Magnetic Resonance Imaging FRCR Physics Lectures Anna Beaumont

MRI Artefacts

MRI Artefacts Aspect of image that is not in original object/patient May degrade image or be more subtle Often can be related to a particular encoding direction May be known as alternative names, some more common than others… MRI Artefacts

Ghosting Due to instabilities in system or motion Part of anatomy will change its position relative to encoding resulting in spatial blurring Ghosts can be discrete (periodic motion: cardiac beats, arterial pulsations, respiration) or diffuse (non- periodic), e.g. bowel peristalsis Principally occurs in PE direction due to disparity in sampling times for FE (ms) and PE (s to min) Also, motion along any magnetic field gradient results in abnormal phase accumulation, which mismaps the signal along the phase encoding gradient.

Example of Ghosting Examples in a test object and patient The patient image has been windowed to reveal ghosting: Non-periodic: Eye movement Periodic: pulsatile motion PE

Example of Ghosting Ghosts may be dark or bright depending on the phase of the pulsating structure w.r.t phase of background. If they are in phase will be bright, out of phase will be dark.

Ghosting: Elimination The separation of ghosts is given by: SEP = (TR) (Ny) (NEX) T( motion) Example The aorta pulsates according to the heart rate. If HR = 60bpm, T(motion) = 1s If TR=0.5s, NEX=1, Ny =256, SEP =(0.5 x 256)/1 = 128/1 = 128 pixels. → Get two ghosts in the image.

Ghosting: Elimination Restrict patient motion (compression devices/ sedation) Periodic motion can be ‘gated’ Cardiac/respiratory triggered scanning Longer scan times Alternatively use ‘breath hold’ Modern scanners have very fast acquisition times Non-periodic motion more difficult Anti-peristalsis drugs PE & FE direction swap Increase separation between ghosts by increasing TR, Ny or NEX (increases scan time) Saturation bands Can suppress the signal from moving tissues with signal suppression techniques

Wraparound Sometimes called wrapping or aliasing or ‘foldover’ Predominantly ‘phase wrap’ Occurs when anatomy extends beyond FOV Part of object outside FOV has same phase as a part inside Outer anatomy is mapped into FOV after reconstruction

Wraparound There is a gradient in a direction, with a max. frequency (fmax) at one end of the FOV and a min. f at the other end, (Nyquist frequency) Any frequency higher than fmax cannot be detected correctly. The computer can’t recognise frequencies outside the bandwidth (which determines the FOV). Any frequency outside of this frequency range is going to get “aliased” to a frequency that exists within the bandwidth.

Wraparound The “perceived” frequency will be the actual frequency -2f Nyq f (perceived) = f (true) – 2f(Nyquist) Example: If frequency bandwidth is 32 kHz (±16kHz). If we have a frequency in the arm (outside the FOV) of +17kHz f (perceived) = +17 – 2(16) = -15 kHz Now the arm will be identified on the opposite side of the image, the low frequency side.

FOV FOV 0° 360°

Why do we see phase wrap in the PE direction? Phase-wrap in a test-object and a patient with the arm placed outside of FOV and wrapped in at bottom of image Why do we see phase wrap in the PE direction? No. of PE steps is related to scan time, so time can be shortened by reducing FOV. If it’s shortened by too much can get wraparound.

Phase-wrap: Elimination Increase FOV size Reduces spatial resolution Use of surface coils Saturation of pulses from outside the FOV Use of ‘No Phase-Wrap’ (NPW) Doubles FOV in PE direction Doubles no. of PE steps (to maintain spatial resolution) Halving no. of excitations (same scan time, but lower SNR) Displaying image on user-defined FOV

Gibbs Artefact Also called ringing or truncation artefact Appears as parallel lines adjacent to high contrast interfaces (skull/ brain, cord/CSF). Cause is inability to approximate exactly steplike change in signal intensity Consequence of using FT to reconstruct signals Signal represented by infinite sum of sine waves (Fourier theory) Series is truncated due to finite sampling FT produced with ripples→ parallel bands Pronounced at high-low signal interfaces

Ringing Example in a test object Artefact is only at edge This example has 256  64 matrix Reduced by increasing the number of PE steps (64 → 256) Increase sampling time Common in spine, where can mimic appearance of syringomyelia

Chemical Shift (1st kind) Difference in resonant frequency of water and fat (chemical-shift effect) At 1.5 T the difference is about 220 Hz Spatial location in FE direction assumed to be consequence of FE gradient. Due to shift voxels containing fat will not have expected frequency and will be mis-registered. Results in signal void and build-up on opposite sides

Chemical Shift (1st kind) Chemical shift is increased by A stronger magnetic field A lower bandwidth. If we decrease BW→ lower BW / pixel, fewer frequencies/ pixel Example: BW ↓ from 32 to 16 kHz. BW/ pixel = 160/256= 62.5 Hz per pixel 220 Hz/ 62.5 Hz/ pixel ~ 4 pixels misregistration, as opposed to ~ 2pixels at 32kHz BW

Here the effect is shown in an egg Example in kidney Here the effect is shown in an egg FE

Chemical Shift (1st kind) Solutions Fat suppression Increase pixel size by keeping FOV same & decreasing No. of frequency encoding steps (but resolution ↓) B0 ↓ BW ↑ (but SNR ↓) Use long TE (less signal from fat)

Chemical Shift (2nd kind) Only occurs in GE sequences The absence of 180 refocusing pulse causes phase shift between fat and water when echo is formed. Difference in resonant frequency of water and fat leads to phase differences when echo is formed. At 1.5T f shift is 225 Hz = period of 4.4ms Every 4.4ms they will be IN phase At 2.2ms they are exactly out of phase For TE → out of phase voxels containing same amount of fat & water will cancel and produce a black line at tissue/fat borders. In-phase Out-of-phase

Chemical Shift (2nd kind) If TE is 2.25, 6.75,11.25ms etc., fat and water will be out of phase and a dark boundary will be seen around organs surrounded by fat, e.g. kidneys and muscles. Called boundary effect, or India Ink etching Does not just occur along FE axis (as with chemical shift of the first kind) because of fat and water proton phase cancellation in all directions. Does not occur in SE because of 180 refocusing pulse. To reduce: Choose appropriate TE so in phase Increase BW (but decreases SNR) Fat suppression

Susceptibility Artefact natural property of all tissues. Measure of how magnetised tissue becomes when placed in strong magnetic field Depends on arrangement of electrons in tissue Diamagnetic Weak susceptibility Produces internal field in opposite direction to applied field. Most body tissues are diamagnetic; air & dense bone have almost zero susceptibility Paramagnetic Stronger susceptibility Produce a field in direction as main field E.g. gadolinium, deoxy-haemaglobin and met-haemoglobin Ferromagnetic strongly magnetised Experience large force when placed in an external field. E.g. metal alloys containing iron, nickel and cobalt. Q: is blood ferromagnetic? A: No. The exchange interaction which produces ferromagnetism does not occur unless iron is found in bulk form. In the body, iron is distributed in various chemical compounds such as haemoglobin (RBC), serum ferritin (iron stores) and haemosiderin (e.g. in clotted blood). These compounds are not ferromagnetic; they are weakly paramagnetic.

Susceptibility Artefact Although the magnetic susceptibility of tissues is small, the differences between the tissues & air are big enough to set up local magnetic field gradients. Hydrogen atoms on either side of the boundary will be in different magnetic fields & will relax more quickly as they interact with each other Artefacts occur at interfaces between materials of very different susceptibilities as local distortions are caused in the magnetic field. Natural boundaries e.g. air in sinuses/ tissue or tissue/ trabecular bone Also any metal (not just ferromagnetic) Form part of the static field inhomogeneities seen in T2* Results in local distortions Local field distortion can lead to precessional frequency shift → when Gss is performed there is an absence of spin excitation and absence of signal When signal is acquired → shift of spatial localisation which causes signal loss and/or image distortion.

Susceptibility Examples (Right) a dental device has produced a huge susceptibility artefact Signal void plus distortion of anatomy An air bubble in this phantom creates susceptibility effects -worse at long TE (short TE allows less time for signal loss) TE = 5 ms TE = 42 ms - Can be reduced by using SE (not GRE) or high bandwidths - Susceptibility artefacts are used to detect haematomas: blood breakdown products (hemosiderin..) cause artefacts that are responsible for a signal loss in T2*-weighted GE images.

RF or ‘Zipper’ Artefact Causes a discrete zip- like line in PE direction Caused by RF interference Usually a breakdown in RF integrity of system Severe artefact on right FE PE

Herringbone artefact Also known as “corduroy” artefact. Series of high and low intensity stripes across the image. Often appears on just one or two images in a multislice set. Caused by spike noise in the raw data, whose F.T. is then convolved with all the image information. A single image with this artefact could be bad luck, but issues with several different scans is a symptom of breakdown of either the RF coils or the decoupling system.

Herringbone artefact Due to interferences while filling‐in the k‐space during read‐out;  These interferences can be RF spikes which lead to points in k‐space with high intensity;  In any of these cases, the image will be contaminated by a zebra pattern, usually oblique, which affects the whole image; The spatial frequency of the pattern depends on which points of k‐space were affected;