I think it's worth pointing out that there exists a function for this purpose in ggplot2, stat_function:
library(tidyverse)
decay <- function(t, t2) exp(-(t/t2))
signal_loss <- list(water = 2500, white_matter = 100)
# Adapted from function reference:
# https://ggplot2.tidyverse.org/reference/stat_function.html
ggplot(data.frame(x = c(0, 10000)), aes(x)) +
stat_function(fun = function(x) decay(x, signal_loss$water), colour = "blue") +
stat_function(fun = function(x) decay(x, signal_loss$white_matter), colour = "red")

Created on 2019-05-20 by the reprex package (v0.2.0).
However, I think if you are plotting more than two lines from a formula, I think something like @Hlynur's approach is more idiomatic as it more naturally allows you to adjust the line's properties (colour etc).