Let $ ABC$ be an isosceles triangle with $ \left|AB\right| \equal{} \left|AC\right| \equal{} 10$ and $ \left|BC\right| \equal{} 12$. $ P$ and $ R$ are points on $ \left[BC\right]$ such that $ \left|BP\right| \equal{} \left|RC\right| \equal{} 3$. $ S$ and $ T$ are midpoints of $ \left[AB\right]$ and $ \left[AC\right]$, respectively. If $ M$ and $ N$ are the foot of perpendiculars from $ S$ and $ R$ to $ PT$, then find $ \left|MN\right|$. 

$\textbf{(A)}\  \frac {9\sqrt {13} }{26} \qquad\textbf{(B)}\  \frac {12 \minus{} 2\sqrt {13} }{13} \qquad\textbf{(C)}\  \frac {5\sqrt {13} \plus{} 20}{13} \qquad\textbf{(D)}\  15\sqrt {3} \qquad\textbf{(E)}\  \frac {10\sqrt {13} }{13}$
1. **Identify the coordinates of the points:**
   - Let \( B = (0, 0) \) and \( C = (12, 0) \).
   - Since \( \triangle ABC \) is isosceles with \( AB = AC = 10 \) and \( BC = 12 \), the coordinates of \( A \) can be found using the distance formula. Let \( A = (6, h) \). Using the distance formula:
     \[
     AB = AC = 10 \implies \sqrt{(6-0)^2 + h^2} = 10 \implies 36 + h^2 = 100 \implies h^2 = 64 \implies h = 8
     \]
     Thus, \( A = (6, 8) \).

2. **Find the midpoints \( S \) and \( T \):**
   - \( S \) is the midpoint of \( AB \):
     \[
     S = \left( \frac{6+0}{2}, \frac{8+0}{2} \right) = (3, 4)
     \]
   - \( T \) is the midpoint of \( AC \):
     \[
     T = \left( \frac{6+12}{2}, \frac{8+0}{2} \right) = (9, 4)
     \]

3. **Find the coordinates of points \( P \) and \( R \):**
   - \( P \) is 3 units from \( B \) along \( BC \):
     \[
     P = (3, 0)
     \]
   - \( R \) is 3 units from \( C \) along \( BC \):
     \[
     R = (9, 0)
     \]

4. **Find the equation of line \( PT \):**
   - The slope of \( PT \) is:
     \[
     \text{slope of } PT = \frac{4-0}{9-3} = \frac{4}{6} = \frac{2}{3}
     \]
   - The equation of line \( PT \) using point-slope form \( y - y_1 = m(x - x_1) \):
     \[
     y - 0 = \frac{2}{3}(x - 3) \implies y = \frac{2}{3}x - 2
     \]

5. **Find the foot of the perpendiculars \( M \) and \( N \):**
   - The slope of the perpendicular to \( PT \) is the negative reciprocal of \( \frac{2}{3} \), which is \( -\frac{3}{2} \).
   - The equation of the line through \( S = (3, 4) \) perpendicular to \( PT \):
     \[
     y - 4 = -\frac{3}{2}(x - 3) \implies y = -\frac{3}{2}x + \frac{9}{2} + 4 \implies y = -\frac{3}{2}x + \frac{17}{2}
     \]
   - Solving for the intersection of \( y = \frac{2}{3}x - 2 \) and \( y = -\frac{3}{2}x + \frac{17}{2} \):
     \[
     \frac{2}{3}x - 2 = -\frac{3}{2}x + \frac{17}{2} \implies \frac{4}{6}x + \frac{9}{6}x = \frac{17}{2} + 2 \implies \frac{13}{6}x = \frac{21}{2} \implies x = \frac{21}{2} \cdot \frac{6}{13} = \frac{63}{13}
     \]
     \[
     y = \frac{2}{3} \cdot \frac{63}{13} - 2 = \frac{126}{39} - 2 = \frac{126}{39} - \frac{78}{39} = \frac{48}{39} = \frac{16}{13}
     \]
     Thus, \( M = \left( \frac{63}{13}, \frac{16}{13} \right) \).

   - The equation of the line through \( R = (9, 0) \) perpendicular to \( PT \):
     \[
     y - 0 = -\frac{3}{2}(x - 9) \implies y = -\frac{3}{2}x + \frac{27}{2}
     \]
   - Solving for the intersection of \( y = \frac{2}{3}x - 2 \) and \( y = -\frac{3}{2}x + \frac{27}{2} \):
     \[
     \frac{2}{3}x - 2 = -\frac{3}{2}x + \frac{27}{2} \implies \frac{4}{6}x + \frac{9}{6}x = \frac{27}{2} + 2 \implies \frac{13}{6}x = \frac{31}{2} \implies x = \frac{31}{2} \cdot \frac{6}{13} = \frac{93}{13}
     \]
     \[
     y = \frac{2}{3} \cdot \frac{93}{13} - 2 = \frac{186}{39} - 2 = \frac{186}{39} - \frac{78}{39} = \frac{108}{39} = \frac{36}{13}
     \]
     Thus, \( N = \left( \frac{93}{13}, \frac{36}{13} \right) \).

6. **Calculate the distance \( MN \):**
   - Using the distance formula:
     \[
     MN = \sqrt{\left( \frac{93}{13} - \frac{63}{13} \right)^2 + \left( \frac{36}{13} - \frac{16}{13} \right)^2} = \sqrt{\left( \frac{30}{13} \right)^2 + \left( \frac{20}{13} \right)^2} = \sqrt{\frac{900}{169} + \frac{400}{169}} = \sqrt{\frac{1300}{169}} = \frac{\sqrt{1300}}{13} = \frac{10\sqrt{13}}{13}
     \]

The final answer is \(\boxed{\frac{10\sqrt{13}}{13}}\).