diff --git a/src/system/Asense.jl b/src/system/Asense.jl
index ca15569..39642c2 100644
--- a/src/system/Asense.jl
+++ b/src/system/Asense.jl
@@ -19,7 +19,7 @@ and coil sensitivity maps `smaps`.
 
 The input `smaps` can either be a `D+1` dimensional array
 of size `(size(samp)..., ncoil)`,
-or a Vector of `ncoil` arrays of size `size(samp)`.
+or a Vector (or `Slices`) of `ncoil` arrays of size `size(samp)`.
 
 # Input
 - `samp::AbstractArray{<:Bool}` `D`-dimensional sampling pattern.
@@ -37,7 +37,7 @@ Returns a `LinearMapsAA.LinearMapAO` object.
 """
 function Asense(
     samp::AbstractArray{<:Bool, D},
-    smaps::Vector{<:AbstractArray{<:Number}},
+    smaps::AbstractVector{<:AbstractArray{<:Number}},
     ;
     dims = 1:D,
     T::Type{<:Complex{<:AbstractFloat}} = ComplexF32,
@@ -48,9 +48,9 @@ function Asense(
     kwargs...
 ) where {D, Tw <: Number}
 
-    all(in(1:D), dims) || error("dims $dims")
-    promote_type(Tw, T) == Tw || error("type Tw=$Tw cannot hold T=$T")
-    axes(work1) == axes(work2) == axes(samp) || error("axes mismatch: samp work")
+    all(in(1:D), dims) || throw(DimensionMismath("dims $dims"))
+    promote_type(Tw, T) == Tw || throw("type Tw=$Tw cannot hold T=$T")
+    axes(work1) == axes(work2) == axes(samp) || throw("axes mismatch: samp work")
     all(==(axes(samp)), axes.(smaps)) || throw("axes mismatch: samp smaps")
 
     sdim = size(samp)
@@ -66,12 +66,12 @@ function Asense(
 
     function forw!(y, x)
         for ic in 1:ncoil
-            @. work1 = x * smaps[ic]
+            @. work1 = x * smaps[ic] # apply sensitivity map
             ifftshift!(work2, work1)
-            mul!(work2, pf, work2)
+            mul!(work2, pf, work2) # FFT
             fftshift!(work1, work2)
             if factor == 1
-                y[:,ic] .= work1[samp]
+                y[:,ic] .= work1[samp] # sampling
             else
                 @. y[:,ic] = work1[samp] * factor
             end
@@ -111,13 +111,14 @@ function Asense(
 end
 
 
+# handle typical array version of `smaps` efficiently
 function Asense(
     samp::AbstractArray{<:Bool, D},
     smaps::AbstractArray{<:Number},
     ;
     kwargs...
 ) where D
-    ndims(smaps) == D+1 || error("dimension mismatch")
-    smapv = [eachslice(smaps, dims = D+1)...]
+    ndims(smaps) == D+1 || throw(DimensionMismatch("$(ndims(smaps)) ≠ $(D+1)"))
+    smapv = eachslice(smaps, dims = D+1) # Slices
     return Asense(samp, smapv; kwargs...)
 end