# Virtual scanner

The stand-ins for the scanner: the cache played on a phantom's tissue by the
Fourier engine, the raw data sent as the scanner's reconstruction client sends
them, and the blocks the cache plays, written as a Pulseq file for a simulator
that reads one.

```{eval-rst}
.. currentmodule:: pulserver.virtual
```

The design calls, the IR, the reconstruction proxy and the reconstruction
plugins are the production code; what each stand-in exercises, and the signal
model, are described in {doc}`../explanations/virtual-scanner`. Positions are
in metres and k-space locations in 1/m, along the physical axes: the logical
axes turned by the prescription's rotation, the identity unless one is given.
Chemical shifts are in ppm from water, and frequencies in Hz from the
scanner's centre frequency.

## Fourier engine

Classes of tissue simulated by blochsim's extended phase graphs over the event
stream each excitation plays, their signals spanned by a temporal basis, and
the images of the terms encoded along the trajectory by bartorch's NUFFT.

| Object | Description |
| --- | --- |
| {obj}`~pulserver.virtual.simulate` | Every readout the cache beside a sequence file acquires of a phantom's tissue, demodulated as the playout demodulates. |
| {obj}`~pulserver.virtual.FourierPlayer` | The blocks the cache beside a sequence file plays, acquired of a phantom's tissue: the configuration states shifted where the gradients' moment dephases a voxel, each readout reading the pathway that passes the centre of k-space during it. |
| {obj}`~pulserver.virtual.Tissue` | A phantom sampled for the Fourier engine: each cube's position, proton density, relaxation times and frequency, the transmit field it sees and the coils' receive sensitivities. |
| {obj}`~pulserver.virtual.RigidMotion` | A subject's rigid pose against scan time: a rotation about a centre and a translation, applied to each readout at its echo. |
| {obj}`~pulserver.virtual.Girf` | A gradient impulse response per physical axis, through which the played gradients' moment is filtered. |

## Scan clock and sound

| Object | Description |
| --- | --- |
| {obj}`~pulserver.virtual.Scan` | The cache beside a sequence file acquired of a phantom's tissue by the Fourier engine against a scan clock, in spans carrying their readouts and the sound of their gradients. |
| {obj}`~pulserver.virtual.Chunk` | A span of a scan between two block boundaries: its bounds in scan time, its readouts and its sound. |
| {obj}`~pulserver.virtual.SAMPLE_RATE` | MATLAB Pulseq's audio sample rate, which a scan's sound takes unless given another. |

## External simulators

| Object | Description |
| --- | --- |
| {obj}`~pulserver.virtual.export` | Write the blocks the cache beside a sequence file plays as one Pulseq 1.5.1 file, and return the receiver phase of every readout. |

## Phantom

| Object | Description |
| --- | --- |
| {obj}`~pulserver.virtual.Phantom` | Ellipses whose signals add, received by one coil or several of analytic sensitivity, placed in the physical frame by a rotation and a position; sampled as tissue for the Fourier engine. |
| {obj}`~pulserver.virtual.BrainWeb` | BrainWeb's normal brain, downloaded on first use, placed head first and supine with its tissues' T1, T2 and proton density, in the field its susceptibility adds to B0; sampled as tissue for the Fourier engine. |
| {obj}`~pulserver.virtual.Ellipse` | An ellipse of uniform magnetization in a plane of constant z, of one chemical shift and one pair of relaxation times. |
| {obj}`~pulserver.virtual.localizer` | The axial, coronal and sagittal images of a phantom's proton density through a point, as DICOM, drawn from its ground truth with no sequence played. |

## Coils

| Object | Description |
| --- | --- |
| {obj}`~pulserver.virtual.Coil` | The RF coils of an exam, fixed in the physical frame: the transmit and receive sensitivities of BART's coil models, sampled by bartorch, or of each coil's field maps, the default shim a single-channel pulse plays through, and the VOP limits of mapped transmit coils. |
| {obj}`~pulserver.virtual.COILS` | The virtual scanner's coils by name, `transmit/receive`: the body coil both ways, the body coil with a 48-channel receive head array, and an 8-channel parallel-transmit head coil with a 32-channel receive head array. |

## Reconstruction client

| Object | Description |
| --- | --- |
| {obj}`~pulserver.virtual.send` | Send one series to a reconstruction proxy as the scanner's client sends it, each readout as it is acquired; return what comes back. |
| {obj}`~pulserver.virtual.record` | Write one series to an ISMRMRD file as `send` sends it. |
| {obj}`~pulserver.virtual.Console` | A scanner console's calls answered in process: the design calls on the interpreter's text blocks, an exam's coil and localizer, and a scan of a stored design whose clock and DICOM images stream back, reconstructed by a proxy or in process; `pulserver console` serves them over a WebSocket. |