1
$\begingroup$

Hardware

We have a homemade MRI machine (from the open source project openimaging). It has a single coil and B0 = 0.5 Tesla (enough for teaching).

Sampling Mask

We used a randomly generated sampling mask for a general single slice Spin Echo imaging:

38/128 phase encoding steps

In the sampling mask, each white line represents a phase encoding step. The other lines were zero-padded. There are 38 white lines and 90 black lines in this mask. The sampling matrice is 128 * 256 (if fully sampled).

Reconstruction Algorithm

We've referenced the open source reconstruction code, based on sigpy, to reconstruct. Here are the code and data we used:

import numpy as np
import sigpy as sp
import sigpy.mri as mr
from scipy.io import loadmat
from matplotlib.pyplot import *


k = loadmat(r"Comp121_data_psf0.3.mat")["data"]
ksp = np.expand_dims(k,axis=0) # Dim:1 * 128 *256
img_shape = k.shape
#%% FFT
F = sp.linop.FFT(ksp.shape, axes=(-1,-2))
I0 = np.squeeze(abs(F.H * ksp))

#%% CS
mask = loadmat(r"Comp121_data_psf0.3.mat")["mask"]
P = sp.linop.Multiply(ksp.shape, mask)

mps = np.ones_like(abs(ksp))
S = sp.linop.Multiply(img_shape, mps)

W = sp.linop.Wavelet(img_shape)
wav = W *S.H* F.H * ksp

A = P * F * S * W.H

lamda = 0.0005
proxg = sp.prox.L1Reg(wav.shape, lamda)
alpha = 1
wav_thresh = proxg(alpha, wav)

# optimizing
max_iter = 10
alpha = 0.001

def gradf(x):
    return A.H * (A * x - ksp)

wav_hat = np.zeros(wav.shape, complex)
alg = sp.alg.GradientMethod(gradf, wav_hat, alpha, proxg=proxg, max_iter=max_iter)

while not alg.done():
    alg.update()
    print('\rL1WaveletRecon, Iteration={}'.format(alg.iter), end='')

I_cs = abs(W.H(wav_hat))
imshow(np.hstack((I0/np.max(I0),I_cs/np.max(I_cs))),cmap='gray')
title(f'padzero [left] and L1wav [right]')
figure()
imshow(abs(I0/np.max(I0)-I_cs/np.max(I_cs)))
colorbar()
show()

Problems

In my expectation, the reconstructed image from zero-padding would be worse. But the two images (zero-padding v.s. L1 wavelet regularized reconstruction) seem to be similar, both have a visible phase ghost. L1 wavelet regularization does not look superior to zero-padding in my situation. zero-padding v.s. L1Wav ghosts

Changing lambda, alpha, and max_iter has no effect, the ghost still exists. Changing the fraction of sampled phase encoding lines from 0.3 to 0.7, the ghost still exists.

My questions:

  1. Are there any mistakes in my reconstruction code?
  2. How to set the parameters: lambda, alpha, and max iter? Are there any standards or references for this?
  3. How to remove the ghost in the reconstructed image?
$\endgroup$

1 Answer 1

1
$\begingroup$

I was experiencing a similar problem (CS reco looked the same as the simple FT reco). What helped me was to normalize my data to max=1,

ksp_norm = ksp  / np.max(np.max(np.abs(ksp)))

in your case. Maybe that helps.

$\endgroup$
1
  • $\begingroup$ Visible ghosts still remained after normalize ksp data. $\endgroup$
    – Shannon
    Sep 30, 2022 at 8:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.