MAGNETOM Harmony, Symphony and Sonata Applications Guide Ver syngo MR 2002B

Manufacturer´s note:

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Products, that are bearing a CE mark fulfill the provisions of the Council Directive 93/42/EEC of 14 June 1993 concerning medical devices. 0.0

The CE mark applies exclusively to medical equipment and products that are released under the relevant EU guidelines mentioned above. 0.0

 Siemens AG 2002 All rights reserved

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Siemens AG, Medical Solutions, Magnetic Resonance Henkestraße 127, D-91052 Erlangen, Germany

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Headquarters in Berlin and Munich Siemens AG, Wittelsbacher Platz 2, D-80333 München, Germany

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Print-No. M3-020.630.06.01.02 Printed in the Federal Republic of Germany AG 07.02

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MAGNETOM

Summary of Contents Image quality and parameters

A

Sequence techniques

B

Applications

C

Angiography

D

Cardiac

E

Pulse sequences

F

Index

L

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Summary of Contents

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MAGNETOM

Table of Contents Introduction

A Image quality and parameters A.1

Characteristics and criteria for image quality in MR

A.2

Selecting the measurement parameters

A.3

Understanding flow effects

A.4

Recognizing and correcting artifacts

A.5

Fat/water saturation

A.6

Additional saturation techniques

B Sequence techniques B.1

Imaging with spin-echo sequences

B.2

Inversion Recovery sequences

B.3

Reducing measurement times using TurboSE

B.4

Imaging with gradient-echo sequences

B.5

Fast imaging with TurboFLASH

B.6

Echoplanar Imaging (EPI)

C Applications C.1

MR Mammography

C.2

BOLD Imaging

C.3

Diffusion and perfusion imaging

C.4

Interactive Real-time Imaging (optional)

C.5

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D Angiography D.1

Angiography with MR

D.2

Inflow MR Angiography (ToF)

D.3

Phase Contrast MR Angiography

D.4

Contrast medium enhanced MR Angiography

E Cardiac E.1

MR Cardiac imaging

E.2

ECG-Triggering

E.3

Localization of the heart

E.4

Morphological display of the heart

E.5

Functional evaluation of the heart

E.6

Flow visualization and quantification

E.7

Coronary Angiography

F Pulse sequences F.1

Pulse Sequences: An Overview

L Index

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Introduction Magnetic Resonance Imaging has been developing a large number of new techniques during the past years, expanding the range of diagnostic applications accordingly. This rate of innovation added new parameters and greater complexities to MR imaging. 0.0

Compared to other diagnostic modalities, the user can influence image quality via the selectable measurement parameters. However, the large number of parameters available does not allow for fixed combinations which deliver the most optimal results in every single case. 0.0

As a consequence, every user deals with the question of finding the most optimal compromise between the different criteria for image quality and the avoidance of image artifacts. 0.0

The Applications Guide has been developed as a guideline for clinical routine examinations. The manual addresses the contrast characteristics of the different pulse sequences and discusses their application range. 0.0

We hope that the Applications Guide answers the majority of the questions that may arise during the daily use of MAGNETOM . We appreciate your support and suggestions in connection with the Applications Guide. 0.0

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Introduction

MAGNETOM

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Applications Guide

PART

A

Image quality and parameters A.1 Characteristics and criteria for image quality in MR What is a good image? ... A.1–2 What to look for: ... A.1–2 The components of image quality ... A.1–3 The parameters of the signal intensity in the image .. A.1–3 How is an MR image produced?... A.1–4 The raw data matrix (measurement matrix) ... A.1–4 The image matrix ... A.1–5 The relationship between raw data and image data ... A.1–6 Image resolution ... A.1–8 Area resolution ... A.1–8 Spatial resolution ... A.1–9 Signal and noise ... A.1–10 Noise ... A.1–10 Spatial resolution versus signal-to-noise ... A.1–11 Measuring signal and noise in the image ... A.1–12 Contrast ... A.1–14 What factors contribute to contrast? ... A.1–15 The relationship between contrast and noise ... A.1–15 The influence of contrast media ... A.1–16 How do the Gadolinium compounds work? ... A.1–16 Administering contrast medium ... A.1–17 Summary ... A.1–18

A.2 Selecting the measurement parameters Slice parameters ... A.2–2 Number of slices ... A.2–2 Slice thickness ... A.2–3 Slice distance ... A.2–4 0.0

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A–1

Table of Contents

Image quality and parameters

Slice position ... A.2–5 Excitation sequence ... A.2–6 Slice orientation... A.2–8 Slab ... A.2–9 Field of view (FOV) ... A.2–10 Square FOV ... A.2–10 Rectangular FOV ... A.2–12 Matrix size ... A.2–14 Combined reduced raw data matrix and rectangular FOV ... A.2–20 Partial Fourier matrices ... A.2–22 Summary ... A.2–24 Oversampling ... A.2–25 In the frequency-encoding direction ... A.2–25 In the phase-encoding direction ... A.2–25 Changing the encoding direction (Swap) ... A.2–27 Acquisition parameters ... A.2–28 Number of averagings ... A.2–28 Number of measurements, delay time ... A.2–29 Concatenating scans... A.2–29 Acquisition time ... A.2–30 Simultaneous excitation ... A.2–33 Reconstruction parameters ... A.2–36 Filter ... A.2–36 Interpolation in the image plane (2-D) ... A.2–39 Phase images ... A.2–40 Monitoring physiological stimulation ... A.2–41

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Applications Guide

Image quality and parameters

Table of Contents

A.3 Understanding flow effects Flow in spin echo imaging ... A.3–2 Signal loss due to fast outflow ... A.3–2 Signal gain due to slower inflow ... A.3–4 Flow in gradient echo sequences ... A.3–6 Signal gain through fast inflow ... A.3–6 Signal loss during laminar flow ... A.3–6 Signal loss due to turbulence ... A.3–7 Flow compensation using GMR ... A.3–8 GMR in angiography ... A.3–9 Limits to GMR ... A.3–10

A.4 Recognizing and correcting artifacts Ghosting and smearing caused by motion ... A.4–2 Causes ... A.4–3 Remedies for motion artifacts ... A.4–5 Chemical shift artifacts ... A.4–7 Causes ... A.4–7 Corrective measures ... A.4–8 Chemical shift contours ... A.4–9 Cause ... A.4–9 Corrective measures ... A.4–10 Ringing due to finite data sampling ... A.4–11 Cause ... A.4–11 Corrective measures ... A.4–12 Distortion due to different magnetic susceptibilities ... A.4–13 Cause ... A.4–13 Corrective measures ... A.4–14 Aliasing artifacts when the object exceeds the FOV ... A.4–15 0.0

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Image quality and parameters

Causes ... A.4–15 Corrective measures ... A.4–16 Distortions through non-linear gradients ... A.4–17 Causes ... A.4–17 Corrective measures ... A.4–18

A.5 Fat/water saturation Image quality problems ... A.5–2 Strong fat signal ... A.5–2 Motion artifacts ... A.5–2 Fat saturation methods ... A.5–3 Spectral saturation ... A.5–3 Inversion Recovery ... A.5–3 Water excitation ... A.5–4 Fat excitation ... A.5–4 Parameter selection ... A.5–5 Prerequisites for satisfactory fat suppression ... A.5–6 Homogeneous magnetic field... A.5–6 Recommendations regarding 3-D Shim ... A.5–6 Minimizing dynamic field effects ... A.5–7 Applications ... A.5–8 Head (including the orbita) ... A.5–8 Neck (spine and soft tissue) ... A.5–10 Lumbar spine ... A.5–11 Shoulder ... A.5–12 Abdomen and pelvis... A.5–13 Knee ... A.5–15 Wrist ... A.5–16 Ankle ... A.5–17

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Image quality and parameters

Table of Contents

A.6 Additional saturation techniques Presaturation ... A.6–2 Parallel saturation ... A.6–4 Tracking SAT ... A.6–6 Magnetization transfer suppression (MTC) ... A.6–7 Range of application ... A.6–8 Contrast between blood, brain, and fat ... A.6–8 Saturation mode ... A.6–9

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Image quality and parameters

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Applications Guide

Characteristics and criteria for image quality in MR

CHAPTER

A.1

A.1

One of your requirements is to produce diagnostically valuable MR images. Image quality in MR is a question of physical and technical capabilities, however, the main concern to you, the user, will be its medical and diagnostic effects.

A.1

Questions

A.1

For this reason, you would like to know:

A.1

which measurement protocol, pulse sequence, and operating parameters to select in order to

A.1

❏ maximize image quality, delivering diagnostically relevant information and ❏ minimize measurement times This chapter will provide you with the basic criteria for image quality. A detailed description of the measurement parameters and their effect on these criteria will be provided in → Chapter A.2.

A.1

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Good Image

Image Quality

What is a good image?

A.1

Disregarding the anatomical contents of an MR image for the moment, we generally evaluate the quality of an MR image based on the following four categories.

A.1

What to look for: Resolution

A.1

Is the image blurred?

A.1

A.1

Adjacent, fine structures will be difficult to differentiate. Borders of contrasting areas will be fuzzy. Small pathologies will be difficult to diagnose.

A.1

Noise

A.1

Is the image noisy?

A.1

If the image is grainy and irregular-like “snow” on the television-it is too noisy.

A.1

Contrast

A.1

Does the lack of contrast in the image conceal a diagnostically important difference in the tissue displayed?

A.1

In this case you will not be able to recognize a small metastasis in the liver.

A.1

Artifacts

A.1

Does the image contain structures not belonging to the anatomy?

A.1

All patterns appearing in images which do not belong to the object examined are called artifacts, with the exception of noise. You have to be able to recognize such effects and eliminate them when possible.

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Applications Guide

Image Quality

Good Image

The components of image quality

A.1

In general, when these components stand up to specific diagnostic requirements, you can say that the image quality is “good”.

A.1

Components

Resolution

Noise

Contrast

Artifacts

The parameters of the signal intensity in the image

A.1

Signal

Tissue parameters

Tissue parameters Measurement parameters

Device parameters

A.1

A.1

A.1

Measurement parameters

System parameters

❏ Density of the hydrogen nuclei (in short: proton density) ❏ Relaxation constants: T1, T2 ❏ Pulse parameter : TR, TE, flip angle, etc. ❏ Slice thickness (refer to → Page A.2–2 onwards) ❏ Matrix parameters: Matrix size, Half-Fourier, etc. (refer to → Page A.2–14 onwards) ❏ Averagings (refer to → Page A.2–28) ❏ Reconstruction parameters: Filter (refer to → Page A.2–36) ❏ Magnetic field strength (1.0 or 1.5 Tesla) ❏ Type of coil used

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A.1–3

MR Image

Image Quality

How is an MR image produced? The image is not directly obtained during measurement; rather, RAW DATA is generated.

the

A.1

MR A.1

The raw data matrix (measurement matrix) Phase-encoding steps

A.1

A.1

In the slice plane, an axis is selected for “phase encoding”. A gradient is switched along the phase axis in such a way that the precessing spins undergo a defined phase shift. However, there is no direct technique for separating the phase from the measured scan. To obtain a resolution of 128 pixels along the phase axis, 128 scans must be performed. During each repetition time TR the phase encoding gradient is incrementally modified, from a maximum value above zero to a minimum value.

A.1

Raw data matrix The axes are labelled kx and ky. The matrix is divided into four quadrants. The plane spread across the axes is called Fourier space or k-space.

Phase

Frequency

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Applications Guide

Image Quality

MR Image

Frequency encoding points (Base resolution)

A.1

During data acquisition, a gradient is switched perpendicular to the phase axis, which superimposes linearly increasing precessing frequencies on the spins. The MR signal which is read out is a combination of all these frequencies. The various frequencies can be individually filtered out using a specific technique: sampling of the data and subsequent (1 dimensional) Fourier transformation. In linear direction, the location of spins can be reconstructed across the frequencies. For this reason, the axis is also called the FREQUENCY ENCODING AXIS or readout direction.

A.1

The image matrix

A.1

The MR-image consists of a multitude of individual image elements, also known as PIXELS. The pixels are arranged into a MATRIX in a chessboard-like pattern. Each pixel in the image matrix has a specific GREY VALUE. Viewed together, the grey value matrix creates an image.

A.1

Matrix Voxel Pixel

FOV

TH

The grey value of a pixel reflects the signal intensity measured for the associated volume element (VOXEL). The signal intensity in turn depends on the transverse magnetization.

A.1

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MR Image

Image Quality

The relationship between raw data and image data

A.1

Each point in the raw data matrix (like a hologram) contains a part of the information required for the complete image. Therefore, a point in the raw data matrix does not correspond directly to a point in the image matrix.

A.1

Pixel size

Raw data matrix

Center raw data lines

A.1

Fourier Transf.

Image matrix

The lines in the center of the raw data matrix determine the basic structure and contrast of the image.

A.1

External raw data lines

A.1

The outer lines of the raw data matrix provide information regarding edges and contours in the image, minute structures, and resolution. They contain almost no information regarding tissue contrast.

A.1

The raw data matrix is transferred to the image matrix by means of a 2 dimensional Fourier transformation. For this reason, the raw data lines are also known as FOURIER LINES.

A.1

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Applications Guide