in the image. (from IUPUI School of Medicine Rad. Lectures) 35

36 Saturation vs. STIR Let s review our options re: fat suppresion 1) Spatial saturation saturation of everything in a region (like an extra slice-select before the initial excitation). Good saturation in a region, but does not discriminate between tissue. 2) Short Tau Inversion Recovery T 1 -based approach. less sensitive to magnetic inhomogeneity, but some loss of SNR. 3) Spectral saturation chemical shift-based approach better SNR, but more dependent on shimming. So, the amount of saturation may vary spatially if shimming isn t good. 36

37 Spectral fat saturation is often also employed in the lumbar spine to facilitate the detection of lesions within the marrow on T 2 -weighted scans. Fat Suppression (Spectral Saturation) Sagittal T 2 -weighted images of the lumbar spine (A) without and (B) with spectral fat saturation. Note the improved visualization of disk hydration (and the loss of hydration at L5-S1, arrow) in the image with fat saturation. 37

38 Fat Suppression (In-Phase, Opposed-Phase) Two main populations of magnetization: adipose (fat) and water-containing tissue. The adipose resonance frequency is about 3.5 ppm lower than that of water (~ -220 Hz on a 1.5 T system, as previously noted). After an initial excitation, the transverse magnetization within adipose tissue will fall behind the transverse magnetization within water. Useful technique: the time when the MR signal is observed can be chosen so that the transverse magnetizations from fat and water are either opposed-phase or in-phase. 38

39 Fat Suppression (In-Phase, Opposed-Phase) The duration of the acquisition window in a pulse sequence is inversely proportional to the bandwidth of the sequence (remember this from the SNR lecture?). If the bandwidth is large enough ( ( the acquisition window is very short), it is possible to acquire opposed- phase and in-phase images simultaneously with a double echo gradient echo sequence (Figure 2). This approach is typically employed with TE, TR, and tip angle chosen to provide T 1 -weighting. 39

40 Fat Suppression (In-Phase, Opposed-Phase) Thus voxels containing fat are high signal intensity and those containing water low signal intensity. However, in voxels in which there is both fat and water, there is a cancellation (loss) of signal on opposed-phase images, due to the transverse magnetization from fat and water being of opposite phase and contained in the same voxel. This leads to signal loss at the interface between fat and water containing structures, for example at the margin of the liver (or spleen) and adjacent intra- abdominal fat. 40

41 Figure 2 (from Ref #1) 41

42 Fat Suppression (In-Phase, Opposed-Phase) Example: Nonhyperfunctioning adrenal adenoma, on (A) in-phase and (B) opposed-phase images (reprinted with permission from Clinical Magnetic Resonance Imaging, VM Runge (editor), W.B. Saunders Company, Philadelphia, 2002). A round, sharply demarcated, homogenous lesion of the left adrenal gland is noted (arrow). In-phase: the lesion is nearly isointense with normal liver parenchyma. Opposed-phase: the lesion is markedly hypointense. 80% of adrenal adenomas contain sufficient lipid to show a marked signal intensity reduction on opposed-phase imaging, which is not seen for the other major lesions considered in differential diagnosis metastasis, pheochromocytoma, and adrenal carcinoma. 42