What it does

TR (repetition time) is the time between excitations of the same slice. It decides how far magnetisation recovers before the next excitation, so it controls T1 weighting, overall signal and how many slices fit into one TR. TE (echo time) is when the echo is read. A longer TE allows more T2 decay before the read, so it controls T2 weighting and signal.

On the console

Every scanner shows TE and TR with the contrast settings, with a choice between letting the system use the shortest value it can reach or typing a value.

On the Contrast tab. TE for spin echo and TSE: shortest or user defined. For FFE: shortest (the minimum, which can shift as you replan), short (a very short value that varies less with gradient settings than shortest), user defined, in-phase and out-phase. With the last two you still type a value, and the system rounds it to the nearest echo time where water and fat are in or out of phase. Multi-echo scans such as mDIXON also show a second TE and a delta TE (the gap between echoes). TR: shortest, user defined or range (next card). The values actually used are on the Info page.

TE and TR are typed values on the Routine card. The protocol header shows the acquisition time (TA) and relative SNR. See the Siemens parameter guide.

Turn it up, turn it down

• Longer TR: more recovery, so more signal and less T1 weighting; more slices fit into each TR; the scan takes longer.
• Shorter TR: more T1 weighting and less signal; fewer slices per TR; faster.
• Longer TE: more T2 weighting, so fluid and oedema stand out more; less signal overall.
• Shorter TE: more signal, less T2 weighting (a T1 or PD look).

Practical picks

• Starting points the departmental notes use for spin-echo and turbo spin echoTSETSE scans: T1-weighted TE 7 to 12 ms with TR 400 to 800 ms; PD-weighted TE 25 to 35 ms with TR 2000 to 3500 ms; T2-weighted TE 80 to 120 ms with TR 2000 ms or more. They sit close to a published review’s spin-echo ranges (T1: TR 400 to 800 ms, TE 6 to 25 ms; T2: TE 60 to 100 ms), and published turbo spin echoTSETSE protocols use longer TRs. Your own protocolExamCardprotocol values come first.
• On Philips, for in-phase and opposed-phase gradient echo, choose the in-phase or out-phase TE setting rather than user defined, so the echo lands where water and fat really are in or out of phase for that field strength. At 1.5 T water and fat come back into phase about every 4.4 to 4.6 ms (opposed at about 2.2 to 2.3 ms and 6.6 to 6.9 ms); at 3 T about every 2.2 to 2.3 ms. The exact values depend on the fat frequency assumed, and fat has several peaks, so no TE is perfect for all of it.

Watch for

• Dark outline around organs and muscles on a gradient-echo scan: an opposed-phase TE cancels signal in voxels that hold both fat and water. Expected on opposed-phase imaging; choose an in-phase TE if you did not want it. Spin echo and turbo spin echoTSETSE refocus this, so it does not apply to their TE.

Good to know

The ranges above are starting points for routine multislice imaging at 1.5 T and 3 T. Fat suppression, driven equilibriumDRIVERESTORE, reduced refocusing angles and long echo trains all shift the contrast a given TE and TR produce, so judge the images, not the numbers.

Sources: PhParams56; ADNI3PhR56; Department notes; Ridgway 2010; Del Grande 2014; Huang 2015; Yu 2008; SHKneeB17

What it does

In a multislice scan the system excites the other slices while each one recovers, so the number of slices that fit into one TR depends on how long each slice takes. When the slices do not fit, some scanners split them into groups measured one after another, each with its own full set of TRs, and others make you choose the number of groups.When the slices do not fit, Philips splits them into packages: groups of slices measured one after another, each with its own full set of TRs. A TR range lets the scanner stretch TR within limits you set as you add slices, instead of opening a new package.On Siemens you split the slices yourself by setting Concatenations: each concatenation is measured one after another with its own full set of TRs. If the slices do not fit into one TR, the minimum TR rises instead.

On the console

Some scanners let TR follow the number of slices within limits and add slice groups automatically; others ask you to type TR and to set the number of groups yourself.

TR = range (multislice scans only): you set a lower and an upper TR, and the TR used is calculated from the number of slices and shown on the Info page. shortest also changes with the number of slices, TE and technique; user defined fixes TR. Minimum number of packages is on the Geometry tab, just below the Stacks settings; the system may use more, and the Info page shows the number used (Packages) with Act. TR and Min. TR.

Siemens has no TR range: TR is typed. The nearest setting to Philips packages is Concatenations on the Routine card, a number you set directly rather than a minimum. See the Siemens parameter guide.

Turn it up, turn it down

• More slice groups at a fixed TR: the scan time multiplies by the number of groups.
• Fewer slices in each group: the shortest possible TR can fall, so signal drops and T1 weighting rises unless TR is held.
• On Philips, a wider TR range: fewer surprise packages, but contrast can drift as TR changes with the slice count.

Practical picks

• On Philips, after adding or removing slices, read the Info page: the actual TR, the number of packages, the scan time and the relative SNR.
• On Philips, set the TR range so that one or two extra slices stretch TR a little rather than forcing a second package. A second package shares the slices out, halving the number in each, and with TR at shortest that drops TR and signal.
• Scan time for a multislice turbo spin echoTSETSE scan grows with TR, the number of shots per slice (phase-encoding lines divided by the echo train lengthTSE factorturbo factor), the number of averages and the number of slice groups.

Watch for

• On Philips, a PD or T2 scan that loses signal after you added slices: a package was probably added, so TR was recalculated and fell; or the TR range hit its upper limit. Check before you accept the change.

Sources: PhParams56; ADNI3PhR56; Department notes; Ridgway 2010; SHKneeB17; ADNI3SiPrismaD13

What it does

At the end of each echo train, a flip-back pulse tips the transverse magnetisation that is left back onto the longitudinal axis. Tissues with a long T2, such as joint fluid and CSF, still have plenty left at that point, so they start the next TR with more magnetisation. Fluid is brighter at a given TR, or the same T2 contrast is reached with a shorter TR and a shorter scan.

On the console

An on or off option on turbo spin echoTSETSE scans, usually listed with the contrast settings.

On the Contrast tab of TSE and GRASE scans: DRIVE = no, yes or anti. Set to anti, the pulse pushes the other way (anti-DRIVE), lowering long-T2 signal for stronger T1 weighting; it is meant for T1-weighted TSE and GRASE only. DRIVE combines with SPIR, SPAIR and REST slabs.

Restore magn. (On or Off, the RESTORE technique) on the Contrast card does the same job on Siemens TSE scans; there is no counterpart to anti-DRIVE. GE calls its version FRFSE.

Turn it up, turn it down

• Driven equilibriumDRIVERESTORE on: brighter fluid at the same TR, or a shorter TR and scan for the same T2 contrast (one cranial nerve study cut a 3D T2 turbo spin echoTSETSE scan by about a quarter).
• On Philips, anti-DRIVE: darker fluid and stronger T1 weighting in T1-weighted TSE.

Practical picks

• The departmental notes switch DRIVE on for musculoskeletal PD scans so joint fluid stays bright at a PD TR of about 2500 ms. Published evidence for driven equilibrium on PD knee scans comes from one research group, using GE’s version (FRFSE): fluid was brighter, but SNR was lower and some soft-tissue contrast was weaker when there was no effusion. Treat it as a local choice and check your images.
• If a T2 turbo spin echoTSETSE scan is too long, driven equilibriumDRIVERESTORE plus a shorter TR is a way to keep fluid bright while cutting time.

Watch for

• Driven equilibriumDRIVERESTORE changes how bright fluid is relative to other tissue, so check against your reference images before adding it to an established protocol.

Sources: PhParams56; ADNI3PhR56; van Uijen 1984; Ciftci 2004; Radlbauer 2010; Woertler 2005; Tokuda 2012a; Tokuda 2012b; Department notes; SHAcronyms2020; SHKneeB17

What it does

Every echo in a turbo spin echoTSETSE echo train is made by a refocusing pulse. Using angles below 180 degrees cuts the RF energy (SAR) sharply. In long echo trains, stimulated echoes add back most of the signal, so the image loses little. Swept angle schedules settle into a pseudo-steady state that keeps the signal steadier along the train, which also sharpens the image.

On the console

Turbo spin echoTSETSE scans let you lower the refocusing angle, either to one fixed value or to a schedule that sweeps the angle along the echo train.

On the Contrast tab: Refocusing control, with values that include constant, T1-optimized and T2-optimized (older releases show yes with an angle). constant: after a short sweep for the first echo, every pulse uses the angle you set (30 to 180 degrees). T1-optimized and T2-optimized switch on Pseudo-Steady-State TSE, with a sweep set by a minimum angle and a maximum angle. Philips suggests a minimum of about 40 to 60 degrees for short-TE (PD and T1) scans and about 60 to 90 degrees for long-TE (T2) scans; the minimum angle affects flow signal most, and the maximum angle affects SAR most. The Info page shows TEeff / TEequiv; with reduced angles TEequiv is shorter than TEeff.

The refocusing angle of a TSE scan is the Flip angle on the Contrast card. The Sequence card adds Flip angle mode (for example T2 var in 3D SPACE) and a separate Hyperecho on or off. No published mapping links these one to one with the Philips sweeps. See the flip-angle modes section of the Siemens parameter guide.

Turn it up, turn it down

• Lower angle, or a lower minimum angle: lower SAR (often a shorter TR or scan when SAR was the limit); weaker signal from moving spins such as flowing blood, which also removes some flow ghosts; less magnetisation transfer between slices; contrast that matches a shorter TE.
• Higher angle: more signal and contrast closer to a classic 180 degree train, more SAR. When SAR is already at its limit, raising the angle can lengthen the scan because the system lengthens TR to stay within the SAR limit.

Practical picks

• A SAR-limited turbo spin echoTSETSE scan whose TR or time keeps climbing: lowering the refocusing angle is one of the most effective levers, alongside fewer slices or fewer averages.
• With reduced or swept angles, the T2 contrast matches a shorter equivalent TE than the TE you set; for typical swept trains one group put it at about half to four-fifths of the echo time. To keep the weighting you are used to, raise the TE., and check TEequiv on the Info page..
• The departmental notes keep a constant angle at or above about 120 degrees in brain and thoracic spine scans, where CSF flow matters. That number is local practice, not a published rule. The published point is that lower angles, and the minimum angle of a sweep above all, darken moving blood and other moving tissue; CSF is expected to behave the same way, and Philips notes that the minimum angle has the most effect on flow signal.
• T1-weighted turbo spin echoTSETSE uses short echo trains, and the stimulated-echo contribution only builds up along the train, so a T2 sweep costs a T1 scan more signal. Philips offers a separate T1 setting for short-TE scans.

Watch for

• CSF or vessels patchy or unexpectedly dark on a low-angle scan, for example around the thoracic cord: this can be flow-related signal loss from the low refocusing angles. Raise the angle, or the minimum angle, and compare before reading anything into it.

Sources: PhParams56; ADNI3PhR56; ADNI3SiPrismaD13; HCPUPennD13; Hennig 2001; Hennig 2003; Alsop 1997; Busse 2006; Busse 2008; Weigel 2006; Weigel 2010; Lebel 2009; Storey 2010; Loening 2015; Department notes

What it does

The number of k-space lines filled after each excitation, and so the factor by which the scan is shorter than conventional spin echo. For example, an image with 320 phase-encoding lines and a factor of 16 needs 20 shots per slice instead of 320. The shot is the whole echo train for one slice, roughly the factor multiplied by the echo spacing; once a slice’s shot ends, the other slices’ shots follow within the same TR.

On the console

A number on turbo spin echoTSETSE scans: the echo train length, or the number of echoes per excitation.

TSE factor on the Contrast tab, under the TSE fast imaging mode. With linear or asymmetric order, a longer TE leaves room for more echoes before the centre of k-space, so the console allows a higher TSE factor. The echo spacing usually follows from the TE, TSE factor and profile order; the console Help describes a TE spacing setting with asymmetric profile order, and ultrashort (TSE mode) shortens it. The Info page shows the result as TSE es / shot.

Turbo factor on the Sequence card, with Echo spacing as its own field beside it. See the Siemens parameter guide.

Turn it up, turn it down

• Higher factor: faster (fewer shots); a longer shot, so more T2-decay blurring along the phase direction and fewer slices per TR. In short-TE scans the train also reaches echoes with much longer echo times, which is why one published 3D PD knee protocol capped the train length.
• Lower factor: sharper images and truer T1 or PD contrast; slower.

Practical picks

• The departmental notes keep the shot short for T1 (about 50 ms), around 120 ms for PD, and let T2 shots run to 200 ms or more. Treat these as starting points for your own scanner.
• T1-weighted turbo spin echoTSETSE: keep the factor low. The notes use no more than about 5 with low-high ordering.
• After switching a turbo spin echoTSETSE scan to Dixon water-fat imagingmDIXONDixon, check the shot length and scan time again: the departmental notes found Dixon scans take longer.

Watch for

• A T1 or PD scan that looks blurred: the factor, and with it the shot, is probably too long.
• A T1 scan with fluid too bright: check three things, a factor that is too long, driven equilibriumDRIVERESTORE switched on, or a TR that has grown with the slice count.

Sources: PhParams56; ADNI3PhR56; Ridgway 2010; Busse 2008; Constable 1992a; Woertler 2005; Department notes; SHKneeB17

What it does

Decides the order in which each echo train fills k-space. The centre of k-space (the low-order profiles) sets the contrast, so the echo time at which the centre is filled is the effective TE; echoes either side of it fill the outer lines, which carry edges and detail.

On the console

Most scanners place the centre of k-space at the TE you ask for and arrange the other echoes around it. Philips lets you choose the arrangement.

Profile order on the Contrast tab: linear, low-high, asymmetric, or reverse linear (only with halfscan).

low-high: the first echo fills the centre and later echoes work outwards, so the echo spacing equals the TE and the shot is the TE multiplied by the TSE factor. Example: TE 9 ms with a factor of 5 gives echoes at 9, 18, 27, 36 and 45 ms, a 45 ms shot. For T1 and short-TE PD.

linear: the train sweeps from one edge of k-space to the other, so the centre falls in the middle of the train. The echo spacing is about 2 × TE ÷ (factor + 1) and the shot is a little under twice the TE. Example: TE 108 ms with a factor of 11 gives echoes every 18 ms, from 18 to 198 ms, with the 6th echo at 108 ms. With an even factor the centre falls between the two middle echoes. For T2.

asymmetric: the centre is filled early in the train, after a few outer lines, and the rest of the train fills the remaining lines. The departmental notes use it for PD, and the console Help table they record lists it for short-TE PD with a higher TSE factor than low-high allows.

Siemens 2D TSE has no profile order setting: you type the TE you want (the effective TE) and the scanner arranges the echoes around it. Reordering on the Sequence card exists only for some 3D sequences, such as MPRAGE. See the Siemens parameter guide.

Turn it up, turn it down

• Low-high: shortest effective TE and the cleanest T1 contrast, but only a short train fits.
• Linear: long effective TE for T2 with a long train. For a fixed TE about half the train must fit before the centre echo, so the factor tops out at about 2 × TE ÷ echo spacing.
• Asymmetric: short or intermediate TE with a longer train than low-high; the outer lines are filled unevenly.

Practical picks

• On Philips, in a linear scan where the TSE factor will not go any higher at the TE you need, switching to asymmetric can fit a longer train.
• Shorter echo spacing means less blurring. After changing the k-space order or the echo train length, check the echo spacing.After changing the profile order or the TSE factor, check the echo spacing on the Info page.After changing the turbo factor or echo spacing, check the TE and the acquisition time.

Watch for

• Same TE, different look: on Philips, changing the profile order changes blurring and contrast even when the TE number does not move. Compare TEeff and echo spacing on the Info page before and after.

Sources: PhParams56; ADNI3PhR56; Ridgway 2010; Moratal 2008; Busse 2008; Loening 2015; Department notes; SHKneeB17; ADNI3SiPrismaD13

What it does

Chooses which in-plane direction is phase encoded; the other is the frequency (measurement) direction. Anatomy outside the field of view wraps back in along the phase direction, and motion and flow ghosts spread across the image in that direction too.

On the console

Every scanner lets you swap the phase-encoding and frequency-encoding directions for each stack.

Fold-over direction under Stacks on the Geometry tab: AP, RL or FH, depending on the slice orientation. A separate Fat shift direction chooses which way fat is shifted against water; Philips notes that susceptibility artefacts shift the opposite way to fat, so reversing the setting moves both.

Phase enc. dir. on the Routine card (repeated on the Geometry card), for example A>>P, R>>L or H>>F. The arrow also sets the polarity; there is no separate fat shift direction field. See the Siemens parameter guide.

Turn it up, turn it down

• Phase across the shorter dimension of the body part: a smaller phase field of view covers it without wrap, so fewer phase lines are needed for the same pixel size and the scan is faster.
• Phase chosen to keep ghosts off the anatomy: for example, ghosts from breathing, swallowing, vessels or CSF fall beside the region you need rather than across it.

Practical picks

• Check the phase-encoding directionfold-over directionphase-encoding direction first when you open a scan: it tells you which way is phase and which is frequency. The departmental notes point out that the acquired voxel can change when you change the field of view, the echo train lengthTSE factorturbo factor, parallel imagingSENSEiPAT (GRAPPA), compressed sensingCompressed SENSECompressed Sensing or oversampling, so check it after each change. (ACQ voxel size on the Geometry tab). (on the Resolution card).
• With parallel imagingSENSEiPAT (GRAPPA), the phase-encoding directionfold-over directionphase-encoding direction is also the acceleration direction, so the coil elements must be spread along it.

Watch for

• Ghosts or wrap across the anatomy you need: swap the phase-encoding directionfold-over directionphase-encoding direction, then recheck fat shift and the scan time, which can change with it.

Sources: PhParams56; ADNI3PhR56; Huang 2015; Zaitsev 2015; Ferreira 2013; Erasmus 2004; Moratal 2008; Department notes; SHKneeB17

What it does

Stops tissue outside the field of view in the phase direction wrapping into the image. Oversampling measures a wider phase field of view than you see and discards the edges, so the wrapped tissue lands in the part that is thrown away. In a 3D scan, slice oversampling measures extra slices at each end of the slab so that the end slices do not wrap.

On the console

Phase oversampling (an extra percentage of phase field of view) and, for 3D, slice oversampling. A saturation slab over the tissue outside the field of view is another option.

Fold-over suppression on the Geometry tab: no; oversampling (you enter the amount in the phase direction; the scan gets longer); rest (REST slabs over the tissue causing the problem, also used against motion and flow artefacts); or zoom (a reduced field-of-view technique for high-resolution diffusion in brain, spine and prostate, with a cardiac variant). Slice oversampling (3D): the system measures more slices than it shows; defaults are 1.28 for TFE and 1.8 for TSE, or user defined. On dStream systems, dS Zoom uses the SENSE reference scan to prevent fold-over in small fields of view, with fold-over suppression set to no.

Phase oversampling (%) and, for 3D, Slice oversampling (%) on the Routine card; Sat. region slabs on the Geometry card. Note the units: Philips slice oversampling is a factor (1.0 means none), Siemens a percentage (0 % means none). See the Siemens parameter guide.

Turn it up, turn it down

• More oversampling: no wrap; more phase-encoding steps, so a longer scan, but more data collected, so SNR does not fall.
• A saturation bandRESTsaturation region slab instead: little extra time, but it adds RF pulses (and SAR), and it darkens the outside tissue rather than removing it from the data.

Practical picks

• Before adding oversampling, see whether swapping the phase-encoding directionfold-over directionphase-encoding direction removes the wrap. A wider phase field of view also works, but at the same voxel size it costs about the same time as oversampling.
• With image-based parallel imaging (SENSE-typeSENSEmSENSE), the reconstructed field of view must still cover the whole body in the phase direction; otherwise the fold-over appears in the middle of the image, not at the edge. K-space-based methods such as GRAPPA wrap at the edge, as an ordinary scan would.Siemens GRAPPA, by contrast, wraps at the edge as an ordinary scan would.GRAPPA, the usual Siemens mode, wraps at the edge as an ordinary scan would.

Watch for

• Wrap at the first and last slices of a 3D slab: slice oversampling is too low for the slab; raise it.
• With Philips Compressed SENSE, one group found that streaky fold-over in a 3D PD knee scan with too small a field of view cleared fully only with a REST slab placed right against the knee plus 10% oversampling.

Sources: PhParams56; ADNI3PhR56; Huang 2015; Ferreira 2013; Krupa 2015; Erasmus 2004; Deshmane 2012; Goldfarb 2004; Griswold 2004; Sartoretti 2018; SHKneeB17; ADNI3SiPrismaD13; SHAcronyms2020

What it does

Sets the order in which the slices of a multislice scan are excited. Slice profiles are never perfectly square, so each pulse partly excites the slices next to it (cross-talk), lowering their signal and changing contrast, T2 contrast most. Exciting slices out of order, so neighbours are not excited back to back, reduces it.

On the console

Most scanners excite slices in an interleaved order by default (for example odd slices first, then even), with linear ascending and descending orders as options.

Slice scan order on the Geometry tab, just below the Stacks settings; default and ascend appear on published ExamCards. The fuller list below is from the console Help, as recorded in the departmental notes; check it against your own console. default spaces neighbouring slices as far apart in time as it can (odd then even within each package; with several packages the slices are shared out first), except that respiratory-triggered and inversion recovery scans use a linear order within each package. ascend and descend run in a straight line whose direction depends on the slice orientation; AP, PA, RL, LR, FH and HF name the direction outright. central starts in the middle and works outwards and rev. central does the opposite (M2D only). interleaved (MS and M2D) jumps through the stack with a step close to the square root of the slice count.

Series on the Geometry card, for example Ascending or Interleaved, next to Multi-slice mode (Interleaved, Sequential or Single shot). See the Siemens parameter guide.

Turn it up, turn it down

• Interleaved or default order: less cross-talk between neighbouring slices.
• Linear order: neighbours are excited one after another, so cross-talk is greater, especially with small slice gaps.

Practical picks

• The default order is built to keep neighbours apart in time, so it suits routine multislice scans.
• With small slice gaps, keep an interleaved order; if neighbouring slices still look different from each other, a larger slice gap helps. Splitting the slices into two separate interleaved passes is the most complete fix, but doubles the scan time.

Watch for

• Slices that alternate bright and dark through the stack: cross-talk. Check the slice order and slice gap.

Sources: ADNI3PhR56; ADNI3PhR5; Department notes; Kneeland 1986; Kucharczyk 1988; Schwaighofer 1989; Filippi 1998; Erasmus 2004; SHKneeB17; ADNI3SiPrismaD13

What it does

A preparation step that measures small field errors in the patient and corrects them to make B0 as uniform as possible over the region. Spectral fat suppression only works if fat and water sit at their expected frequencies (about 3.4 ppm apart) across the whole region, so it depends on a good shim. Field uniformity is worst far from the isocentre, over large fields of view and at air-tissue boundaries; balanced gradient echo (dark bands where the field is off) and EPI are also sensitive to it.

On the console

Automatic shimming over the scan region, or over a shim box that you place on the anatomy. Some systems add higher-order shimming.

Shim on the Contrast tab: default (shims only when SPIR, SPAIR, ProSet or balanced FFE/TFE is used; otherwise none); auto (first-order shim over each stack; not with radial stacks); volume (first-order shim over a shim box you draw; works with parallel and radial stacks); PB-auto and PB-volume (pencil-beam shimming with first- and second-order corrections, 3.0 T only; PB-volume uses a cube you place, so planning matters); IB-volume (image-based, cardiac) and IB-smart (breast). The shim box position is shown on the Offc/Ang tab.

Shim mode (B0 Shim mode on newer software), for example Tune up, Standard or Advanced, under the adjustments on the System card, with an Adjust volume box placed automatically or by you on the anatomy. See the Siemens parameter guide.

Turn it up, turn it down

• A small shim boxshim boxadjust volume tight on the anatomy: the field is evened out where it matters.
• A large one, or one that takes in air (sinuses, lungs, bowel gas) or the edge of the body: the correction is averaged over tissue you do not need and fat suppression gets patchy.

Practical picks

• For fat-suppressed scans, make the shim boxshim boxadjust volume cover only the anatomy of interest; for large or off-centre regions, a higher-order shim helps where your system has it.
• The departmental notes check the frequency spectrum when it appears after the preparation of a fat-suppressed scan: the system’s centre frequency (F0) line should sit on the water peak, which is the left-hand peak on the Philips display (higher frequency). Drag it onto the water peak if it is on fat, then proceed.

Watch for

• Fat suppression patchy or failed: check the centre frequency is on water and the shim boxshim boxadjust volume is tight on the anatomy. Then consider STIR (not after gadolinium, because it also suppresses enhancing tissue) or Dixon water-fat imagingmDIXONDixon, which depend less on an even field; Dixon water-fat imagingmDIXONDixon can still swap fat and water where the field is very uneven.

Sources: PhParams56; ADNI3PhR56; Huang 2015; Graves 2022; Del Grande 2014; Gruetter 1993; Zhu 2014; Department notes; SHKneeB17; SHAcademyField

What it does

At 3 T the RF wavelength in water-rich tissue (about 26 cm) is close to the width of the body, so the transmit field (B1) becomes uneven and images show bright and dark shading. Larger body regions, ascites and pregnancy make it worse; one review notes that slim patients with an oval cross-section can be affected as much as larger patients. Philips MultiTransmit drives two independent RF sources and sets their amplitude and phase for each patient from a short B1 calibration scan, which two independent research groups found makes B1 more uniform.

On the console

On 3 T systems with more than one RF transmit channel, a choice between a fixed preset and a patient-specific setting from a calibration scan.

RF shims on 3.0 T MultiTransmit systems: fixed (preset settings, no B1 calibration, quicker where B1 is not a problem); adaptive (patient-specific settings from an automatic B1 calibration scan, mostly used against shading in the abdomen); Smart (SmartExam breast; shims over breast and armpit tissue, leaving out the lungs, heart and silicone); volume (cardiac and interactive scans with the dS Torso coil; the RF shim volume matches the B0 shim box). Philips does not allow MultiTransmit scans with the patient lying on their side, and does not make them available for patients with MR Conditional implants or for children.

Siemens 3 T B1 shimming is TimTX TrueForm, set as the B1 Shim mode under the adjustments on the System card; patient-specific B1 shimming, from a B1 map of each patient, is part of TimTX TrueShape. See the Siemens parameter guide.

Turn it up, turn it down

• Adaptive: more even B1 in the body, and because local SAR is estimated more accurately some protocols can run with a shorter TR (shown mainly by one group); costs a calibration scan.
• Fixed: no calibration, so quicker, for anatomy where shading is not a problem.

Practical picks

• On Philips, use adaptive for body work at 3 T, where shading is most likely and where MultiTransmit is permitted for that patient.
• The departmental notes find adaptive RF shimming less even when the field of view is larger than about 300 mm, and split large regions into two stacks or stations rather than going above about 400 mm. Those numbers are local practice. The published principle behind them: a two-channel RF shim is a compromise over the whole region it is set for, and a smaller target region can be made more uniform.

Watch for

• Shading that stays after adaptive RF shimming: check the field of view is not larger than the region the shim can handle.
• Implants, children and side-lying patients: Philips does not allow MultiTransmit scanning for patients with MR Conditional implants, for children, or with the patient on their side. Scan only within the implant’s labelled conditions and local policy.

Sources: PhParams56; ADNI3PhR56; Huang 2015; Graves 2022; Willinek 2010; Rahbar 2012; Childs 2013; Mao 2006; Sprinkart 2014; Department notes; SHAcronyms2020; SHAcademyField; SHTrueShape

What it does

Skips k-space lines (fewer phase-encoding steps) to save time. Skipping lines shrinks the field of view, so each coil element sees a folded image; the scanner unfolds it using how sensitive each element is at each point, measured in a reference scan. K-space methods such as GRAPPA instead fill in the skipped lines from calibration lines, usually measured within the scan.Siemens GRAPPA instead fills in the skipped lines in k-space from calibration lines.GRAPPA, the usual Siemens mode, instead fills in the skipped lines in k-space from calibration lines, usually measured within the scan. Scan time falls by about the reduction factor, and SNR falls with it.

On the console

Parallel imaging: an acceleration factor in the phase direction (and, in 3D, the slice direction), calibrated from coil-sensitivity information.

SENSE on the Geometry tab, with P reduction (phase direction) and S reduction (slice direction, 3D only, when coil elements lie along that axis). Siemens’ cross-vendor chart lists it as SENSE or dS SENSE. The SENSE reference scan is acquired before the SENSE scans (the console Help says it runs automatically in the ExamCard preparation), and the CLEAR uniformity correction uses it too. PlanAlign keeps double-oblique scans free of in-plane rotation, which avoids SENSE artefacts in cardiac planning.

PAT mode (usually GRAPPA; Siemens also offers the image-based mSENSE) with Accel. factor PE and, in 3D, Accel. factor 3D on the Resolution card; Siemens calls the family iPAT. GRAPPA usually calibrates from reference lines acquired within the scan (Reference scan mode); a separate reference is also offered for some sequences. See the Siemens parameter guide.

Turn it up, turn it down

• Higher reduction: faster; shorter echo trains in single-shot turbo spin echoTSETSE and EPI, so less blurring and distortion; lower SNR (by the square root of the factor, times a coil-geometry penalty that is usually worst in the middle of the image); more risk of artefacts.
• Lower reduction: more SNR and fewer artefacts; slower.

Practical picks

• Spread the coil elements along the phase-encoding directionfold-over directionphase-encoding direction: the reduction factor cannot in general be higher than the number of coil elements, and it works best along the direction with the most elements.
• With SENSE (and Siemens mSENSE), keep the field of view covering the whole body in the phase direction. A field of view smaller than the body gives fold-over in the middle of the image (intrinsic backfolding), not at the edge. GRAPPA wraps at the edge instead.
• With SENSE, or a separate GRAPPA reference, keep the patient still between the reference and the accelerated scan; if they have moved, repeat the reference.

Watch for

• A noisy patch in the middle of the image at high reduction factors: lower the reduction factor.

Sources: PhParams56; ADNI3PhR56; Pruessmann 1999; Deshmane 2012; Huang 2015; Goldfarb 2004; Griswold 2004; SHAcronyms2020; SHKneeB17; ADNI3SiPrismaD13

What it does

Measures fewer k-space lines in a pseudo-random pattern that samples the centre more densely. The skipped lines then show up as a fine, noise-like haze instead of clean fold-over. An iterative reconstruction uses the coil sensitivities and the fact that images can be stored compactly (for example as wavelets) to remove the haze. Philips combines it with SENSE. The time saved can go on speed, resolution, coverage or SNR.

On the console

Compressed sensing: an acceleration factor and, on some systems, a denoising strength.

CS-SENSE on the Geometry tab, in place of SENSE, with a reduction factor (the amount of acceleration) and denoising (no, weak, medium (the system default) or strong). Release 5.6 lets dS Zoom and Compressed SENSE work together. Some scan types cannot use it; the Conflicts tab says so. On later Philips software (not in the R5.6 manual), SmartSpeed builds on Compressed SENSE, adding an AI step to the reconstruction.

Compressed Sensing is offered within specific Siemens applications (for example CS SPACE, CS cardiac cine, CS TOF and GRASP-VIBE). See the Siemens parameter guide.

Turn it up, turn it down

• Higher reduction factor: faster; more noise, more risk of artefacts and a smoother look. Philips Compressed SENSE studies from two separate brain centres, and one group’s knee studies in volunteers (each with Philips co-authors), kept diagnostic quality at moderate factors while cutting scan time by roughly a quarter to a half. The usable factor depends on the sequence, so test it locally.
• Stronger denoising: less noise but more artificial smoothing, which can soften fine detail. Weaker denoising: sharper but noisier.

Practical picks

• Change one setting at a time. If the image is noisy, lower the reduction factor or step the denoising up; if it looks waxy or over-smoothed, step the denoising down.
• Compare a new compressed sensing scan with your reference images before replacing a scan in the protocolExamCardprotocol.

Watch for

• A waxy, over-smoothed look with lost fine detail, or streaky or grainy artefacts: too much acceleration or denoising. Step back and compare.

Sources: PhParams56; ADNI4PhR57; PhCSPage; PhSmartSpeedMSK; Lustig 2007; Feng 2017; Jaspan 2015; Geerts-Ossevoort 2018; Sartoretti 2018; Vranic 2019; Eichinger 2019; Iuga 2020; Dratsch 2024; SHAcronyms2020; SHCS